SharpGL and RenderBuffers

Question

I'm attempting to port a pathtracer to GLSL, and to do this I need to modify a shader sample program to use a texture as the framebuffer instead of the backbuffer.

This is the vertex fragment

#version 130

out vec2 texCoord;

// https://rauwendaal.net/2014/06/14/rendering-a-screen-covering-triangle-in-opengl/
void main()
{
 float x = -1.0 + float((gl_VertexID & 1) << 2);
 float y = -1.0 + float((gl_VertexID & 2) << 1);
 texCoord.x = x;
 texCoord.y = y;
 gl_Position = vec4(x, y, 0, 1);
}

This is the setup code

gl.GenFramebuffersEXT(2, _FrameBuffer);
gl.BindFramebufferEXT(OpenGL.GL_FRAMEBUFFER_EXT, _FrameBuffer[0]);

gl.GenRenderbuffersEXT(2, _RaytracerBuffer);
gl.BindRenderbufferEXT(OpenGL.GL_RENDERBUFFER_EXT, _RaytracerBuffer[0]);
gl.RenderbufferStorageEXT(OpenGL.GL_RENDERBUFFER_EXT, OpenGL.GL_RGBA32F, (int)viewport[2], (int)viewport[3]);

And this is the runtime code

//  Get a reference to the raytracer shader.
var shader = shaderRayMarch;

// setup first framebuffer (RGB32F)
gl.BindFramebufferEXT(OpenGL.GL_FRAMEBUFFER_EXT, _FrameBuffer[0]);
gl.Viewport((int)viewport[0], (int)viewport[1], (int)viewport[2], (int)viewport[3]); //0,0,width,height)
gl.FramebufferRenderbufferEXT(OpenGL.GL_FRAMEBUFFER_EXT, OpenGL.GL_COLOR_ATTACHMENT0_EXT, OpenGL.GL_RENDERBUFFER_EXT, _RaytracerBuffer[0]);
gl.FramebufferRenderbufferEXT(OpenGL.GL_FRAMEBUFFER_EXT, OpenGL.GL_DEPTH_ATTACHMENT_EXT, OpenGL.GL_RENDERBUFFER_EXT, 0);
uint [] DrawBuffers = new uint[1];
DrawBuffers[0] = OpenGL.GL_COLOR_ATTACHMENT0_EXT;
gl.DrawBuffers(1, DrawBuffers);

shader.Bind(gl);
shader.SetUniform1(gl, "screenWidth", viewport[2]);
shader.SetUniform1(gl, "screenHeight", viewport[3]);
shader.SetUniform1(gl, "fov", 40.0f);
gl.DrawArrays(OpenGL.GL_TRIANGLES, 0, 3);
shader.Unbind(gl);

int[] pixels = new int[(int)viewport[2]*(int)viewport[3]*4];
gl.GetTexImage(_RaytracerBuffer[0], 0, OpenGL.GL_RGBA32F, OpenGL.GL_INT, pixels);

But when I inspect the pixels coming back from GetTexImage they're black. When I bind this texture in a further transfer shader they remain black. I suspect I'm missing something in the setup code for the renderbuffer and would appreciate any suggestions you have!

Answer 1

Renderbuffers are not textures. So when you do glGetTexImage on your renderbuffer, you probably got an OpenGL error. When you tried to bind it as a texture with glBindTexture, you probably got an OpenGL error.

If you want to render to a texture, you should render to a texture. As in glGenTextures/glTexImage2D/glFramebufferTexture2D.

Also, please stop using EXT_framebuffer_object. You should be using the core FBO feature, which requires no "EXT" suffixes. Not unless you're using a really ancient OpenGL version.