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What is the range of gl_FragCoord.xy ? I know gl_FragCoord s are screen coordinates but what is the actual range? ( 0, width - 1 ) or ( 1, width ) ? And what kind of error (if any) would glsl compiler generate say I did something like gl_FragCoord.x - 5 when i do some fragcoord based calculation?

ps: I don't need normalization.

I did a test to the actual range of gl_FragCoord.xy , by using the following shader code and glReadPixels(0, 0, 1024, 1024, GL_RED, GL_FLOAT, xxx) to get the output of the shader from my framebuffer object, and the FBO had attached a texture which's internal format is GL_R32F .

out highp vec4 Output;
void main()
    Output = vec4(gl_FragCoord.xy, 0.0, 1.0);
The actual result is: gl_FragCoord.xy is in the range [0.5, 1023.5], not [0.0, 1023.0].
                From the spec: "By default, gl_FragCoord assumes a lower-left origin for window coordinates and assumes pixel centers are located at half-pixel coordinates. For example, the (x, y) location (0.5, 0.5) is returned for the lower-left-most pixel in a window" Pay attention to pixel centers concept.
– Ripi2
                Aug 15, 2018 at 12:31
                khronos.org/opengl/wiki/… There are two layout qualifiers that can affect the result. origin_upper_left and pixel_center_integer. pixel_center_integer will be shifted by a half-pixel, so that the center of each pixel is an integer value.
– KAlO2
                Jun 2, 2021 at 10:12
                thankx, i use vec4 coord=gl_FragCoord-0.5;         int index = int(coord.y * width + coord.x);
– roberto
                Jun 23, 2021 at 10:02
        
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