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OpenGL strangeness w / transparency and Z buffer (2d map editor)

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hi, ive been working on my map editor lately for my tile based 2d RPG. it's made w /C++ and SDL. anyway, on to the point. ive recently added a "show solidity" check-box to my editor. basically, this will show if a tile is Solid or Nonsolid by drawing an S or an N over the tile. anyway, to do this was simple. each frame, when i draw the visible tiles of the map, i check if "show solidity" is checked off, and if so, i check what type of tile this is. if its SOLID, i draw a quad here with the SOLID texture. if its NONSOLID, i draw the quad here with the nonsolid texture. anyway,im getting a very weird problem which bothers me and makes me think i still dont fully understand the very basics of OpenGL. first i have to give more info. Each tile can contain up to 5 different layers, each layer having a varying Z value. i have znear = -10 and zfar = 10, so i can use any Z value bettweeen -10 and 10. i stick to values between -2 and 2 for tiles and objects, though, since 5 layers is more then enough. for the buttons and the S and N tiles, and everything else, i draw with a Z value of 5 so that they are automatically drawn on top of the other stuff. besides having tiles, im able to place a "free floating object". this free floating map object is not a tile, but a regular quad with a texture, drawn each frame. this free floating object also can have a Z value ranging from -2 to 2, which represents its layer. here comes the problem: when "show solidity" is checked off, all of a suddent my free floating map objects disapear!!! it doesnt make any sence! ive ran through my code a dozen times, and i know that all the variables are what i think they are. ive tried disabling different states to see if that effected anything. basically, for some reason, if "show solidity" is checked off, my free floating objects just vanish. first, heres a pic of the S and N textures. these are what i bind to a quad and render over a quad if "show solidity" is marked off. now, heres a screenshot of the editor, with "show solidity" not checked off: everything is perfect. the building you see there is a free floating map object. its Z value is 0. also note that all the tiles in the scene are also 0, too. so everything here's Z value is 0, EXCEPT the blue quad you see drawn in the center of the building. this is a blue quad, whos Z value is 5, and i draw him in the center of the building so that i can know where the objects "center" is (for sorting purposes). now, look what happends when i check off "show solidity" see what happends? all of a sudden, my building disapears! it doesnt make sence... i mean, if the building disapeared, why didnt the tiles disapear too? they all share the same Z value. i dont get. i should also note that the 'S' and 'N' textures are ALWAYS drawn with the Z value of 5. this means that it *should* be drawn over everything. apparently, it is, but where is my building? since the texture is transparent, the S and N images should simply just be drawn over the building, like they are drawn over the rest of the tiles... i dont get it. in psuedo code, heres what my rendering loop looks like:

for(int y = 0; y < visable_y_tiles; y++)
{
     for(int x = 0; x < visable_x_tiles; x++)
     {
        
        //DRAWN WITH A Z VALUE OF 0
        draw_tile(map[y][x]);
  
        if(show_solidity)
        {
           //THESE ARE DRAWN WITH A Z VALUE OF 5
            if(map[y][x] IS SOLID)
               draw_quad(solid_texture,x*32,y*32);
           else draw_quad(nonsolid_texture,x*32,y*32);
         }
      }
}

for(each free floating map object)
{
    obj.Render();  <--------- DRAWS WITH A Z VALUE OF 0
}



 ---------- NOW SOMEWHERE ELSE IN THE CODE ---------------


Free_Floating_Map_Object::Render()
{

   //DRAWN WITH A Z VALUE OF 0
    draw_quad(my_texture,x,y);
   
     //DRAWN WITH A Z VALUE OF 5
    draw_blue_quad(my_center_position);
}




sorry for such a long post. this is one of those weird bugs that take a long time to explain. i hope im missing something obvious. in the psuedo code i show the order in which i drawn things and the Z value each thing has. the way i have it set up is closer to 10 means closer to the screen, and further from 10 means further into the screen. thanks alot for any help!!

