[png][alpha_channel]PNG Specification: Rationale : 12.8. Non-premultiplied alpha : PNG uses "unassociated" or "non-premultiplied" alpha so that images with separate transparency masks can be stored losslessly. Another common technique, "premultiplied alpha", stores pixel values premultiplied by the alpha fraction; in e
[agg][alpha_channel]Anti-Grain Geometry - Version 2.2 Release Notes : pre-multiplied result on the canvas. It has even less number of calculations than agg::pixfmt_rgba32_pre. Format agg::pixfmt_rgba32_plain is slow because of division operations. APIs allowing for alpha-blending require premultiplied colors.
