Premultiplied alpha and straight alpha with a free math lab

A transparent image can look correct on one background and develop a dark fringe in another application. Before changing the matte, check how the color and alpha are interpreted. Premultiplied alpha and straight alpha describe the relationship between those values, not whether the visible subject is a good selection.

For a repeated transparent surface image, use the tileable texture inspector to review joins and preserve the source alpha.

What premultiplied alpha means

With straight or unassociated alpha, the stored color has not already been multiplied by opacity. With premultiplied or associated alpha, it has. A color value of 200 at one-half opacity becomes 100 in the premultiplied representation, before adding the background contribution.

The W3C compositing specification sets out source-over alpha compositing. For an opaque background, the association examples here compare the same foreground and background values in the same numeric space. Color-space conversion is a separate decision; the lab is not a color-managed linear-light renderer.

Compare the four cases in the lab

Choose a foreground color, background color and opacity. The first two rows use their matching representations: straight color multiplied once, or already-premultiplied color added without multiplying it again. Their numeric results match. The last two rows deliberately use the wrong interpretation so you can see the consequence.

The swatches are calculated opaque output colors for one pixel. They are not samples measured from an uploaded file. Download the current SVG and numeric JSON to keep the selected inputs and results together, or inspect the original calculation source.

Why a PNG is not a premultiplied-file detector

The PNG specification states that PNG color samples are not premultiplied by alpha. Software can still use premultiplied representations internally while decoding, filtering or displaying an image. A canvas preview does not reveal every internal buffer convention or the original settings of another application.

Do not diagnose a file solely from a dark edge. A black matte baked into the color, an inaccurate cutout, resampling, color management or an incorrect interpretation can produce different problems. Inspect the original image and the settings in the application where the fringe appears.

Use a controlled edge test

1. Keep an untouched copy of the source and note its format.
2. View the edge against contrasting light and dark backgrounds.
3. Check the application's source interpretation and alpha handling.
4. Compare a small known test before changing an entire sequence.
5. Export and re-import the result into the intended destination.

Change one setting at a time. If an interpretation correction fixes the fringe without changing the silhouette, retain the source and record that setting. If the shape itself is missing detail, address the matte separately using the rotoscoping and matte-review workflow.

Understand the zero-alpha limit

When opacity is greater than zero, an ideal unpremultiply operation divides the associated color by opacity. At zero opacity, that division cannot recover a unique original color. The lab returns a null recovered value at zero rather than inventing a color from missing information.

Near zero, quantization and rounding can also matter. Small associated values have limited precision in a finite-bit representation. Keep higher-quality source material when the pipeline requires it and inspect the actual export rather than assuming every conversion is reversible byte for byte.

Keep the tasks in the right order

For an AI-generated character or product, approve the source image in Clout Image and Video, then use suitable editing tools for selection and compositing. A transparent-looking background in a preview is not proof of a usable alpha channel.

The light-wrap tool changes inside-edge color while preserving the selected PNG alpha. It is a different finishing step from correcting association. The image-sequence converter flattens alpha onto a chosen background for its MP4 output; retain the original PNGs for later compositing.