Alpha Channel

An alpha channel is an extra image channel — beyond the standard red, green and blue — that encodes per-pixel transparency information, ranging from fully opaque (alpha = 1.0 or 255) to fully transparent (alpha = 0). The alpha channel allows compositing operations: a foreground element can be overlaid on a background, with the alpha channel controlling exactly which pixels show foreground and which let the background through, including partial transparency for soft edges, motion blur, hair, and semi-transparent objects like glass or smoke. Image formats supporting alpha include OpenEXR (essential for VFX, supports half-float alpha), PNG (8-bit alpha for web graphics), TIFF (8 or 16-bit alpha), PSD (multiple alpha channels for Photoshop layers), and ProRes 4444 (alpha video). Video formats supporting alpha include ProRes 4444 family, DNxHR 444, GoPro Cineform RGB, and Animation codec. Alpha can be pre-multiplied (RGB values already multiplied by alpha at the source, used in CG and EXR) or straight (RGB unchanged, alpha as separate matte, used in some compositing pipelines). Mismatching pre-multiplied and straight alpha during compositing causes visible black or white fringing on transparent edges. In VFX pipelines, alpha channels are sometimes called mattes or masks (the terms overlap but matte usually implies the alpha resulting from a compositing operation like keying, mask implies a hand-painted or generated isolation channel). Reference: OpenEXR documentation for alpha pre-multiplication conventions.