# OpenGL Terrain Lighting Per Vertex - Looks Terrible

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I made a height map loader, and then calculated normals and turned on OpenGL lighting. When I do lighting per triangle it looks just like you would expect. When I do lighting per vertex, it looks like crap. Maybe someone has seen this before and help me out. I've checked everything I can think to check and it all seems right.
Here's some screen shots:
Height Coloring

Per Triangle Lighting

Per Vertex Lighting

Per Vertex Lighting + Normals

I appreciate it,
David

edit: ps. Sorry about the wide images, but I think you can see better that way.

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It looks like a bug in the way you average vertex normals. Can you post the relevant code snippet?

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It will help to see this:
std::vector< std::vector<float> > heightMap_;std::vector< std::vector< std::vector<Vec3f> > > triangleNormals_;

It is important that you understand how I am using triangle Normals_. It is a 3D vector, where its 2D components represent the vertex. The 3rd component only has a size of 3. So, triangleNormals_ holds 3 normals for each vertex: one for each triangle that makes up the square to its(the vertex) upper right, and the third normal is the vertex normal. So when I render using 1 normal per triangle I use the firsts 2 normals, and when I render using per vertex lighting I use the 3rd normal. If this is too difficult to understand I will draw a picture :)

Here I initialize heightMap_ which is a 2D vector of floats
    hmImage_.loadTextureFromFile(source);    if (!hmImage_.isReady()) {        std::cout << "Failed to load terrain image. Quitting." << std::endl;        return false;    }    heightMap_.resize(hmImage_.getHeight());    for (unsigned int i = 0; i < heightMap_.size(); i++) {        heightMap_[i].resize(hmImage_.getWidth());    }    for (int i = 0; i < hmImage_.getHeight(); i++) {        for (int j = 0; j < hmImage_.getWidth(); j++) {            heightMap_[j][i] = (float)hmImage_.getData()[j + hmImage_.getWidth()*i];        }    }

Then I compute a normal for each triangle:
void Terrain::computeTriangleNormals(){    triangleNormals_.resize(hmImage_.getHeight());    for (unsigned int i = 0; i < triangleNormals_.size(); i++) {        triangleNormals_[i].resize(hmImage_.getWidth());        for (unsigned int j = 0; j < triangleNormals_[i].size(); j++) {            triangleNormals_[i][j].resize(3);        }    }    for (unsigned int i = 0; i < triangleNormals_.size() - 1; i++) {        for (unsigned int j = 0; j < triangleNormals_[i].size() - 1; j++) {            Vec3f t1(1.0f * MAP_SCALE_AREA, (heightMap_[i+1][j+1] - heightMap_[i][j])*MAP_SCALE_HEIGHT, 1.0f * MAP_SCALE_AREA);            Vec3f t2(1.0f * MAP_SCALE_AREA, (heightMap_[i+1][j] - heightMap_[i][j])*MAP_SCALE_HEIGHT, 0.0f);            triangleNormals_[i][j][0] = t1.cross(t2);            Vec3f t3(0.0f, (heightMap_[i][j+1] - heightMap_[i][j])*MAP_SCALE_HEIGHT, 1.0f * MAP_SCALE_AREA);            Vec3f t4(1.0f * MAP_SCALE_AREA, (heightMap_[i+1][j+1] - heightMap_[i][j])*MAP_SCALE_HEIGHT, 1.0f * MAP_SCALE_AREA);            triangleNormals_[i][j][1] = t3.cross(t4);        }    }}

Lastly I compute the vertex normals:
void Terrain::computeVertexNormals(){    Vec3f t1;    //gooey center    for (unsigned int i = 1; i < triangleNormals_.size() - 1; i++) {        for (unsigned int j = 1; j < triangleNormals_[i].size() - 1; j++) {            t1 = triangleNormals_[i][j][0] + triangleNormals_[i][j][1] + triangleNormals_[i-1][j-1][0] //            + triangleNormals_[i-1][j-1][1] + triangleNormals_[i-1][j][0] + triangleNormals_[i][j-1][1];            triangleNormals_[i][j][2] = t1;        }    }    //Chewy Edges    for (unsigned int i = 1; i < triangleNormals_.size() - 1; i++) {        for (unsigned int j = 0; j < triangleNormals_[i].size(); j += triangleNormals_[i].size() - 1) {            if (j == 0) {                t1 = triangleNormals_[i][j][0] + triangleNormals_[i][j][1] + triangleNormals_[i-1][j][0];                triangleNormals_[i][j][2] = t1;            }            else {                t1 = triangleNormals_[i-1][j-1][0] + triangleNormals_[i-1][j-1][1] + triangleNormals_[i][j-1][1];                triangleNormals_[i][j][2] = t1;            }        }    }    for (unsigned int j = 1; j < triangleNormals_.size() - 1; j++) {        for (unsigned int i = 0; i < triangleNormals_.size(); i += triangleNormals_.size() - 1) {            if (i == 0) {                t1 = triangleNormals_[i][j][0] + triangleNormals_[i][j][1] + triangleNormals_[i][j-1][1];                triangleNormals_[i][j][2] = t1;            }            else {                t1 = triangleNormals_[i-1][j-1][0] + triangleNormals_[i-1][j-1][1] + triangleNormals_[i-1][j][0];                triangleNormals_[i][j][2] = t1;            }        }    }    //Chrunchy Corners    t1 = triangleNormals_[0][0][0] + triangleNormals_[0][0][1];    triangleNormals_[0][0][2] = t1;    t1 = triangleNormals_[0][triangleNormals_[0].size()-2][1];    triangleNormals_[0][triangleNormals_[0].size()-1][2] = t1;    t1 = triangleNormals_[triangleNormals_.size()-2][0][0];    triangleNormals_[triangleNormals_.size()-1][0][2] = t1;    t1 = triangleNormals_[triangleNormals_.size()-2][triangleNormals_[0].size()-2][0] //    + triangleNormals_[triangleNormals_.size()-2][triangleNormals_[0].size()-2][1];    triangleNormals_[triangleNormals_.size()-1][triangleNormals_[0].size()-1][2] = t1;}

Also Vec3f is a 3d vector class. If it will help you to see it, then I will post it.

