A single despill effect for improving efects of chroma keying. Works for Green, Blue and Red backgrounds, no other controls yet. Tested.

effects/internal/despill.frag

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+/***
+    Despill - for cleaning up keying residue.
+    Implementation by unfa for Olive-Editor Community
+***/
+
+uniform sampler2D tex;
+varying vec2 uTexCoord;
+uniform vec2 resolution;
+uniform int channel;
+
+const vec2 renderScale = vec2(1,1);
+
+void main(void)
+{
+    vec2 uv = gl_FragCoord.xy/resolution.xy;
+
+    vec4 col = texture2D(tex, uv);
+    
+    // separate channels
+    
+    float r = col.r;
+    float g = col.g;
+    float b = col.b;
+    float a = col.a;
+    
+    // Select despilled channel: 0 = RED; 1 = GREEN; 2 = BLUE;
+    
+    if (channel == 0) // RED
+    { 
+        // replace new green channel by avaraging between existing red and blue channels
+        float r2 = (g + b) * 0.5;
+        
+        // now composite the original and new gree channel using the "darken" mode
+        r = min(r, r2);
+    }
+    
+    if (channel == 1) // GREEN
+    { 
+        // replace new green channel by avaraging between existing red and blue channels
+        float g2 = (r + b) * 0.5;
+        
+        // now composite the original and new gree channel using the "darken" mode
+        g = min(g, g2);
+    }
+    
+    if (channel == 2) // BLUE
+    { 
+        // replace new green channel by avaraging between existing red and blue channels
+        float b2 = (r + g) * 0.5;
+        
+        // now composite the original and new gree channel using the "darken" mode
+        b = min(b, b2);
+    }
+        
+
+    // and return the result
+
+    gl_FragColor = vec4(r, g, b, a);
+}

effects/internal/despill.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<effect name="Despill" category="Keying">
+    <row name="Channel">
+		<field type="combo" default="1" id="channel">
+			<option>Red</option>
+			<option>Green</option>
+			<option>Blue</option>
+		</field>
+	</row>
+        <shader vert="../COMMON/common.vert" frag="despill.frag"/>
+</effect>