PremultiplyAlpha

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ceramic.PremultiplyAlpha (Class)

Utilities for converting between straight and premultiplied alpha in image data.

Premultiplied alpha is a technique where RGB color values are multiplied by their alpha channel value before storage or rendering. This is crucial for correct alpha blending in GPU rendering pipelines.

In premultiplied alpha:

  • RGB values are scaled by alpha: (RA, GA, B*A, A)
  • Fully transparent pixels have RGB values of (0, 0, 0, 0)
  • Prevents color bleeding artifacts during filtering
  • Required by many GPU blend modes for correct results

In straight (non-premultiplied) alpha:

  • RGB values are stored independently of alpha: (R, G, B, A)
  • Transparent pixels can have any RGB values
  • More intuitive for image editing
  • Used by most image file formats
// Load image pixels
var pixels = texture.fetchPixels();

// Convert to premultiplied for GPU rendering
PremultiplyAlpha.premultiplyAlpha(pixels);

// Convert back to straight alpha for saving
PremultiplyAlpha.reversePremultiplyAlpha(pixels);
texture.saveToFile("output.png");
See: Texture.fetchPixels For accessing pixel data, Blending For alpha blending modes

Static Members

premultiplyAlpha(pixels: UInt8Array): Void

Converts pixel data from straight alpha to premultiplied alpha format.

Multiplies each RGB component by its corresponding alpha value. This operation modifies the pixel data in-place for efficiency.

The conversion formula for each pixel:

  • R' = R * (A / 255)
  • G' = G * (A / 255)
  • B' = B * (A / 255)
  • A' = A (unchanged)
Name Type Description
pixels UInt8Array The pixel data array in RGBA format (4 bytes per pixel). Must have length divisible by 4. Modified in-place. * haxe // Prepare pixels for GPU rendering var pixels = loadImagePixels("sprite.png"); PremultiplyAlpha.premultiplyAlpha(pixels); var texture = Texture.fromPixels(width, height, pixels);
reversePremultiplyAlpha(pixels: UInt8Array): Void

Converts pixel data from premultiplied alpha back to straight alpha format.

Divides each RGB component by its alpha value to restore original colors. This operation modifies the pixel data in-place for efficiency. Pixels with zero alpha are left unchanged to avoid division by zero.

The conversion formula for each pixel (when A > 0):

  • R' = R / (A / 255)
  • G' = G / (A / 255)
  • B' = B / (A / 255)
  • A' = A (unchanged)
Name Type Description
pixels UInt8Array The pixel data array in premultiplied RGBA format. Must have length divisible by 4. Modified in-place. * haxe // Convert back for image editing or saving var pixels = texture.fetchPixels(); PremultiplyAlpha.reversePremultiplyAlpha(pixels); savePixelsAsPNG(pixels, "output.png"); * Note: Due to rounding during premultiplication, this operation may not perfectly restore original values, especially for low alpha values.