sample GL_DEPTH24_STENCIL8 depth texture error in some IOS device

Question

I have a depth texture which is GL_DEPTH24_STENCIL8( or GL_DEPTH_COMPONENT24), and I can correctly sample this texture on some devices(iPhone5s iPad1), but fail with some invalid pixels. Following is bound gpu texture (the depth) and the format info captured by xcode :

Note that I've clip the value into [0.999, 1] since the homo depth are mostly in the set. I am sampling the texture and clip the value in my shader also.

uniform sampler2D tex0;
varying mediump vec2 TexCoord0;
void ps_main()
{
  float bias = 0.0;
  lowp vec4 zb = texture2D(tex0, TexCoord0, bias);
  const mediump float mag = 20.0;
  mediump float linearz = (zb - 0.999) / (1.0 - 0.999)
  gl_FragColor = vec4(linearz, linearz, linearz, 1.0);
}

And this shader gives a wrong result on the devices mentioned above:

The device and driver info is:

Driver: OpenGLES2
Description: Apple A8 GPU
Version: OpenGL ES 2.0 Apple A8 GPU - 77.14
Vendor: Apple Inc.
Width: 2048, Height: 1536, BitDepth: 32

Any clues to this problem? Or some other debug suggestions?

Answer 1

You are relying on more precision than the API guarantees to provide. For example, the variable zb is lowp which would only guarantee 1 part in 256 accuracy, but you then try to use in a computation in which needs at least 1 part in 1000 in when computing the value of linearz.

Try increasing precision to highp to get above the critical precision threshold.