Sign in to follow this  
Followers 0
Grumple

OpenGL
2D icons in a 3D projection?

4 posts in this topic

Hi,

I'm working on a project where I need to draw 2D icons in a 3D perspective projection. The idea is to position the icons correctly in the 3D scene, but to draw them with consistent pixel size regardless of distance from the camera. If you picture 'points of interest' in google maps, you will see what I'm after. Regardless of the map zoom level or camera 'distance from map', the points of interest icons maintain constant pixel size.

The icons themselves will just be standard textured billboards, always oriented to face the camera directly.

As far as I can tell, there are two basic approaches....one being to draw them as part of the perspective scene render, but calculate their height/width to cancel out distance from camera (ie make them bigger the farther from the camera they are using a reversal of the perspective projection), or to do a second render pass in an actual Ortho projection, where the widths and heights can be consistent. However, if my head is wrapped around the problem correctly, I wouldn't really save effort with an ortho projection as I would then have to do math to position things in a perspective projection manner within the viewport.

Would I be correct to say the logical method is to render the icons in perspective mode with the rest of the scene, but adjust the width/height of the billboards in a 'reverse perspective' calculation to compensate for distance from camera? Is there any other simple way to do this?

Thanks!

Edit: In hindsight this probably should have gone in the "Graphics Programming and Theory" forum instead of OpenGL.. Edited by Grumple
0

Share this post


Link to post
Share on other sites
So, what you need to do is:
1. To project the vertex of your icon's position in 3d-space (you can use gluProject, for example)
2. Now you have got a position in window space. Go to orthogonal projection and draw it:
[code]
glOrtho(0,vp[2],0,vp[3],0,1); // NOTE: vp - is "int viewport[4]" which you used in gluProject
//double wpos[3] - output from gluProject (window position)
//double iconSize; - in pixels
glBegin(GL_QUADS); // simple actions
glVertex3d(wpos[0] - 1 * iconSize, wpos[1] - 1 * iconSize, wpos[2]);
glVertex3d(wpos[0] + 1 * iconSize, wpos[1] - 1 * iconSize, wpos[2]);
glVertex3d(wpos[0] + 1 * iconSize, wpos[1] + 1 * iconSize, wpos[2]);
glVertex3d(wpos[0] - 1 * iconSize, wpos[1] + 1 * iconSize, wpos[2]);
glEnd();
// this would draw quadratic at 3D position with fixed size in pixels
[/code]
That's little example on how to do that.

Best wishes, FXACE. Edited by FXACE
0

Share this post


Link to post
Share on other sites
Thanks a lot, FXACE....gluProject definitely looks useful for what I want.

Just as a follow up question, I assume window Z coord returned by gluProject would be depth into the screen from camera perspective? I still want my 2D icons to be depth tested and culled by any geometry that was rendered in perspective projection and is 'closer' to the camera. Would the depth testing still work for the billboards while rendering in Ortho using the winZ value from gluProject?

Edit:
Upon doing some gluProject reading, it seems the returned z coordinate is in 'normalized' depth buffer coordinates. Here is an example I've found describing my main problem quite accurately, with what looks like a good solution.
[url="http://stackoverflow.com/questions/8990735/how-to-use-opengl-orthographic-projection-with-the-depth-buffer/8991624#8991624"]http://stackoverflow.com/questions/8990735/how-to-use-opengl-orthographic-projection-with-the-depth-buffer/8991624#8991624[/url]

That thread has a formula for converting between the intial gluProject winZ and a z value matching that depth in the Ortho mode. However, if I follow the formula correctly, setting my intial ortho znear to 0, and zfar to 1, I can directly use the z value returned by gluProject as my billboard Z, just negated to fall down the -z axis (opengl).

Does that make sense? The only part that is still confusing me is whether or not the billboard corners can correctly have the same z value as the center point that was returned through gluProject? I think I might be imagining it wrong, but in perspective mode if I ran a ray from the camera to the center of the billboard, it would get a different distance than a ray from camera to a corner, so I would think the depth values would be different as well.

Does the perspective depth buffer somehow make the depths 'normalized' from a plane the camera is on perpendicular to its line of sight, or do the depths represent distance from the actual spot the camera sits? Edited by Grumple
0

Share this post


Link to post
Share on other sites
This formula would be useful for you in near future but for this you don't really need that.
Let me show you complete example:
[code]
// Set up projection
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(...);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
// place your view to look at
DrawScene();
double proj[16];
double mv[16];
int vp[4];
glGetDoublev(GL_MODELVIEW_MATRIX, mv);
glGetDoublev(GL_PROJECTION_MATRIX, proj);
glGetIntegerv(GL_VIEWPORT, vp);
double pos[3] = {0, 0, 0}; // position of your icon in 3D space
double wpos[3];

gluProject(pos[0], pos[1], pos[2], mv, proj, vp, &wpos[0], &wpos[1], &wpos[2]);
wpos[2] *= -1; // negate z value because OpenGL's forward axis is '-Z'
double iconSize = 50;


// go to orthogonal projection
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0, vp[2], 0, vp[3], 0,1);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();

glColor3f(<your color>);
glBegin(GL_QUADS); // simple actions
glVertex3d(wpos[0] - 1 * iconSize, wpos[1] - 1 * iconSize, wpos[2]);
glVertex3d(wpos[0] + 1 * iconSize, wpos[1] - 1 * iconSize, wpos[2]);
glVertex3d(wpos[0] + 1 * iconSize, wpos[1] + 1 * iconSize, wpos[2]);
glVertex3d(wpos[0] - 1 * iconSize, wpos[1] + 1 * iconSize, wpos[2]);
glEnd();
[/code]
And finally you will get nice quad which would be drawn as you wanted (depth tested and culled).