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Guest Anonymous Poster
What do you have your near/far values set in gluPerspective()?

This could be a z-buffer resolution issue. If your far plane is set to something really large, or near is something really small, then the precision of your z-values near your image plane might be much worse than 1. If your quad at 5 was getting mapped to the same depth as a quad at 0, that could explain it.

This seems a little weird, but that was my first thought.

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Guest Anonymous Poster
When you said you have znear = -10, zfar = 10, are those the near/far plane values for gluPerspective?

If they are, then you are misunderstanding what they are. znear can NEVER be zero or negative. You can set it to something like .001.

What this means is the distance from the camera to the image plane. It doesn't make sense to have an image plane behind the camera. If you want a range from -10 to 10, set your camera at -11, near at 1, and far at 21.

If this is what you have done, I appologize. If not, try this and see if it helps.

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hey anonymous, thanks for the reply. actually, im using those values in the call to glOrtho() and set my zNear to -10 and zFar to 10,and i make 0,0 the top left corner and w/h of screen res the bottom right corner.... I dont use gluPerspective at all.. i probably should have mentioned that in the post [smile]. thanks for anymore help.

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hi renderer, i think your a little confused on whats happening. i think its due to the shrunken screenshot. anyway, no tiles are disapearing. the building is not a bunch of tiles, it is a single quad that represents a "free floating map object". this FFMO is disapearing when i want to "view solidity". checking off "view solidity" just means to draw an S or an N quad over each tile based on if they were solid or non solid. it just allows me to see if a tile is solid or not.

anyway, when i im showing solidity, for some reason my building disapears. im not sure why, since, the S and N quads are on a much higher Z level (meaning they should be drawn over the building), so if they are above the building, how come i cant see the building given that the S and N textures are transparent? in fact, i can even see the tiles that are underneath the building. the building and the tiles both have a Z of 0 and the S and N have a Z of 5. also, the tiles are drawn first, so the building *should* be on top of the tiles. it just doesnt really make sence... where did my building go?

to give you a better idea, heres a non shrunked screeny. first, this is what the map looks like in normal view, without "showing solidity"



next, heres what it looks like when i "view solidity"



do you see what im talking about? the building just vanishes! given the fact the building and tiles both have the same Z value, and the building is drawn after the tiles, i should be seeing a building right now!!

anyway, heres what it should look like, although in the real version there would be no tiles drawn over the building since the building and tiles share the same Z and the building is drawn last. i made this in Paint Shop Pro:



thanks for any help

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ok, wow, im so confused...

i took your advise and tried rendering the S/N quad AFTER i render the building. this worked! i can see my building now, the S/N is drawn over the building, and i can still see the building since the S/N has a transparent background.

this doesnt make any sence! why would i have to draw the S/N AFTER the building? why cant i draw it before? i have verified the Z values, so i dont get it... to sum it up:

-draw my tiles at 0 Z value
-draw my S/N at 5 Z value
-draw my building at 0 Z value

the building disapears, even through the S/N has a transparent background!

-draw my tiles at 0 Z value
-draw my building at 0 Z value
-draw my S/N at 5 Z value

this works! the S/N is drawn over the building, and since it has a transparent background, i can still see the building.

why does my building disapear using the first method? why dont my tiles disapear, too, since they share the same Z value? well, i guess they dont disapear because they are drawn before the S/N, but like i said, the S/N is transparent, so anything under it, i should see... it just doesnt make any sence.

thanks a lot for any further help. i'd really like to figure out why this is happening. i dont like not understanding something as important as the basics of rendering. does it have to do with transparency or something? the other thing is, i dont want to have to draw my S/N after i draw the buildings. I'd like to draw the S/N immediately after i draw the tiles, since they share the same rendering loop. i dont want to have to draw in 2 passes. thanks again.

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Quote:
this doesnt make any sence! why would i have to draw the S/N AFTER the building? why cant i draw it before? i have verified the Z values, so i dont get it... to sum it up:


You do have depth checking enabled, right?

