Bandwidth Efficient Graphics with ARM Mali GPUs - GPU Pro: Advanced Rendering Techniques - page 276

This section will introduce two extensions:

• GL_ARM_shader_framebuffer_fetch

•

GL_ARM_shader_framebuffer_fetch_depth_stencil

3.2.1 GL ARM shader framebuffer fetch
This extension enables reading of the existing framebuffer color. This enables
use-cases such as programmable blending and other operations that may not be
possible to implement using fixed-function blending.
The extension is enabled by adding

# extension GL ARM shader framebuffer fetch : enable

to the very start of a fragment shader.
See Table 3.1 for a list of the built-in variables added by the extension and
Listing 3.1 for a simple programmable blending example using the extension.

Variabl e Type Description

Reads the existing framebuffer color.

gl_LastFragColorARM vec4

Tab l e 3. 1. New built-in variables introduced with the GL_ARM_shader_frame
buffer_fetch extension.

#extension GL ARM shader framebuffer fetch : enable
precision mediump float ;

uniform vec4 uBlend0 ;
uniform vec4 uBlend1 ;

void main ( void )
{ vec4 Color = gl_LastFragColorARM ;

Color = lerp ( Color , uBlend0 , Color . w uBlend0 . w );

Color ￿ = uBlend1 ;
gl FragColor = Color ;

}

Listing 3.1. Programmable blending using GL_ARM_shader_framebuffer_fetch .