SkSL_PremultipliedAlpha.py, Based on chrome/m131:docs/examples/SkSL_PremultipliedAlpha.cpp

Author: HinTak

SkSL_PremultipliedAlpha.py

@@ -0,0 +1,47 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+#
+#  Copyright 2024 Hin-Tak Leung
+#  Distributed under the terms of the new BSD license.
+#
+
+# Based on chrome/m131:docs/examples/SkSL_PremultipliedAlpha.cpp
+#     - We check effect being non-NULL here, unlike the c++ example.
+
+from skia import RuntimeEffect, Paint, ColorGRAY
+
+sksl = \
+  "const half3 iColor = half3(0, 0.5, 0.75);"                 + \
+  "half4 main(float2 coord) {"                                + \
+  "  float alpha = 1 - (coord.y / 150);"                      + \
+  "  if (coord.x < 100) {"                                    + \
+  "    /* Correctly premultiplied version of color */"        + \
+  "    return iColor.rgb1 * alpha;"                           + \
+  "  } else {"                                                + \
+  "    /* Returning an unpremultiplied color (just setting alpha) leads to over-bright colors. */" + \
+  "    return half4(iColor, alpha);"                          + \
+  "  }"                                                       + \
+  "}"
+
+def draw(canvas):
+    autoResult = RuntimeEffect.MakeForShader(sksl)
+
+    if (autoResult.effect is None):
+        raise RuntimeError(autoResult.errorText)
+
+    myShader = autoResult.effect.makeShader(None)
+
+    canvas.drawColor(ColorGRAY)
+
+    p = Paint()
+    p.setShader(myShader)
+    canvas.drawPaint(p)
+
+if __name__ == '__main__':
+    from skia import Surface, kPNG
+    surface = Surface(200, 200)
+    with surface as canvas:
+        draw(canvas)
+    surface.flushAndSubmit()
+    image = surface.makeImageSnapshot()
+    image.save('./output.png', kPNG)