Thanks!
edit: ps. Again, sorry for really screwing over the width of this page. That line of code is way too long, hah.

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OK, there no way anybody could follow this. Hopefully this will help:

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Your results look correct. The "seams" are due to the fact that a given terrain quad is made up of 2 triangles, each of which only have access to 3 of the four vertices, thus interpolation errors occur when there is a large enough discrepancy between the tri's 3 verts and the inaccessible fourth vert. Sorry, that's a rubbish way of explaining it but it's late over here, I'll try again in the morning.

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Quote:
 Original post by JackTheRapperYour results look correct. The "seams" are due to the fact that a given terrain quad is made up of 2 triangles, each of which only have access to 3 of the four vertices, thus interpolation errors occur when there is a large enough discrepancy between the tri's 3 verts and the inaccessible fourth vert.
The simplest way to fix this, is to derive your normal directly from the heightmap (via central distance, or similar), rather than from the triangles.

In fact, I would highly recommend that you use a much higher-resolution heightmap than you use vertex grid, and produce from that a similarly high-resolution normal map, allowing you to display much more detail.

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Quote:
 Original post by JackTheRapperYour results look correct. The "seams" are due to the fact that a given terrain quad is made up of 2 triangles, each of which only have access to 3 of the four vertices, thus interpolation errors occur when there is a large enough discrepancy between the tri's 3 verts and the inaccessible fourth vert. Sorry, that's a rubbish way of explaining it but it's late over here, I'll try again in the morning.

This actually makes a lot of sense to me, I guess I wasn't really understanding how openGL colors things, or blends colors, or something. I thought if you had two triangles that share an edge (they share two vertices) then along that edge each triangle would have the same color (no seam). To me it seems strange that the other two vertices could affect that. Thanks for bringing it to my attention. I guess I should really read up on how that works.

Quote:
 Original post by swiftcoderThe simplest way to fix this, is to derive your normal directly from the heightmap (via central distance, or similar), rather than from the triangles.In fact, I would highly recommend that you use a much higher-resolution heightmap than you use vertex grid, and produce from that a similarly high-resolution normal map, allowing you to display much more detail.

When I was searching I read something about calculating normals directly from the height map, but I don't remember where that was and I didn't think much of it at the time. I can't really think (thought I haven't given it a lot of thought yet) how exactly you could calculate normals from a height field. It seems like you still need to create surfaces and calculate their normals.

Maybe you could point me in the right direction, please.

Thank you both very much.

EDIT: Never mind. I just needed a little bit more time to think about it. I think I know what I want to try now. If it doesn't work out I'll let you know.
You guys are awesome!

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Try this:
do this:

glColor3f ( <your terrain data x >/255.0f, <your terrain data y >/255.0f, <your terrain data z >/255.0f );
...
.....

that should work.

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I am very disappointed to report that what I tried did not fix the problem. This time I took each point in the height field and draw a vector from it to each adjacent point on the height field (in including diagonals). This produces eight triangles with a vertex at the point of interest. I calculate the cross product of each of those triangles and add them all together and then normalize. This means that every vertex normal is taking into account the 8 vertices around it.

I didn't not change the triangle pattern (maybe that's the problem) and it looks exactly the same as before. Do you recommend that I draw triangles in a different pattern? or is this one acceptable?

Maybe I should take into account more than just the 8 nearest vertices?

HELP, I'm going crazy.

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Quote:
 Original post by GMA965_X3100Try this:when finnally displaying your terrain, and when colouring your terrain,do this:glColor3f ( /255.0f, /255.0f, /255.0f );........that should work.

I don't really understand why I should do this. This will make the map black at the origin and lighter the further a point is from the origin?

Or maybe you mean something different than I am thinking when you say x data and z data.

In the first picture I posted the coloring was done like this:
glColor3f ( <your terrain data y >/255.0f, <your terrain data y >/255.0f, <your terrain data y >/255.0f );

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maybe I made an error in reply. What i really meant was:

What method do you use to create terrain may I ask?

The actual colouring of my terrain looks like this:
glColor3f ( ter [ x ] [ z ] [ 1 ]/255.0f, ter [ x ] [ z ] [ 1 ]/255.0f, ter [ x ] [ z ] [ 1 ]/255.0f ) );

If what you say is true, we might actually be creating our terrains in the same way, using y vertices to first draw flat plane, consisting of x, y and z vertices;
so the [ 1 ] above is the y index that changes it's heihgt based on heightmap, and ofcourse that would be in for statements that account for each and every vertex created by terrain x and z size.

so indeed, it is glColor3f ( <your y data here>/255.0f, <your y data here>/255.0f, <your y data here>/255.0f ) sort of; but in truth involves all axis, but in reality using height axis ( y ) as index into array of terrain data.

Maybe I'm not advanced openGL wise enough to correct you. I am but merely a 2 month openGL programmer after all, and probably 3 good weeks out of that went towards learning openGL.

