２枚の画像を合成するシェーダー（上の画像に依存）

shader.frag

#version 120

uniform sampler2DRect tex0;
uniform sampler2DRect foreground;

varying vec2 texCoordVarying;

void main()
{
    vec4 bgtex = texture2DRect(tex0, texCoordVarying);
    vec4 fgtex = texture2DRect(foreground, texCoordVarying);

    gl_FragColor = bgtex * (1.0 - fgtex.a) + fgtex * fgtex.a;
}

shader.vert

#version 120

varying vec2 texCoordVarying;

void main()
{
    texCoordVarying = gl_MultiTexCoord0.xy;
	  gl_Position = ftransform();
}