OpenGL Full Depth With Partial Color Write

Question

I am making a program which renders terrain in two separate passes, with an intermediate clearing of depth in between (there is no way to change this requirement). In the first pass I render the skybox and terrain really far away using depth testing. I clear the depth buffer bits to 1.0. Then, I render all my close terrain with depth testing as well. So the code looks something like this:

// My State
glEnable(GL_BLEND);
glBlendFunc(GL_ONE, GL_ZERO);
glEnable(GL_DEPTH_TEST);
glDepthMask(GL_TRUE);
glDepthFunc(GL_LESS);
glClearColor(0, 0, 0, 0);
glClearDepth(1.0);

// Pass 1
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
DrawSkybox();
DrawFarTerrain();

// Pass 2
glClear(GL_DEPTH_BUFFER_BIT);
DrawNearTerrain(); // IMPORTANT: All done in one draw command

The following images show the color values and depth (white = 1.0) at the end of each pass:

Pass 1 Pass 1 Color Pass 1 Depth Pass 2 Pass 2 Color Pass 2 Depth

As you can see, within that single draw command for the second pass, the pixel values are being written in the depth buffer, but the color buffer is only partially written. The white area causes some weird problem.

According to GDebugger, this was my state during that draw command:

GL_BLEND: TRUE
GL_BLEND_EQUATION_ALPHA: GL_FUNC_ADD
GL_BLEND_EQUATION: GL_FUNC_ADD
GL_BLEND_EQUATION_RGB: GL_FUNC_ADD
GL_BLEND_DST: 0
GL_BLEND_DST_ALPHA: 0
GL_BLEND_DST_RGB: 0
GL_BLEND_SRC: 1
GL_BLEND_SRC_ALPHA: 1
GL_BLEND_SRC_RGB: 1
GL_DEPTH_FUNC: GL_LESS
GL_DEPTH_TEST: TRUE
GL_STENCIL_FUNC: GL_ALWAYS
GL_STENCIL_TEST: FALSE
GL_BLEND_COLOR: {0, 0, 0, 0}

Can anyone tell me what is going on? I feel like the GPU is gaslighting me.

EDIT:

I currently have one GL Context that is alive.
My card is an Intel HD 4000 with the latest drivers running on an Intel i7 with 8 GB RAM.
MY FBO's color attachment has an internal format of GL_RGBA16F and the depth attachment is GL_DEPTH_COMPONENT32F.
Stencil testing is never used.

According to docs (opengl.org/sdk/docs/man/docbook4/xhtml/glDepthFunc.xml) and my experience, there's no GL_LESS_THAN tag. — Kromster
– Kromster, Commented Nov 24, 2014 at 9:23
I see you set GL_STENCIL Off, but where and how do you set it On? — Kromster
– Kromster, Commented Nov 24, 2014 at 9:25
1. what gfx card do you use? ATI,Intel? did you try it on nVidia? 2. check for memory leaks of your app some gl drivers like ATI are extremly sensitive to them an then provide funny behavior 3. do you use more then one GL context ? — Spektre
– Spektre, Commented Nov 24, 2014 at 11:18
Do you see something in the bottom half of the color picture with depth testing disabled? — dari
– dari, Commented Nov 24, 2014 at 15:49

Darth Zaloj · Accepted Answer · 2014-11-24 14:19:07Z

2

Well, I talked to someone who encountered an error like this and I resolved the problem in the process.

It turns out that everything in the white part at the end of the first pass is not white, but NaN in reality. GDEBugger displays the values as (255, 255, 255, 255) because it must not have support for displaying any other texture formats. It just says: "NaN, you must mean white :P".

Well, why does NaN matter if we are doing pixel replacement - glBlendFunc(GL_ONE, GL_ZERO). It's because operations on NaN still result in NaN the whole way down through the pipeline. The GL implemention still performs the "Result = SourceFactor * Source + DestFactor * Dest" - no optimizations are performed.

In regards to the fix... it turns out that performing "outFragColor = clamp(outFragColor , vec4(0.0), vec4(3000000000000.0))" in the fragment stage (outFragColor is the pixel return value) will not result in a NaN even though outFragColor contains NaNs. Go figure.

answered Nov 24, 2014 at 14:19

Darth Zaloj

1536 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

derhass Over a year ago

Why are you generating NaNs in the first place? You should fix the root issue, instead of trying to repair it later - which will behave completely implementation-dependent.

Collectives™ on Stack Overflow

OpenGL Full Depth With Partial Color Write

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related