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I see what your getting at, but the problem I'm having doesn't necessarily have to do with coloring. I can turn color material off and the problem persists. This has to do with the calculation of the normals

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Quote:
Original post by Geared
Quote:
 Original post by swiftcoderThe simplest way to fix this, is to derive your normal directly from the heightmap (via central distance, or similar), rather than from the triangles.
When I was searching I read something about calculating normals directly from the height map, but I don't remember where that was and I didn't think much of it at the time. I can't really think (thought I haven't given it a lot of thought yet) how exactly you could calculate normals from a height field. It seems like you still need to create surfaces and calculate their normals.
I don't know if your math background includes calculus, but if it does, recall that to find the line normal to a curve, you take the first derivative of that curve. For a given point on the surface of a heightmap, we can regard it as the intersection of a curve in the x direction, and a curve in the z direction (assuming y is the vertical axis). One can approximate the derivative of a curve using finite differences (i.e. the difference between two nearby samples), and from this you can derive the normal.

That actually isn't as hard as it sounds, but we can also cheat a little. We can treat any point on the surface as belonging to a triangle of arbitrary size, and compute its normal by finding the vectors for two of its sides, and computing the cross product. Since our heightmap forms a grid, right triangles make the most sense, leading to something like this:

h00 = height(x, y)h10 = height(x + 1, y)h01 = height(x, y + 1)normal = vec3(1, h10 - h00, 0) cross vec3(0, h01 - h00, 1)normal.normalise()

Where 1 represents the distance to the next pixel.

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I think this is just a problem with using quads to represent curved spaces.

If the two tris making the quad aren't planar (and they're usually not for stuff like fractal ground) then you will see different results for when the edge is connected to the other two vertices instead, which proves a quad doesn't cut it. Even tools like 3DS Max have an "edge turn" facility for manually fixing cases like you highlight.

It's better to use 4 tris per quad with a centroid vertex (letter envelope) but this only minimises it. You'll still get problems at the shared outer edges of your quad with the next one along, so you need to split the edges as well and you end up with millions of triangles. Think of this as an aliasing problem.

A hexagonal arrangement with 6 tris per hex might work out more efficient for solving this, but I've never tried it. It sure feels like it should work though.

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As Rubicon said, I'm not sure you can really fix the problem by changing the way normals are calculated.

Personally I'd suggest the best method of "fixing" it would be to add a nice LOD system to your terrain renderer, so that the number of triangles near the viewer and on the more sharply angled terrain is a lot higher. This should minimise any visible artifacts.

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Quote:
 Original post by sprite_houndAs Rubicon said, I'm not sure you can really fix the problem by changing the way normals are calculated.
Wait... what? Of course you can. Do you think all lighting looked like this before normal mapping was invented?

The current issues are a form of aliasing, caused by taking a (fairly) continuous representation (the heightmap), and mapping it to a discrete representation (triangles), which is then used to calculate the normals. Calculating the normals directly from the continuous representation sidesteps the issue.

Even though two adjacent triangles are not generally planar, the shared edge is comprised of two vertices, both of which are shared between the two triangles. Since in each triangle it has the same end vertices, they will be interpolated identically, and the lighting will be consistent. This is true for all three edges of a triangle, which leads to continuous lighting across neighbouring faces.

***

In this day and age, you should honestly try to forgo vertex normals entirely, in favour of normal mapping with per-pixel lighting, at which point you can increase the detail immensely, at very little cost...

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Quote:
Original post by swiftcoder
Quote:
 Original post by sprite_houndAs Rubicon said, I'm not sure you can really fix the problem by changing the way normals are calculated.
Wait... what? Of course you can. Do you think all lighting looked like this before normal mapping was invented?

No, I suppose I should have said "while using per vertex lighting". My point was more that while using per vertex lighting it doesn't matter how you calculate the normal and send it to the graphics card, it'll still be interpolated linearly between the vertices, which causes those artifacts.

I agree that using a detail normal map and per pixel lighting would be a good idea.

(I should probably also amend my LOD suggestion to be "if it's feasible for you / your terrain creation method and you really want to stick to per vertex lighting")

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Quote:
 Original post by sprite_houndMy point was more that while using per vertex lighting it doesn't matter how you calculate the normal and send it to the graphics card, it'll still be interpolated linearly between the vertices, which causes those artifacts.

Yes, the normals are linearly interpolated, but provided that your vertex normals are of unit length (and they should be), the differences are very small. Certainly not enough to cause the bright/dark lines the OP is experiencing.

The OP's normal generation code is bjorked enough that I can't tell at a glance if it is correct or not, but I *can* tell that he is missing the final normalise - and this is likely the cause of the issue.

I also don't see much point in calculating vertex normals by summing triangle normals, given that your triangle normals are already created by central differencing - you might as well just use central differencing at each vertex.

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Well I tried some things. I'm pretty convinced that this does not have to do with normal calculation.

I got better results when I (and this was before I read your new suggestions) changed the triangle pattern, changed normal calculation to look at the nearest 24 vertices, and used a higher resolution heightmap.

Now the seam effect on some flat slopes is gone, and it is mostly apparent at light-dark boundaries.

Essentially by making the triangle pattern bigger, any lighting errors are less noticeable because they do not repeat so frequently.

Top left: I first tried calculating normals by doing
n= a x b + b x c + c x d + d x e + e x f + f x g + g x h + h x a

Top right: Then I decided to take into account more vertices. I took the previous normal and added:
n += (a x b) * RATIO + (b x c) * RATIO ... and then normalized it.

