Editing PNG (section)

=== Transparency of image ===
PNG offers a variety of transparency options. With true-color and grayscale images either a single pixel value can be declared as transparent or an [[alpha channel]] can be added (enabling any percentage of partial transparency to be used). For paletted images, alpha values can be added to palette entries. The number of such values stored may be less than the total number of palette entries, in which case the remaining entries are considered fully opaque.

The scanning of pixel values for binary transparency is supposed to be performed before any color reduction to avoid pixels becoming unintentionally transparent. This is most likely to pose an issue for systems that can decode 16-bits-per-channel images (as is required for compliance with the specification) but only output at 8 bits per channel (the norm for all but the highest end systems).

Alpha ''storage'' can be "associated" ("[[Alpha compositing|premultiplied]]") or "unassociated", but PNG standardized<ref name="w3.org">{{cite web|url=http://www.w3.org/TR/PNG-Rationale.html|title=PNG Specification: Rationale|work=w3.org}}</ref> on "unassociated" ("non-premultiplied") alpha, which means that imagery is not alpha ''encoded''; the emissions represented in RGB are not the emissions at the pixel level. This means that the over operation will multiply the RGB emissions by the alpha, and cannot represent emission and occlusion properly.