Opengl novice here so sorry ahead of time! Just hoping to get some questions answered about sampling textures. I was messing around with rendering a scene to textures so that they could be used later. I then wanted to make a shader to view a depth texture by linearizing the depth values and then just bliting it on screen using a single quad. I can see that everything is being drawn into the texture correctly through gDEBugger (debugger.png) but am having some trouble sampling it .

First, to render the texture I output a single quad covering the screen. Then in vertex shader I multiply by modelviewproj matrix and save the xy value. My understanding is this should perfectly match the screen coordinates but when I check these values I am only getting the top right corner (output.png).

Second, I always seem to get values of 0 when sampling my texture. I can see in the debugger data view that (0,0) has a value of like ~.9997. However I sample it, I always seem to get 0s. It is a floating point texture with GL_DEPTH_ATTACHMENT.

Here's the fragment and vertex shader code (my graphics card doesn't have geometry shader):

ShadowVisualDataOutput ShadowVisualVertexMain(ShadowVisualDataInput vdi, uniform float4x4 modelViewProj) {
  ShadowVisualDataOutput ret;
  ret.projv = mul(modelViewProj, vdi.vertex); 
  ret.saved = mul(modelViewProj, vdi.vertex).xy;
  return ret;
}


ShadowVisualDataPixelOutput ShadowVisualFragmentMain(
	ShadowVisualDataOutput pdi, 
	uniform sampler2D depthTex,
	uniform float camNear, 
	uniform float camFar) {
		
	ShadowVisualDataPixelOutput ret;	
	if (pdi.saved.x < 0 || pdi.saved.x > 720) {
		ret.color = float4(0,0,1,1);
		return ret;
	}
	if (pdi.saved.y < 0 || pdi.saved.y > 480) {
		ret.color = float4(0,0,1,1);
		return ret;
	}	

	float2 lookup = float2(0, 0);
	float depth = tex2D(depthTex, lookup).x;
	if (depth == 0.0f) {
			ret.color = float4(0,0,0,0);
			return ret;
	}

	ret.color = float4(1,1,1,1);
	return ret;

}

Thanks for the response!

I don’t know what you mean by this being the only thing you are “getting”. This is the only region over which you are not drawing blue. See image.

I am rendering a single quad that should cover up the whole screen exactly. If I use a fragment shader that just outputs a single color, it fills the screen completely. But when I multiply the vertex coordinates by model view projection matrix, store them, and then check in the fragment shader if they are exactly the screen (0 < x < 720 and 0 < y < 480) only the top right half projects into this. I am misunderstanding something about the projection?

(0.5f, 0.5f) and (1.0f, 1.0f) also return 0. Maybe I am setting something up wrong:

// Creation

glGenTextures(1, &depthTexture);
glBindTexture(GL_TEXTURE_2D, depthTexture);	
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT24, width, height, 0, GL_DEPTH_COMPONENT, GL_FLOAT , 0);	
glBindTexture(GL_TEXTURE_2D, 0);

...

// Attach to fbo

glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHMENT, GL_TEXTURE_2D, depthTexture, 0); glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, colorTexture, 0);

checkStatus() <-- returns complete okay!

// Bind to fbo, render scene once, then switch back to default

glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, depthFramebuffer);

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

renderHWLoop();

glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);

...

//switch to visualizing programs

//eventually these get called

fragmentDepthText = cgGetNamedParameter(fragmentProgram, "depthTex");

cgGLSetTextureParameter(fragmentDepthText, depthTexture);

Sorry for the confusion. Let me try rewording it. With the simplest shader just outputing a color, it fills the screen. But when I project the points by multiplying by the modelviewproj and check if they are within the bounds of the screen, only the top corner projects into this range. Am I misunderstanding something about projection?

And I tried this fragment shader with just pass-through texture coordinates and am still getting all black.

ShadowVisualDataPixelOutput ret;

float depth = tex2D(depthTex, pdi.texture).x;
if (depth == 0.0f) {
	ret.color = float4(0,0,0,0);
	return ret;
} else {
	ret.color = float4(depth, depth, depth, 1);
	return ret;
}

Awesome! Thank you!