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Showing content from https://developer.mozilla.org/en-US/docs/Web/API/OES_standard_derivatives below:

OES_standard_derivatives extension - Web APIs

Constants

This extension exposes one new constant, which can be used in the hint() and getParameter() methods.

ext.FRAGMENT_SHADER_DERIVATIVE_HINT_OES: A GLenum indicating the accuracy of the derivative calculation for the GLSL built-in functions: dFdx, dFdy, and fwidth.

GLSL built-in functions

The following new functions can be used in GLSL shader code, if this extension is enabled:

genType dFdx(genType p)
genType dFdy(genType p)
genType fwidth(genType p)

dFdx(): Returns the derivative in x using local differencing for the input argument p.
dFdy(): Returns the derivative in y using local differencing for the input argument p.
fwidth(): Returns the sum of the absolute derivative in x and y using local differencing for the input argument p. That is, abs(dFdx(p)) + abs(dFdy(p)).

dFdx() and dFdy() are commonly used to estimate the filter width used to anti-alias procedural textures.

Examples

Enabling the extensions:

gl.getExtension("OES_standard_derivatives");
gl.getExtension("EXT_shader_texture_lod");

Shader code that avoids artifacts when wrapping texture coordinates:

<script type="x-shader/x-fragment">
  #extension GL_EXT_shader_texture_lod : enable
  #extension GL_OES_standard_derivatives : enable

  uniform sampler2D myTexture;
  varying vec2 texCoord;

  void main(){
    gl_FragColor = texture2DGradEXT(myTexture, mod(texCoord, vec2(0.1, 0.5)),
                                    dFdx(texCoord), dFdy(texCoord));
  }
</script>

Specifications Browser compatibility

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