[quote name='Grumple' timestamp='1337019764' post='4940156']
Does that make sense? The only part that is still confusing me is whether or not the billboard corners can correctly have the same z value as the center point that was returned through gluProject? I think I might be imagining it wrong, but in perspective mode if I ran a ray from the camera to the center of the billboard, it would get a different distance than a ray from camera to a corner, so I would think the depth values would be different as well.

Does the perspective depth buffer somehow make the depths 'normalized' from a plane the camera is on perpendicular to its line of sight, or do the depths represent distance from the actual spot the camera sits?
[/quote]
Nope, any vertex is clipped by planes (left, right, bottom, top, near, far). In orthogonal projection distance of center, corner of billboard to near (&far) plane are equivalent. OpenGL doesn't have a camera, it has a view port (where you move/transform all vertices into, by using of matrices, etc)
But with shaders you can construct a depth buffer as you wish...

Best wishes, FXACE. Edited by FXACE
1

Share this post


Link to post
Share on other sites
Thanks for the detailed explanation! I was definitely mixing up the difference between the concept of a camera and the OpenGL viewport. Cheers!
0

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now
Sign in to follow this  
Followers 0

  • Similar Content

    • By recp
      Hi,
      I'm working on new asset importer (https://github.com/recp/assetkit) based on COLLADA specs, the question is not about COLLADA directly
      also I'm working on a new renderer to render (https://github.com/recp/libgk) imported document.
      In the future I'll spend more time on this renderer of course, currently rendering imported (implemented parts) is enough for me
      assetkit imports COLLADA document (it will support glTF too),
      importing scene, geometries, effects/materials, 2d textures and rendering them seems working
      My actual confusion is about shaders. COLLADA has COMMON profile and GLSL... profiles,
      GLSL profile provides shaders for effects so I don't need to wory about them just compile, link, group them before render

      The problem occours in COMMON profile because I need to write shaders,
      Actually I wrote them for basic matrials and another version for 2d texture
      I would like to create multiple program but I am not sure how to split this this shader into smaller ones,

      Basic material version (only colors):
      https://github.com/recp/libgk/blob/master/src/default/shader/gk_default.frag
      Texture version:
      https://gist.github.com/recp/b0368c74c35d9d6912f524624bfbf5a3
      I used subroutines to bind materials, actually I liked it,
      In scene graph every node can have different program, and it switches between them if parentNode->program != node->program
      (I'll do scene graph optimizations e.g.  view frustum culling, grouping shaders... later)

      I'm going to implement transparency but I'm considering to create separate shaders,
      because default shader is going to be branching hell
      I can't generate shader for every node because I don't know how many node can be exist, there is no limit.
      I don't know how to write a good uber-shader for different cases:

      Here material struct:
      struct Material { ColorOrTexture emission; ColorOrTexture ambient; ColorOrTexture specular; ColorOrTexture reflective; ColorOrTexture transparent; ColorOrTexture diffuse; float shininess; float reflectivEyety; float transparency; float indexOfRefraction; }; ColorOrTexture could be color or 2d texture, if there would be single colorOrTex then I could split into two programs,
      Also I'm going to implement transparency, I am not sure how many program that I needed

      I'm considering to maintain a few default shaders for COMMON profile,
      1-no-texture, 2-one of colorOrTexture contains texture, 3-........

      Any advices in general or about how to optimize/split (if I need) these shaders which I provied as link?
      What do you think the shaders I wrote, I would like to write them without branching if posible,
      I hope I don't need to write 50+ or 100+ shaders, and 100+ default programs

      PS: These default shaders should render any document, they are not specific, they are general purpose...
             I'm compiling and linking default shaders when app launched

      Thanks
    • By CircleOfLight97
      Hi guys,
      I would like to contribute to a game project as a developer (open source possibly). I have some experiences in C/C++ in game development (perso projects). I don't know either unreal or unity but I have some knowledges in opengl, glsl and shading theory as I had some courses at university regarding to that. I have some knowledges in maths and basic in physics. I know a little how to use blender to do modelling, texturing and simple game assets (no characters, no animation no skinning/rigging). I have no game preferences but I like aventure game, dungeon crawler, platformers, randomly generated things. I know these kind of projects involve a lot of time and I'd be really to work on it but if there are no cleary defined specific design goals/stories/gameplay mechanics I would like to not be part of it x) and I would rather prefer a smaller but well defined project to work on that a huge and not 'finishable' one.
      CircleOfLight97
    • By gamesthatcouldbeworse
      Hi, I finally released KILL COMMANDO on gamejolt for free. It is a retro-funsplatter-shooter with C64 style. Give it a try.
    • By phil67rpg

      void TimerFunction(int value) {  glutPostRedisplay();  glutTimerFunc(1000, TimerFunction, 1); } void drawScene() {  glClear(GL_COLOR_BUFFER_BIT);      drawScene_bug();  TimerFunction(1);  eraseScene_bug(); // drawScene_bug_two(); // eraseScene_bug_two(); drawScene_ship(); drawScene_bullet();  glutSwapBuffers(); }
  • Popular Now