Doing all this made surprisingly little difference. So I changed the pattern to the one seen at the bottom of the image. This helped a lot, but it still seems fuzzy at light-dark boundaries (because of a lack of triangles, I now realize)

Here's an example of what it looks like now. I think it's better, it just looks kinda fuzzy:

So with LOD you need to detect the curvature of of the terrain and use more triangles for greater curvature and less for less. right?
Since I already have a normal field for the map, calculating the curvature at any point isn't that difficult.

Maybe I can start with 2 tris per poly for low curvature, and if the curvature is higher then start splitting the tris in half (using some maths to predict new vertices.

I feel like that would be a pretty good solution, though I can already see issues with implementation... I haven't done any reading over LOD terrain, but this is how it works right?

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Looks a helluva lot better now you've tesselated it better ;)

Whilst you're in the mood to dick about, why not try my hexagonal suggestion? If you add half a unit sideways to every second row of vertex positions you should be mostly there.

This is meant to look fuzzy - that just proves there are no sharp edges, which there shouldn't be. Adding a bit more ambient will make it look more natural.

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The normals are being normalized. I enabled GL_NORMALIZE anyway.

This is the code for central differencing normal calculation:
    Vec3f t1, t2;    for (unsigned int i = 0; i < triangleNormals_.size(); i++) {        for (unsigned int j = 0; j < triangleNormals_[i].size(); j++) {            if (i > 1 && j > 1 && i < triangleNormals_.size()-2 && j < triangleNormals_[i].size()-2) {                //gooey center                /*First add in the nearest 8 normals */                t1.setX(1.0f * MAP_SCALE_AREA);                t1.setY((heightMap_[i+1][j] - heightMap_[i][j]) * MAP_SCALE_HEIGHT);                t1.setZ(0.0f);                t2.setX(1.0f * MAP_SCALE_AREA);                t2.setY((heightMap_[i+1][j-1] - heightMap_[i][j]) * MAP_SCALE_HEIGHT);                t2.setZ(-1.0f * MAP_SCALE_AREA);                triangleNormals_[i][j][2] = t1.cross(t2);                t1.setX(0.0f);                t1.setY((heightMap_[i][j-1] - heightMap_[i][j]) * MAP_SCALE_HEIGHT);                t1.setZ(-1.0f * MAP_SCALE_AREA);                triangleNormals_[i][j][2] += t2.cross(t1);                t2.setX(-1.0f * MAP_SCALE_AREA);                t2.setY((heightMap_[i-1][j-1] - heightMap_[i][j]) * MAP_SCALE_HEIGHT);                t2.setZ(-1.0f * MAP_SCALE_AREA);                triangleNormals_[i][j][2] += t1.cross(t2);                t1.setX(-1.0f * MAP_SCALE_AREA);                t1.setY((heightMap_[i-1][j] - heightMap_[i][j]) * MAP_SCALE_HEIGHT);                t1.setZ(0.0f);                triangleNormals_[i][j][2] += t2.cross(t1);                t2.setX(-1.0f * MAP_SCALE_AREA);                t2.setY((heightMap_[i-1][j+1] - heightMap_[i][j]) * MAP_SCALE_HEIGHT);                t2.setZ(1.0f * MAP_SCALE_AREA);                triangleNormals_[i][j][2] += t1.cross(t2);                t1.setX(0.0f);                t1.setY((heightMap_[i][j+1] - heightMap_[i][j]) * MAP_SCALE_HEIGHT);                t1.setZ(1.0f * MAP_SCALE_AREA);                triangleNormals_[i][j][2] += t2.cross(t1);                t2.setX(1.0f * MAP_SCALE_AREA);                t2.setY((heightMap_[i+1][j+1] - heightMap_[i][j]) * MAP_SCALE_HEIGHT);                t2.setZ(1.0f * MAP_SCALE_AREA);                triangleNormals_[i][j][2] += t1.cross(t2);                t1.setX(1.0f * MAP_SCALE_AREA);                t1.setY((heightMap_[i+1][j] - heightMap_[i][j]) * MAP_SCALE_HEIGHT);                t1.setZ(0.0f);                triangleNormals_[i][j][2] += t2.cross(t1);

This is just like in the picture I gave in my last post, except in the code I did not start with the vertices to the right, instead I started at the vertices above and worked CCW.
x and z components are either 1 or 0 obviously. And the y component is the difference between the central vertice and the near vertice.
All the values are scaled by constants as you can see.

I know its a bit clumsy, but I think you should be able to follow it pretty easily. I didn't include the next 16 nearest vertices, but it works the same way, except that the cross product gets reduced because it's not weighted as much. (Also because it these triangles have a larger area and therefor a normal with a greater magnitude.

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Quote:
 Original post by RubiconLooks a helluva lot better now you've tesselated it better ;)Whilst you're in the mood to dick about, why not try my hexagonal suggestion? If you add half a unit sideways to every second row of vertex positions you should be mostly there.This is meant to look fuzzy - that just proves there are no sharp edges, which there shouldn't be. Adding a bit more ambient will make it look more natural.

I do plan on trying it. Later today I should have some free time to try it. I'll let you know how it goes. I might also try adding a little LOD action.

Also I'm still partial to swifts thoughts that something is still wrong. I think maybe I've just masked it here. I don't think it should have looked like that even with the other triangle pattern. If you don't mind swift, let me know if that is not the right way to calculate normals.

Thanks guys

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Why wouldn't it look fuzzy? Vertex lighting was never meant to be smooth, try per-pixel lighting.