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you mean depth testing, right? then yes, i do have that enabled, otherwise i dont think all the other stuff that messes with the Z axis would be working (and it is...). heres all the code i use to set-up OpenGL.


void System::Init_OpenGL()
{

/* Enable Texture Mapping ( NEW ) */
glEnable(GL_TEXTURE_2D);

/* Enable smooth shading */
glShadeModel(GL_SMOOTH);

/* Set the background black */
glClearColor(0.0f, 0.0f, 0.0f, 0.0f);

/* Depth buffer setup */
glClearDepth(1.0f);

/* Enables Depth Testing */
glEnable(GL_DEPTH_TEST);

/* The Type Of Depth Test To Do */
glDepthFunc(GL_LEQUAL);

/* Really Nice Perspective Calculations */
glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST);
glHint(GL_POINT_SMOOTH_HINT,GL_NICEST);

glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

/* Setup our viewport. */
glViewport(0,0,SCREEN_RW,SCREEN_RH);

/* change to the projection matrix and set our viewing volume. */
glMatrixMode(GL_PROJECTION);

glLoadIdentity();


glOrtho(0,SCREEN_RW,SCREEN_RH,0,-10.0f,10.0f);


glMatrixMode(GL_MODELVIEW);

/* Reset The View */
glLoadIdentity();


// DISABLE V SYNC !!!
typedef void (APIENTRY * WGLSWAPINTERVALEXT) (int);
WGLSWAPINTERVALEXT wglSwapIntervalEXT = (WGLSWAPINTERVALEXT) wglGetProcAddress("wglSwapIntervalEXT");

if (wglSwapIntervalEXT)
wglSwapIntervalEXT(0); // disable vertical synchronisation


}

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Guest Anonymous Poster
Quote:
Original post by graveyard filla
ok, wow, im so confused...


-draw my tiles at 0 Z value
-draw my S/N at 5 Z value
-draw my building at 0 Z value

the building disapears, even through the S/N has a transparent background!

-draw my tiles at 0 Z value
-draw my building at 0 Z value
-draw my S/N at 5 Z value

this works! the S/N is drawn over the building, and since it has a transparent background, i can still see the building.


Hmm, when you draw the S/N at 5, do the transparent parts change the depth buffer? Are you using alpha blending, or alpha test to throw away transparent pixels? This would make all the difference. If you are blending an alpha of 0 with the background, you will see the background, but the depth value will be set at 5, and when you draw your building at depth 0, all pixels will fail the depth test. If you use alpha test to throw away pixels with alpha of 0, then it won't set the depth buffer for those pixels.

I think this makes sense...but I'm not strong on alpha blending...

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Yes that's it, look at the init code. Instead of doing blending, you can do
glEnable(GL_ALPHA_TEST);
glAlphaFunc(GL_EQUAL, 1);

That will cause any transparent parts to not be drawn.

If you want to smooth out the edges of your S/N, you might want to change 1 to something like .5 and use GL_GREATER. If you still enable blending, the partially opaque edges will be blended with whatever pixel color is there when it draws (i.e. the tile). When you draw the building later, it won't look like it blended with the building, but instead some of the tile will show through (i.e. the portion that blended with the S/N). This may not look very good.

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hi,

thanks a lot anonymouse and renderer. im still confused though. first of all, i MUST keep blending enabled. this is because i use blending in a lot of different things.

secondly, i still dont understand why this is happening. i really just dont get it. what is meant exactly by "all pixels will fail the depth test" ??

lastly, how could i get everything working with blending still enabled? liked i said, i need it still for things. thanks a lot guys for all your help.

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First of all, put those 2 lines of code in your init code and see if it fixes everything.

Now for the explanation..

When you say something is transparent and blended, as each pixel is being drawn into the frame buffer, it gets blended with whatever pixel is already there, according to the blending rules (i.e. SRC_ALPHA, ONE_MINUS_SRC_ALPHA). If you draw something else with a z value behind that pixel at a later time, OpenGL doesn't "re-blend" your pixels. Think about this...it would have to keep track of every single transparent pixel drawn.

