Fragment shader seems to floor() imprecisely

Posted by Peter K. on Game Development See other posts from Game Development or by Peter K.
Published on 2013-11-13T10:16:07Z Indexed on 2013/11/13 16:19 UTC
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Filed under:

shaders

|

glsl

|

opengl-es2

I'm trying to interpolate coordinates in my fragment shader. Unfortunately if close to the upper edge the interpolated value of fVertexInteger seems to be rounded up instead of beeing floored. This happens above approximately fVertexInteger >= x.97.

Example:

floor(64.7) returns 64.0 --> correct
floor(64.98) returns 65.0 --> incorrect

The same happens on ceiling close above x.0, where ceil(65.02) returns 65.0 instead of 66.0.

Q: Any ideas how to solve this?

Note:

GL ES 2.0 with GLSL 1.0
highp floats are not supported in fragment shaders on my hardware
flat varying hasn't been a solution, because I'm drawing TRIANGLE_STRIP and can't redeclare the provoking vertex (only OpenGL 3.2+)

Fragment Shader:

varying float fVertexInteger;
varying float fVertexFraction;
void main() {

    // Fix vertex integer
    fixedVertexInteger = floor(fVertexInteger);

    // Fragment color
    gl_FragColor = vec4(
        fixedVertexInteger / 65025.0,
        fract(fixedVertexInteger / 255.0),
        fVertexFraction,
        1.0 );

}

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