Also, I am loving your screen shots. I always like seeing live examples of terrain generation :)

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Quote:
 Original post by GearedIf you don't mind swift, let me know if that is not the right way to calculate normals.
I don't see anything wrong per se, but it is a ridiculous degree of overkill. Lets try the simplest possible method of calculating vertex normals, and see how that compares...

We want the normal at point P, so we sample along the grid in each direction, at locations h1-h4. The formula for the resulting normal is:

normal = vec3( h3 - h1, 1.0, h2 - h4 )

Where 1.0 is the same as the horizontal offset used to find h1-h4 (this is just a simplification of the central difference method).

This ought to eliminate at least three possible locations for error, and we can see if the results come out any different...

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OK, so the problem was, like Swift said, with the normals. The reason I was so confused was because when I started calculating normals directly from the height field, I thought that it didn't fix the problem. I probably didn't compile or something. It seems the two different tessellation patterns that I used do not make any difference. The improvement came from my normal calculations, I just didn't notice it. Anyway, I did a bunch of tests. I computed normals using nearest 4, nearest 8, and nearest 24 heights. I also compared with and without a good neighbor modification (isn't that what it's called when you make the vertices more like their neighboring vertices?)

I'm getting that the good neighbor modification always improves quality. And that using nearest 24 is better than using nearest 8 is better than using nearest 4. So these are good results in my mind.

I still think the lighting looks pretty terrible though, but I'm sure it takes lots of tweaking and messing with it to make it look good.

If you want I will post the results of my tests.

@Rubicon, I'm not really motivated to try another tessellation, since the two I used were visually identical. I'm thinking it has more to do with the number of triangles and curvature.