So if you want to use an overlay with completely transparent pixels, but still want to draw something "behind" it at a later point, the only way to do this is to not have drawn that pixel at all. That's what the GL_ALPHA_TEST does. Based on the alpha value of each pixel, you can either draw it or throw it away. If you still want to gave blending, you can indeed use both.

Try this:
glEnable(GL_ALPHA_TEST);
glAlphaFunc(GL_GREATER, 0);

That will blend anything with alpha greater than 0, and throw away anything that's completely transparent.

The key issue is that when you blend a transparent pixel with the background, the depth value for that pixel is now the depth of the transparent pixel. If you try and draw something "behind it", it will fail the depth test (i.e. it won't overwrite the pixel because the depth of the new pixel is "behind" what's already there).

If this is still confusing, I suggest you read up on how the depth buffer works (OpenGL red book is a good place to start).

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that did it... thanks a lot man. i think i understand what your saying... theres only a few parts i didnt get..

"If you draw something else with a z value behind that pixel at a later time, OpenGL doesn't "re-blend" your pixels. Think about this...it would have to keep track of every single transparent pixel drawn."

did you mean draw something else with a z value closer to the screen then that pixel ?

also,
"That will blend anything with alpha greater than 0, and throw away anything that's completely transparent."

what do you mean by alpha greater then 0, exactly?

last, what does it mean exactly by "Failing the depth test"?

im gunna have to start checking out the Red Book again. thanks a lot for your help.

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"If you draw something else with a z value behind that pixel at a later time, OpenGL doesn't "re-blend" your pixels. Think about this...it would have to keep track of every single transparent pixel drawn."

What i mean is that if you draw something at say -2, then draw a 50% transparent object at +2, you will see half of the color of the pixel from -2 and half from +2...right?
Now if you draw something at 0 afterwards, you see no change.
BUT, if you draw something at -2, then something at 0, then something at +2 with 50% transparency, the you will see 50% of the pixel at 0 and 50% at 2.

Why does drawing order matter? Because of the depth buffer. Each pixel that gets rasterized for every single primative goes through the "depth test". OpenGL looks at the depth of the pixel currently at that location and check is the new one is closer to the camera or farther. If it is farther away, it doesn't have to do anything, because it is covered by the closer pixel. This is why you can draw your triangles in any order for a normal mesh (no transparency). If you turn off depth test, you will see that whatever primatives you drew last end up in front.

So the catch is that blending order matters! If you want to draw 4 semi-transparent objects one on top of another, you have to draw the furthest one away first, becuase if you draw a closer on first, the farther one will fail the depth test (i.e. not get drawn at all).


"That will blend anything with alpha greater than 0, and throw away anything that's completely transparent."

what do you mean by alpha greater then 0, exactly?

Alpha values range from 0 to 255 (or in float, 0 to 1), with zero being completely transparent and 1 being completely solid (opaque). If you want to use blending, you need to keep pixels with alpha values greater than 0 (because they are semi opaque), but if you don't want to worry about drawning order for your S/N with completely transparent pixels, you can throw away all pixels with alpha values of 0 (using GL_ALPHA_TEST).



last, what does it mean exactly by "Failing the depth test"?
The current pixel being drawn is further away from the camera than a pixel in the same location that's already been drawn. That pixel fails the depth test and as a result does not get drawn.

im gunna have to start checking out the Red Book again. thanks a lot for your help.

Yup, I've read through several sections at least 4 or 5 times and I'm only finally starting to get a really good handle of what's goin on.

