Nonpremultiplied

Nonpremultiplied, also called straight alpha, refers to image data in which the red, green, and blue color channels are stored as the original colors independent of the alpha channel. The alpha channel represents opacity, but the RGB values are not multiplied by alpha. This is in contrast to premultiplied alpha, where the RGB values are stored as color multiplied by alpha.

In compositing, the difference affects blending. For straight alpha, blending a source over a destination involves multiplying the source color by its alpha and the destination color by the remaining transparency, then combining them and adjusting by the resulting alpha. In premultiplied alpha, the RGB values are already multiplied by alpha, which simplifies the math and can avoid certain artifacts such as color halos during blending.

Common formats and usage: PNG and TIFF often store straight alpha. Many graphics APIs and GPU pipelines operate with premultiplied alpha for performance reasons, so images may be converted on load or during rendering. When working with nonpremultiplied data, filtering, color adjustments, and resizing can introduce halos or color shifts near edges if the alpha is not handled carefully.

Practical notes: If a workflow involves multiple layered composites, converting to premultiplied form before blending can be advantageous. Some tools expose both modes, and it is important to match the alpha handling to the target pipeline when saving or exporting. Understanding whether data is straight or premultiplied helps ensure correct color results during compositing and display.