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Introduction
Graphics APIs have come a long way from small set of basic commands allowing limited control of configurable stages of early 3D accelerators to very low-level programming interfaces exposing almost every aspect of the underlying graphics hardware. Next-generation APIs, Direct3D12 by Microsoft and Vulkan by Khronos are relatively new and have only started getting widespread adoption and support from hardware vendors, while Direct3D11 and OpenGL are still considered industry standard. New APIs can provide substantial performance and functional improvements, but may not be supported by older hardware. An application targeting wide range of platforms needs to support Direct3D11 and OpenGL. New APIs will not give any advantage when used with old paradigms. It is totally possible to add Direct3D12 support to an existing renderer by implementing Direct3D11 interface through Direct3D12, but this will give zero benefits. Instead, new approaches and rendering architectures that leverage flexibility provided by the next-generation APIs are expected to be developed.
There are at least four APIs (Direct3D11, Direct3D12, OpenGL/GLES, Vulkan, plus Apple's Metal for iOS and osX platforms) that a cross-platform 3D application may need to support. Writing separate code paths for all APIs is clearly not an option for any real-world application and the need for a cross-platform graphics abstraction layer is evident. The following is the list of requirements that I believe such layer needs to satisfy:
Lightweight abstractions: the API should be as close to the underlying native APIs as possible to allow an application leverage all available low-level functionality. In many cases this requirement is difficult to achieve because specific features exposed by different APIs may vary considerably. Low performance overhead: the abstraction layer needs to be efficient from performance point of view. If it introduces considerable amount of overhead, there is no point in using it. Convenience: the API needs to be convenient to use. It needs to assist developers in achieving their goals not limiting their control of the graphics hardware. Multithreading: ability to efficiently parallelize work is in the core of Direct3D12 and Vulkan and one of the main selling points of the new APIs. Support for multithreading in a cross-platform layer is a must. Extensibility: no matter how well the API is designed, it still introduces some level of abstraction. In some cases the most efficient way to implement certain functionality is to directly use native API. The abstraction layer needs to provide seamless interoperability with the underlying native APIs to provide a way for the app to add features that may be missing. Diligent Engine is designed to solve these problems. Its main goal is to take advantages of the next-generation APIs such as Direct3D12 and Vulkan, but at the same time provide support for older platforms via Direct3D11, OpenGL and OpenGLES. Diligent Engine exposes common C++ front-end for all supported platforms and provides interoperability with underlying native APIs. It also supports integration with Unity and is designed to be used as graphics subsystem in a standalone game engine, Unity native plugin or any other 3D application. Full source code is available for download at GitHub and is free to use.
Overview
Diligent Engine API takes some features from Direct3D11 and Direct3D12 as well as introduces new concepts to hide certain platform-specific details and make the system easy to use. It contains the following main components:
Render device (IRenderDevice  interface) is responsible for creating all other objects (textures, buffers, shaders, pipeline states, etc.).
Device context (IDeviceContext interface) is the main interface for recording rendering commands. Similar to Direct3D11, there are immediate context and deferred contexts (which in Direct3D11 implementation map directly to the corresponding context types). Immediate context combines command queue and command list recording functionality. It records commands and submits the command list for execution when it contains sufficient number of commands. Deferred contexts are designed to only record command lists that can be submitted for execution through the immediate context.
An alternative way to design the API would be to expose command queue and command lists directly. This approach however does not map well to Direct3D11 and OpenGL. Besides, some functionality (such as dynamic descriptor allocation) can be much more efficiently implemented when it is known that a command list is recorded by a certain deferred context from some thread.
The approach taken in the engine does not limit scalability as the application is expected to create one deferred context per thread, and internally every deferred context records a command list in lock-free fashion. At the same time this approach maps well to older APIs.
In current implementation, only one immediate context that uses default graphics command queue is created. To support multiple GPUs or multiple command queue types (compute, copy, etc.), it is natural to have one immediate contexts per queue. Cross-context synchronization utilities will be necessary.
Swap Chain (ISwapChain interface). Swap chain interface represents a chain of back buffers and is responsible for showing the final rendered image on the screen.
Render device, device contexts and swap chain are created during the engine initialization.
Resources (ITexture and IBuffer interfaces). There are two types of resources - textures and buffers. There are many different texture types (2D textures, 3D textures, texture array, cubmepas, etc.) that can all be represented by ITexture interface.
Resources Views (ITextureView and IBufferView interfaces). While textures and buffers are mere data containers, texture views and buffer views describe how the data should be interpreted. For instance, a 2D texture can be used as a render target for rendering commands or as a shader resource.
Pipeline State (IPipelineState interface). GPU pipeline contains many configurable stages (depth-stencil, rasterizer and blend states, different shader stage, etc.). Direct3D11 uses coarse-grain objects to set all stage parameters at once (for instance, a rasterizer object encompasses all rasterizer attributes), while OpenGL contains myriad functions to fine-grain control every individual attribute of every stage. Both methods do not map very well to modern graphics hardware that combines all states into one monolithic state under the hood. Direct3D12 directly exposes pipeline state object in the API, and Diligent Engine uses the same approach.
Shader Resource Binding (IShaderResourceBinding interface). Shaders are programs that run on the GPU. Shaders may access various resources (textures and buffers), and setting correspondence between shader variables and actual resources is called resource binding. Resource binding implementation varies considerably between different API. Diligent Engine introduces a new object called shader resource binding that encompasses all resources needed by all shaders in a certain pipeline state.
API Basics
Creating Resources
Device resources are created by the render device. The two main resource types are buffers, which represent linear memory, and textures, which use memory layouts optimized for fast filtering. Graphics APIs usually have a native object that represents linear buffer. Diligent Engine uses IBuffer interface as an abstraction for a native buffer. To create a buffer, one needs to populate BufferDesc structure and call IRenderDevice::CreateBuffer() method as in the following example:
BufferDesc BuffDesc; BufferDesc.Name = "Uniform buffer"; BuffDesc.BindFlags = BIND_UNIFORM_BUFFER; BuffDesc.Usage = USAGE_DYNAMIC; BuffDesc.uiSizeInBytes = sizeof(ShaderConstants); BuffDesc.CPUAccessFlags = CPU_ACCESS_WRITE; m_pDevice->CreateBuffer( BuffDesc, BufferData(), &m_pConstantBuffer ); While there is usually just one buffer object, different APIs use very different approaches to represent textures. For instance, in Direct3D11, there are ID3D11Texture1D, ID3D11Texture2D, and ID3D11Texture3D objects. In OpenGL, there is individual object for every texture dimension (1D, 2D, 3D, Cube), which may be a texture array, which may also be multisampled (i.e. GL_TEXTURE_2D_MULTISAMPLE_ARRAY). As a result there are nine different GL texture types that Diligent Engine may create under the hood. In Direct3D12, there is only one resource interface. Diligent Engine hides all these details in ITexture interface. There is only one  IRenderDevice::CreateTexture() method that is capable of creating all texture types. Dimension, format, array size and all other parameters are specified by the members of the TextureDesc structure:
TextureDesc TexDesc; TexDesc.Name = "My texture 2D"; TexDesc.Type = TEXTURE_TYPE_2D; TexDesc.Width = 1024; TexDesc.Height = 1024; TexDesc.Format = TEX_FORMAT_RGBA8_UNORM; TexDesc.Usage = USAGE_DEFAULT; TexDesc.BindFlags = BIND_SHADER_RESOURCE | BIND_RENDER_TARGET | BIND_UNORDERED_ACCESS; TexDesc.Name = "Sample 2D Texture"; m_pRenderDevice->CreateTexture( TexDesc, TextureData(), &m_pTestTex ); If native API supports multithreaded resource creation, textures and buffers can be created by multiple threads simultaneously.
Interoperability with native API provides access to the native buffer/texture objects and also allows creating Diligent Engine objects from native handles. It allows applications seamlessly integrate native API-specific code with Diligent Engine.
Next-generation APIs allow fine level-control over how resources are allocated. Diligent Engine does not currently expose this functionality, but it can be added by implementing IResourceAllocator interface that encapsulates specifics of resource allocation and providing this interface to CreateBuffer() or CreateTexture() methods. If null is provided, default allocator should be used.
Initializing the Pipeline State
As it was mentioned earlier, Diligent Engine follows next-gen APIs to configure the graphics/compute pipeline. One big Pipelines State Object (PSO) encompasses all required states (all shader stages, input layout description, depth stencil, rasterizer and blend state descriptions etc.). This approach maps directly to Direct3D12/Vulkan, but is also beneficial for older APIs as it eliminates pipeline misconfiguration errors. With many individual calls tweaking various GPU pipeline settings it is very easy to forget to set one of the states or assume the stage is already properly configured when in fact it is not. Using pipeline state object helps avoid these problems as all stages are configured at once.
While in earlier APIs shaders were bound separately, in the next-generation APIs as well as in Diligent Engine shaders are part of the pipeline state object. The biggest challenge when authoring shaders is that Direct3D and OpenGL/Vulkan use different shader languages (while Apple uses yet another language in their Metal API). Maintaining two versions of every shader is not an option for real applications and Diligent Engine implements shader source code converter that allows shaders authored in HLSL to be translated to GLSL. To create a shader, one needs to populate ShaderCreationAttribs structure. SourceLanguage member of this structure tells the system which language the shader is authored in:
When sampling a texture in a shader, the texture sampler was traditionally specified as separate object that was bound to the pipeline at run time or set as part of the texture object itself. However, in most cases it is known beforehand what kind of sampler will be used in the shader. Next-generation APIs expose new type of sampler called static sampler that can be initialized directly in the pipeline state. Diligent Engine exposes this functionality: when creating a shader, textures can be assigned static samplers. If static sampler is assigned, it will always be used instead of the one initialized in the texture shader resource view. To initialize static samplers, prepare an array of StaticSamplerDesc structures and initialize StaticSamplers and NumStaticSamplers members. Static samplers are more efficient and it is highly recommended to use them whenever possible. On older APIs, static samplers are emulated via generic sampler objects.
The following is an example of shader initialization:
Creating the Pipeline State Object
After all required shaders are created, the rest of the fields of the PipelineStateDesc structure provide depth-stencil, rasterizer, and blend state descriptions, the number and format of render targets, input layout format, etc. For instance, rasterizer state can be described as follows:
PipelineStateDesc PSODesc; RasterizerStateDesc &RasterizerDesc = PSODesc.GraphicsPipeline.RasterizerDesc; RasterizerDesc.FillMode = FILL_MODE_SOLID; RasterizerDesc.CullMode = CULL_MODE_NONE; RasterizerDesc.FrontCounterClockwise = True; RasterizerDesc.ScissorEnable = True; RasterizerDesc.AntialiasedLineEnable = False; Depth-stencil and blend states are defined in a similar fashion.
Another important thing that pipeline state object encompasses is the input layout description that defines how inputs to the vertex shader, which is the very first shader stage, should be read from the memory. Input layout may define several vertex streams that contain values of different formats and sizes:
// Define input layout InputLayoutDesc &Layout = PSODesc.GraphicsPipeline.InputLayout; LayoutElement TextLayoutElems[] = {     LayoutElement( 0, 0, 3, VT_FLOAT32, False ),     LayoutElement( 1, 0, 4, VT_UINT8, True ),     LayoutElement( 2, 0, 2, VT_FLOAT32, False ), }; Layout.LayoutElements = TextLayoutElems; Layout.NumElements = _countof( TextLayoutElems ); Finally, pipeline state defines primitive topology type. When all required members are initialized, a pipeline state object can be created by IRenderDevice::CreatePipelineState() method:
// Define shader and primitive topology PSODesc.GraphicsPipeline.PrimitiveTopologyType = PRIMITIVE_TOPOLOGY_TYPE_TRIANGLE; PSODesc.GraphicsPipeline.pVS = pVertexShader; PSODesc.GraphicsPipeline.pPS = pPixelShader; PSODesc.Name = "My pipeline state"; m_pDev->CreatePipelineState(PSODesc, &m_pPSO); When PSO object is bound to the pipeline, the engine invokes all API-specific commands to set all states specified by the object. In case of Direct3D12 this maps directly to setting the D3D12 PSO object. In case of Direct3D11, this involves setting individual state objects (such as rasterizer and blend states), shaders, input layout etc. In case of OpenGL, this requires a number of fine-grain state tweaking calls. Diligent Engine keeps track of currently bound states and only calls functions to update these states that have actually changed.
Direct3D11 and OpenGL utilize fine-grain resource binding models, where an application binds individual buffers and textures to certain shader or program resource binding slots. Direct3D12 uses a very different approach, where resource descriptors are grouped into tables, and an application can bind all resources in the table at once by setting the table in the command list. Resource binding model in Diligent Engine is designed to leverage this new method. It introduces a new object called shader resource binding that encapsulates all resource bindings required for all shaders in a certain pipeline state. It also introduces the classification of shader variables based on the frequency of expected change that helps the engine group them into tables under the hood:
Static variables (SHADER_VARIABLE_TYPE_STATIC) are variables that are expected to be set only once. They may not be changed once a resource is bound to the variable. Such variables are intended to hold global constants such as camera attributes or global light attributes constant buffers. Mutable variables (SHADER_VARIABLE_TYPE_MUTABLE) define resources that are expected to change on a per-material frequency. Examples may include diffuse textures, normal maps etc. Dynamic variables (SHADER_VARIABLE_TYPE_DYNAMIC) are expected to change frequently and randomly. Shader variable type must be specified during shader creation by populating an array of ShaderVariableDesc structures and initializing ShaderCreationAttribs::Desc::VariableDesc and ShaderCreationAttribs::Desc::NumVariables members (see example of shader creation above).
