android - Fragment shader seems to floor() imprecisely

Question

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 );

}

Answer 1

我不完全知道mediump floatAndroid 上 OpenGL ES 中 a 的实际精度是多少，但最可能的解释是，文字（或实际代码中的变化值）64.98已经四舍五入65（使用四舍五入到最近）首先，在它被放入之前floor。