Only getting 8-bits of precision with depth tex

OpenGL OpenGL: Advanced Coding
1

I’m using a depth texture in a GLSL program to do some depth-based outlining as a post process. I build the depth texture using glCopyTexImage/SubImage with a GL_DEPTH_COMPONENT parameter (tried GL_DEPTH_COMPONENT24 also)

I’ve checked the depth of the texture using glGetTexLevelPameter and I get 24 as expected.

I’ve searched these forums and found that I need to make sure GL_NEAREST filtering is used on the depth texture to get full precision. I’ve done this but I still only get 8-bits.

Also I’ve made sure that GL_TEXTURE_COMPARE_MODE is set to GL_NONE.

The depth texture is sampled in a GLSL shader as a sampler2D using texture2D.

I’m running on an Nvidia 6800.

Does anyone know of any other states that may affact the precision when sampling a depth texture?

Thanks

2

Question, how do you know you are only getting 8-bits?

I take it you are doing something like…

vec4 depthVec = texture2D(depthImage, gl_TexCoord[0].st);
float depthVal = depthVec.r

3

Good question. To test the bit depth I’ve written a shader that outputs the depth value to the .a value of the color buffer.

When using the value obtained from the .a channel instead of the depth value I get the same results as given by the value in the depth buffer. But, if I render to a 16-bit floating point color buffer, I get the good outlining results I’m hoping for.

4

So what ? I don’t understand what’s a problem, what’s not a problem and so. Could you explain ?

5

MalcolmB: you are being very vague on what you are doing to test the precision so I can’t help you there, but this is my setup which works:

glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT24_ARB, Width, Height, 0, GL_DEPTH_COMPONENT, GL_UNSIGNED_INT, 0);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);	
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);	
glTexParameteri(GL_TEXTURE_2D, GL_DEPTH_TEXTURE_MODE_ARB, GL_LUMINANCE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_COMPARE_MODE_ARB, GL_NONE);
glCopyTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, 0, 0, Width, Height);

Using this texture in a shader gives me a 24bit precision depth texture with values from 0.0 to 1.0.

6

I’m sorry. I’ll try to explain better.
I’m using the depth value to detect edges on objects so I can put black outlines around them (to give it a toon look). In a post process pass I compare the current pixel’s depth value with all it’s neighbour’s depth values and do a difference against a threshold value to determine if it’s an edge or not.

Initially I outputted the depth value to the .a channel in a 16-bit float pbuffer and then copied this to a 16-bit float texture. This gave me good outlines.

I then decided to try out a depth texture instead since that is obviously a smarter way to do it. When I did this instead of nice outlines I get what look like depth contour lines (like on a map) on my objects. Now it’s hard to know exactly whats going on but my assumption is these lines are where the depth values change from one value to the next. For comparision I went back to my original method of outputting the values to the .a channel in a texture, but this time I used a normal 8-bit per channel texture. This gave the same results as the depth texture. This is why I think I’m only getting 8-bits of precision.

To further test this I’ve made a shader that takes the resulting depth buffer and does a comparison between the current pixel and one of it’s neighbours. If their values are different then I set the output to white, otherwise it’s black. On the depth texture I get the same contour lines (most of the image is black) which means very few of the values I’m getting are different.
On the 8-bit texture I get the same contour lines.
On the 16-bit texture I get almost all white, as expected.

Hope I’m clearer now.

7

Originally posted by MalcolmB:
Hope I’m clearer now.
We are getting there… :wink:

Initially I outputted the depth value to the .a channel in a 16-bit float pbuffer and then copied this to a 16-bit float texture. This gave me good outlines.
Are you saying that you are using a shader to render each fragments depth value to the alpha channel and not reading out the framebuffer at all?
If this is the case then there is just a difference in coordinatespace between the two methods.
Try outputting the depth texture to the screen and compare the two methods.

I think the depthtexture values are in screen space while your pbuffer solution gives worldspace depth values.

http://www.opengl.org/resources/faq/technical/depthbuffer.htm

8

Well, the value written to the .a channel comes from the gl_FragCoord.z, which is screen space depth. Remember that I get the same results when using an 8-bit pbuffer as I do when I use the depth buffer.

Yes, the pbuffer method and the depth texture method create textures that look the same visually.

9

Problem solved. I have learnt on Nvidia Geforce 6800 and Quadro 4500 hardware (I won’t vouch for future hardware because who knows what will change) you must use these settings to get full precision.

GL_DEPTH_TEXTURE_MODE_ARB = GL_LUMINANCE
GL_TEXTURE_COMPARE_MODE_ARB = GL_NONE
GL_TEXTURE_MIN_FILTER = GL_NEAREST
GL_TEXTURE_MAG_FILTER = GL_NEAREST

depth texture format must be GL_DEPTH_COMPONENT24_ARB.

As Def said, this is what he uses. Thanks for the help everyone.