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thanks a lot renderer. i didnt know how alpha blending and the Z buffer worked, but now i have a pretty good idea. so OpenGL optimizes by throwing away pixels that *shouldnt* be seen then (this is the depth test?)? i think i get it. just to be sure, i DONT have to do anything special now, right? since now i have it so that things which have an alpha value of zero are just never even drawn to the screen, this will solve my problems, correct?

but this brings up another issue. what about objects who are 2%, or 50%, or 80% transparent? do i have to sort these objects somehow? i really, really hope not. i thought i was finnally done with all my sorting algo's =).

the other thing is, im a little confused when you say things with 50% or xy% transparency. when you say this, do you mean they are given their transparency level by the call to glColor4f(), IE, given their transparency by the quad they are drawn on? OR, do you mean things with per pixel alpha blending? IE, a .png image file which makes use of per pixel blending, having some pixels with 0% trans and others with 50%... ive never used these before, so i could be wrong, but im pretty sure that is possible. are these 2 things treated the same by OpenGL, or are they different? currently i just use 0% transparency in my textures, so i dont know.

thanks again for all your help

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The alpha channel applies to both situations. If you have a colored quad (each vertex has a glColor4f(r,g,b,a)) then it blends the colors between the 4 vertices (including the alpha channel) so each pixel has some alpha value. Similarly, if you have a texture mapped quad, and that texture was loaded from a png with transparency levels, or generated from wherever, then when that texture gets applied to a quad and the quad is drawn, each pixel that gets drawn has some transparency.

As far as needing to sort things for blending...short answer yes, but only in certain situations. Since blending occours only once (when that pixel is drawn), and blending occours with whatever pixel is CURRENTLY behind that pixel, you need to make sure that you have the right pixels already drawn before you draw something to be blended on top. If you are drawing multiple semi-transparent objects on top of one another, then they need to be sorted back-to-front. If you were say drawing a 50% blue tinted window on a house, then you need to make sure everything inside the house is already drawn first, if you are looking from the outside, or that everything outside the house is already drawn, if you are looking from the inside.

Hope this makes things clear. Good luck with your programming.

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i still dont really fully understand how / why its all happening, but i think i get the gist of it. so basically, the only time i should worry is if i wanted to draw 2 partially transparent objects over-lapping and both objects had varying Z values which i was depending on to "sort" them. in your house with blue window example, i would only have to worry if something outside / in the house was also partially transparent, right? if this is the case, then this isnt really that big of a deal. i guess no partial transparency in my tile layering system, kind of a bummer.

i guess its not THAT bad to get around.anything which is partially transparent, ill have to store togeather. then, each frame, ill sort() this list of partially transparent stuff by the Y axis, putting things that are further up on the screen to the front of the list. then i just render front to back. this will make it so something above another object is drawn first. so if i have 2 partially transparent guys and guy A is standing above guy B, the top part of guy B's body will overlap the bottom part of guy A.

the hard part will be figuring out what objects are partially transparent and knowing to put them in that special list. it also sort of makes it sloppy because now i have things grouped togeather, but in that system things would be in different places, which is unorganized.

actually, now that i think about it, instead of sorting the transparent object by their Y value, i should do it by their Z value, right? since their Z value already tells me what order to draw them, and that is whats screwing things up in the first place anyway. so i would sort() by Z value and put all things further from the camera in the front of the list, and then draw front to back. this should stop things from not getting rendered, correct? maybe ill just try to avoid this by just not making anything partially transparent that needs to be sorted for drawing.

thanks again for all your help!!!

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hey renderer (or whoever),

i was thinking, its kind of difficult to group all semi-transparent objects togeather and sort them. does anyone have any suggestions on how to do this?

actually, i was thinking it might just be easier to throw away the whole idea of using the Z axis. because i can get a "layered" effect by just rendering in multiple passes, and i wont have this issue with transparent objects. do you think i should say screw it, and just get rid of the Z axis? or, should i try to figured out a method to sort transparent objects? its really disapointing, too. using the Z axis for a layered effect was VERY convenient, and not only that, but it also would make rendering faster, since i would be able to stick all my tiles into a single vertex array and render them all at once, rather then sticking them in multiple vertex arrays and rendering it multiple passes. same goes for my other objects which are layered besides tiles...

thanks for any help or suggestions.

