OpenGL ES 2.0 Derivative Texture

Question

I have an OpenGL ES 2.0 based iPhone app which I am running to a bit of OpenGL ES trouble.

I am trying to complile a fragment shader which computes/displays the derivative of an input texture. My fragment shader code is:

 varying highp vec2 textureCoordinate;

 uniform sampler2D inputImageTexture;
 uniform sampler2D inputImageTexture2;

 void main(void)
 {
   mediump vec4 derivData = vec4(dFdx(texture2D(inputImageTexture, textureCoordinate).xyz, 0.0);

   gl_FragColor = derivData;
 }

However, this fails the compile. If I take out the dFdX, it compiles just fine.

Does anyone have any experience with this? I'd eventually like to calculate the derivative with respect to Y as well, then merge them seeing how the input texture is an image.

I've been struggling on this for a few days now so any advice you have would be greatly appreciated!

Answer 1

All iOS hardware that supports ES 2.0 will support the GL_OES_standard_derivatives extension: https://developer.apple.com/library/ios/documentation/DeviceInformation/Reference/iOSDeviceCompatibility/OpenGLESPlatforms/OpenGLESPlatforms.html

However, you don't get it "for free". In your fragment shader, you must add the following at the top (from http://www.khronos.org/registry/gles/extensions/OES/OES_standard_derivatives.txt):

#extension GL_OES_standard_derivatives : enable

All the info in the comments to the first answer is mostly accurate, but without this piece you'll continue to get the following error:

ERROR: 0:15: Call to undeclared function 'dFdx'
ERROR: 0:16: Call to undeclared function 'dFdy'

This threw me for a loop, but once I added the enable line, it seems to work on both the device and the simulator (haven't actually evaulate that it works on both, but it does compile).