GL_OES_sample_variables

Supported Hardware

Series6, Series6XE, Series6XT

Valid APIs

OpenGL ES 3.x

Description

This extension is one of three extensions which enhances OpenGL ES's programmability with respect to how multisampling is performed:

  • GL_OES_sample_variables
  • GL_OES_sample_shading
  • GL_OES_shader_multisample_interpolation

GL_OES_sample_variables is required by the other two extensions, and adds a number of variables accessible to a fragment shader that allow control over each individual sample that contributes to the current fragment:

    in lowp int gl_SampleID*  // The ID of the current sample being shaded. 
    in mediump vec2 gl_SamplePosition* // The sub-pixel position of the sample, in the range [0,1], relative to the bottom left of the pixel. 
    in highp int gl_SampleMaskIn[(gl_MaxSamples+31)/32] // The sample mask generated for the current fragment by the fixed function pipeline. 
    out highp int gl_SampleMask[(gl_MaxSamples+31)/32]** // Output sample mask, allowing the shader to generate a different mask, programmably. 

* Any use of these variables will force the fragment shader to run at sample rate, increasing the amount of fragment work.

** gl_SampleMask will be filled in with the value of gl_SampleMaskIn if it is never written to. If any execution path might write to it, all paths must write to it, or the value will be undefined.

Example

#extension GL_OES_sample_variables : require 
void main() 
{	 
	... 
	// Safely invert the sample mask 
	for (uint i = 0; i < ((gl_MaxSamples+31)/32); ++i) 
	{ 
		gl_SampleMask[i] == !gl_SampleMaskIn[i]; 
	} 
	... 
}