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sorry, but im going to shamelessly bump this. i really need your guys opinions on this, before i move on.

what do you think will be cleaner/easier/better ? should i just get this whole Z axis idea out of my head, and render in multiple passes (slower, less convenient). or, should i keep the Z axis, and somehow figure out a way to sort any semi-transparent objects which have varying Z values?

thanks a lot for any help!

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      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.
      Creating Shaders
      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:
      SHADER_SOURCE_LANGUAGE_DEFAULT - The shader source language matches the underlying graphics API: HLSL for Direct3D11/Direct3D12 mode, and GLSL for OpenGL and OpenGLES modes. SHADER_SOURCE_LANGUAGE_HLSL - The shader source is in HLSL. For OpenGL and OpenGLES modes, the source code will be converted to GLSL. SHADER_SOURCE_LANGUAGE_GLSL - The shader source is in GLSL. There is currently no GLSL to HLSL converter, so this value should only be used for OpenGL and OpenGLES modes. There are two ways to provide the shader source code. The first way is to use Source member. The second way is to provide a file path in FilePath member. Since the engine is entirely decoupled from the platform and the host file system is platform-dependent, the structure exposes pShaderSourceStreamFactory member that is intended to provide the engine access to the file system. If FilePath is provided, shader source factory must also be provided. If the shader source contains any #include directives, the source stream factory will also be used to load these files. The engine provides default implementation for every supported platform that should be sufficient in most cases. Custom implementation can be provided when needed.
      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:
      ShaderCreationAttribs Attrs; Attrs.Desc.Name = "MyPixelShader"; Attrs.FilePath = "MyShaderFile.fx"; Attrs.SearchDirectories = "shaders;shaders\\inc;"; Attrs.EntryPoint = "MyPixelShader"; Attrs.Desc.ShaderType = SHADER_TYPE_PIXEL; Attrs.SourceLanguage = SHADER_SOURCE_LANGUAGE_HLSL; BasicShaderSourceStreamFactory BasicSSSFactory(Attrs.SearchDirectories); Attrs.pShaderSourceStreamFactory = &BasicSSSFactory; ShaderVariableDesc ShaderVars[] = {     {"g_StaticTexture", SHADER_VARIABLE_TYPE_STATIC},     {"g_MutableTexture", SHADER_VARIABLE_TYPE_MUTABLE},     {"g_DynamicTexture", SHADER_VARIABLE_TYPE_DYNAMIC} }; Attrs.Desc.VariableDesc = ShaderVars; Attrs.Desc.NumVariables = _countof(ShaderVars); Attrs.Desc.DefaultVariableType = SHADER_VARIABLE_TYPE_STATIC; StaticSamplerDesc StaticSampler; StaticSampler.Desc.MinFilter = FILTER_TYPE_LINEAR; StaticSampler.Desc.MagFilter = FILTER_TYPE_LINEAR; StaticSampler.Desc.MipFilter = FILTER_TYPE_LINEAR; StaticSampler.TextureName = "g_MutableTexture"; Attrs.Desc.NumStaticSamplers = 1; Attrs.Desc.StaticSamplers = &StaticSampler; ShaderMacroHelper Macros; Macros.AddShaderMacro("USE_SHADOWS", 1); Macros.AddShaderMacro("NUM_SHADOW_SAMPLES", 4); Macros.Finalize(); Attrs.Macros = Macros; RefCntAutoPtr<IShader> pShader; m_pDevice->CreateShader( Attrs, &pShader );
      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.
      Binding Shader Resources
      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:
      PixelShader->GetShaderVariable( "g_tex2DShadowMap" )->Set( pShadowMapSRV ); Mutable and dynamic variables are bound via a new Shader Resource Binding object (SRB) that is created by the pipeline state (IPipelineState::CreateShaderResourceBinding()):
      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.
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