Static variables cannot be changed once a resource is bound to the variable. They are bound directly to the shader object. For instance, a shadow map texture is not expected to change after it is created, so it can be bound directly to the shader:
m_pPSO->CreateShaderResourceBinding(&m_pSRB); Note that an SRB is only compatible with the pipeline state it was created from. SRB object inherits all static bindings from shaders in the pipeline, but is not allowed to change them.
Mutable resources can only be set once for every instance of a shader resource binding. Such resources are intended to define specific material properties. For instance, a diffuse texture for a specific material is not expected to change once the material is defined and can be set right after the SRB object has been created:
m_pSRB->GetVariable(SHADER_TYPE_PIXEL, "tex2DDiffuse")->Set(pDiffuseTexSRV); In some cases it is necessary to bind a new resource to a variable every time a draw command is invoked. Such variables should be labeled as dynamic, which will allow setting them multiple times through the same SRB object:
m_pSRB->GetVariable(SHADER_TYPE_VERTEX, "cbRandomAttribs")->Set(pRandomAttrsCB); Under the hood, the engine pre-allocates descriptor tables for static and mutable resources when an SRB objcet is created. Space for dynamic resources is dynamically allocated at run time. Static and mutable resources are thus more efficient and should be used whenever possible.
As you can see, Diligent Engine does not expose low-level details of how resources are bound to shader variables. One reason for this is that these details are very different for various APIs. The other reason is that using low-level binding methods is extremely error-prone: it is very easy to forget to bind some resource, or bind incorrect resource such as bind a buffer to the variable that is in fact a texture, especially during shader development when everything changes fast. Diligent Engine instead relies on shader reflection system to automatically query the list of all shader variables. Grouping variables based on three types mentioned above allows the engine to create optimized layout and take heavy lifting of matching resources to API-specific resource location, register or descriptor in the table.
This post gives more details about the resource binding model in Diligent Engine.
Setting the Pipeline State and Committing Shader Resources
Before any draw or compute command can be invoked, the pipeline state needs to be bound to the context:
m_pContext->SetPipelineState(m_pPSO); Under the hood, the engine sets the internal PSO object in the command list or calls all the required native API functions to properly configure all pipeline stages.
The next step is to bind all required shader resources to the GPU pipeline, which is accomplished by IDeviceContext::CommitShaderResources() method:
m_pContext->CommitShaderResources(m_pSRB, COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES); The method takes a pointer to the shader resource binding object and makes all resources the object holds available for the shaders. In the case of D3D12, this only requires setting appropriate descriptor tables in the command list. For older APIs, this typically requires setting all resources individually.
Next-generation APIs require the application to track the state of every resource and explicitly inform the system about all state transitions. For instance, if a texture was used as render target before, while the next draw command is going to use it as shader resource, a transition barrier needs to be executed. Diligent Engine does the heavy lifting of state tracking.  When CommitShaderResources() method is called with COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES flag, the engine commits and transitions resources to correct states at the same time. Note that transitioning resources does introduce some overhead. The engine tracks state of every resource and it will not issue the barrier if the state is already correct. But checking resource state is an overhead that can sometimes be avoided. The engine provides IDeviceContext::TransitionShaderResources() method that only transitions resources:
m_pContext->TransitionShaderResources(m_pPSO, m_pSRB); In some scenarios it is more efficient to transition resources once and then only commit them.
Invoking Draw Command
The final step is to set states that are not part of the PSO, such as render targets, vertex and index buffers. Diligent Engine uses Direct3D11-syle API that is translated to other native API calls under the hood:
ITextureView *pRTVs[] = {m_pRTV}; m_pContext->SetRenderTargets(_countof( pRTVs ), pRTVs, m_pDSV); // Clear render target and depth buffer const float zero[4] = {0, 0, 0, 0}; m_pContext->ClearRenderTarget(nullptr, zero); m_pContext->ClearDepthStencil(nullptr, CLEAR_DEPTH_FLAG, 1.f); // Set vertex and index buffers IBuffer *buffer[] = {m_pVertexBuffer}; Uint32 offsets[] = {0}; Uint32 strides[] = {sizeof(MyVertex)}; m_pContext->SetVertexBuffers(0, 1, buffer, strides, offsets, SET_VERTEX_BUFFERS_FLAG_RESET); m_pContext->SetIndexBuffer(m_pIndexBuffer, 0); Different native APIs use various set of function to execute draw commands depending on command details (if the command is indexed, instanced or both, what offsets in the source buffers are used etc.). For instance, there are 5 draw commands in Direct3D11 and more than 9 commands in OpenGL with something like glDrawElementsInstancedBaseVertexBaseInstance not uncommon. Diligent Engine hides all details with single IDeviceContext::Draw() method that takes takes DrawAttribs structure as an argument. The structure members define all attributes required to perform the command (primitive topology, number of vertices or indices, if draw call is indexed or not, if draw call is instanced or not, if draw call is indirect or not, etc.). For example:
DrawAttribs attrs; attrs.IsIndexed = true; attrs.IndexType = VT_UINT16; attrs.NumIndices = 36; attrs.Topology = PRIMITIVE_TOPOLOGY_TRIANGLE_LIST; pContext->Draw(attrs); For compute commands, there is IDeviceContext::DispatchCompute() method that takes DispatchComputeAttribs structure that defines compute grid dimension.
Source Code
Full engine source code is available on GitHub and is free to use. The repository contains two samples, asteroids performance benchmark and example Unity project that uses Diligent Engine in native plugin.
AntTweakBar sample is Diligent Engine’s “Hello World” example.

Atmospheric scattering sample is a more advanced example. It demonstrates how Diligent Engine can be used to implement various rendering tasks: loading textures from files, using complex shaders, rendering to multiple render targets, using compute shaders and unordered access views, etc.

Asteroids performance benchmark is based on this demo developed by Intel. It renders 50,000 unique textured asteroids and allows comparing performance of Direct3D11 and Direct3D12 implementations. Every asteroid is a combination of one of 1000 unique meshes and one of 10 unique textures.

Finally, there is an example project that shows how Diligent Engine can be integrated with Unity.

Future Work
The engine is under active development. It currently supports Windows desktop, Universal Windows and Android platforms. Direct3D11, Direct3D12, OpenGL/GLES backends are now feature complete. Vulkan backend is coming next, and support for more platforms is planned.

• I've started building a small library, that can render pie menu GUI in legacy opengl, planning to add some traditional elements of course.
It's interface is similar to something you'd see in IMGUI. It's written in C.
Early version of the library
I'd really love to hear anyone's thoughts on this, any suggestions on what features you'd want to see in a library like this?