I am working inside a test bed project to get some hands on learning experience with OpenGL, and I’ve run into something that I do not understand. I have created a small rectangular texture with format of GL_RGBA32I, to try out a isampler2DRect, which contains just 3 pixels: red, green, and blue. I am using the sampler to sample the red pixel at coordinate (0, 0) and then I am verifying the value of the sample’s red channel by mixing it with a hard coded blue. I noticed that the red channel seems to have multiple values, which does not make sense to me.
Rounding error. The elements of a vec4 are single-precision floating-point values, which only have a 24-bit significand (23 bits stored, leading 1 implied). A comparison between int and float casts the int operand to float, so both 2147483584 (0x7fffffc0) and 2147483647 (0x7fffffff) are being rounded to 2147483648.0 (0x80000000), as that’s the closest representable value to either.
For signed integer textures (isampler*), texture() returns an ivec4. Changing the type of textureSample to ivec4 will avoid the rounding and will also avoid casting the other arguments of the comparisons, as both arguments will be integers.
Similarly, for unsigned integer textures texture() returns a uvec4.
This issue might be happening due to how integer textures are being sampled and interpreted in your shader. From your description, it sounds like you’re working with a GL_RGBA32I texture, which means the texture contains signed integer data. However, how you handle this data in the shader and how the GPU processes it can lead to unexpected results if there’s a mismatch somewhere.
Here are a few things to check:
Sampler Type and Shader Logic
Since your texture is GL_RGBA32I, you should be using an isampler2DRect in your shader, which it looks like you are. Make sure you’re not accidentally using a regular sampler type (sampler2DRect), as it will interpret the data differently, leading to undefined behavior.
Texture Values
Verify that the values in your texture are exactly what you think they are. When you create and upload the texture, ensure there’s no unintentional conversion happening. Debug by reading back the texture data after uploading it using glGetTexImage to confirm the pixel values.
Precision Issues in GLSL
Mixing integer and float data can sometimes cause issues. If your shader mixes hardcoded floats with sampled integer values, GLSL might be converting or interpreting them in ways that produce unexpected results. Make sure the mix() function is used correctly, and avoid mixing integer and floating-point data unless explicitly casting between them.
Testing Coordinates and Sampling
Double-check the texture sampling coordinates. Even a slight offset in coordinates could result in sampling a different pixel or interpolated values, depending on how the texture is set up.
Debugging in the Shader
To debug, output the sampled values directly to the fragment color:
This will help you confirm the actual value you’re sampling from the texture.
In your case, the “pink quad” could be because the red value you expect isn’t purely isolated, or it might be blending with another value due to one of the above issues. Carefully check how data is being passed through the pipeline and verify at each step.