Shader texture values not the same as written when creating the texture

Question

I have created a texture and filled it with ones:

size_t size = width * height * 4;
float *pixels = new float[size];
for (size_t i = 0; i < size; ++i) {
    pixels[i] = 1.0f;
}

glTextureStorage2D(texture_id, 1, GL_RGBA16F, width,
                   height);
glTextureSubImage2D(texture_id, 0, 0, 0, width, height, GL_RGBA,
                    GL_FLOAT, pixels);

I use linear filtering (GL_LINEAR) and clamp to border.

But when I draw the image:

color = texture(atlas, uv);

the last row looks like it has alpha values of less than 1. If in the shader I set the alpha to 1:

color.a = 1.0f;

it draws it correctly. What could be the reason for this?

Answer 1

The problem comes from the combination of GL_LINEAR and GL_CLAMP_TO_BORDER:

• Clamp to border means that every texture coordinate outside of [0, 1] will return the border color. This color can be set with glTexParameterf(..., GL_TEXTURE_BORDER_COLOR, ...) and is black by default.

• Linear filter will take into account pixels that are adjacent to the sampling location (unless sampling happens exactly at texel centers¹), and will thus also read border color texels (which are here black).

If you don't want this behavior, the simplest solution would be to use GL_CLAMP_TO_EDGE instead which will repeat the last row/column of texels to infinity. The different wrapping modes are explained very well explained at open.gl.

_{1) Sampling happens most probably not at pixel centers as explained in this answer.}