Luminance values clipped to [0, 1] during texture transfer? - opengl

Question

I am uploading a host-side texture to OpenGL using something like:
GLfloat * values = new [nRows * nCols];
// initialize values
for (int i = 0; i < nRows * nCols; ++i)
{
values[i] = (i % 201 - 100) / 10.0f; // values from -10.0f .. + 10.0f
}
glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, nRows, nCols, GL_LUMINANCE, GL_FLOAT, values);
However, when I read back the texture using glGetTexImage(), it turns out that all values are clipped to the range [0..1].
First, I cannot find where this behavior is documented (I am using the Red Book for OpenGL 2.1).
Second, is it possible to change this behavior and let the values pass unchanged? I want to access the unscaled, unclipped data in an GLSL shader.

Answer 1

I cannot find where this behavior is documented
In the actual specification, it's in the section on Pixel Rectangles, titled Transfer of Pixel Rectangles.
Second, is it possible to change this behavior and let the values pass unchanged?
Yes. If you want to use "unscaled, unclamped" data, you have to use a floating point image format. The format of your texture is defined when you created the storage for it, probably by a call to glTexImage2D. The third parameter of that function defines the format. So use a proper floating-point format instead of an integer one.