demotool/shaders/debug/alpha-channel.glsl

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#version 450 core
//! type fragment
layout( location = 0 ) uniform sampler2D u_InputTexture;
layout( location = 1 ) uniform int u_LOD;
layout( location = 2 ) uniform vec2 u_OutputSize;
layout( location = 0 ) out vec4 o_Color;
void main( void )
{
vec2 ts = textureSize( u_InputTexture , u_LOD );
ivec2 pos = ivec2( ts * gl_FragCoord.xy / u_OutputSize );
o_Color = vec4( vec3( texelFetch( u_InputTexture , pos , u_LOD ).a ) ,
1 );
}