mkxp-freebird/shader/bitmapBlit.frag

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/* Shader for approximating the way RMXP does bitmap
* blending via DirectDraw */
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uniform sampler2D source;
uniform sampler2D destination;
uniform vec4 subRect;
uniform float opacity;
varying vec2 v_texCoord;
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void main()
{
vec2 coor = v_texCoord;
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vec2 dstCoor = (coor - subRect.xy) * subRect.zw;
vec4 srcFrag = texture2D(source, coor);
vec4 dstFrag = texture2D(destination, dstCoor);
vec4 resFrag;
float ab = opacity;
const float as = srcFrag.a;
const float ad = dstFrag.a;
const float at = ab*as;
resFrag.a = at + ad - ad*at;
resFrag.rgb = mix(dstFrag.rgb, srcFrag.rgb, ab*as);
resFrag.rgb = mix(srcFrag.rgb, resFrag.rgb, ad*resFrag.a);
gl_FragColor = resFrag;
}