把影像存在buffer中，透過shader讀取不同的材質。
uniform sampler2D tex;
uniform vec2 resolution;
uniform int f;
void main (void)
{
vec2 uv=gl_FragCoord.xy/resolution.xy;
vec3 col= texture2D(tex, vec2(uv.x , 1.0-uv.y)).xyz;
gl_FragColor = vec4( col, 1.0 );
}
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