片段着色器似乎不精确地落在floor（）上

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

我不完全知道什么是精度mediump float实际上是在OpenGL ES在Android，但最有可能的解释是，该文本（或实际代码的变化值） 64.98已四舍五入为65 （使用圆最接近的位置），甚至还没有放到floor 。