Question

I want to view a flat fullscreen texture as it is spherical, by transforming it in a postprocess shader.

I figure I have to apply a projectionmatrix to the texture coordinate in the shader.

I found this website: http://www.songho.ca/opengl/gl_projectionmatrix.html which learns me a lot about the inners of the projectionmatrix.

But how do I apply it? I thought I would have to multiply the third row of the projection matrix to the texture coordinate with a calculated z value added to make it spherical. My efforts don’t show any result though.

EDIT: I see the same issue here: http://lists.openscenegraph.org/pipermail/osg-users-openscenegraph.org/2008-April/009765.html

Answer 1

I think after you multiply text coords by projection matrix you have to make a perspective division and move from 3D to 2D (since the texture is 2D). This is the same as with shadow mapping.

// in fragment shader:
vec4 proj = uniformModelViewProjMatrix * tex_coords;
proj.xyz /= proj.w;
proj.xyz += vec3(1.0);
proj.xyz *= 0.5;
vec4 col = texture2D(sampler, proj.xy);

or look at http://www.ozone3d.net/tutorials/glsl_texturing_p08.php (for texture2DProj)