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* `APNG_RENDER_OP_DISPOSE_PREVIOUS`: the frame's region is to be reverted to the previous contents.
`APNG_RENDER_OP_BLEND_FLAG` may be added to any of the above disposal operations. If this flag is 0, all color components of the frame, including alpha, overwrite the current contents of the frame's canvas region. If the APNG_RENDER_OP_BLEND_FLAG is not 0 the frame should be composited onto the canvas based on its alpha, using a simple OVER operation as described in the "Alpha Channel Processing" section of the PNG specification [pngspec]. Notethat Variation 2 of the sample code is applicable.
`APNG_RENDER_OP_BLEND_FLAG` is not valid for color types 0 (greyscale without alpha) or 2 (truecolor without alpha). It is valid for type 3 (indexed) images; however, the resulting pixel values may not be present in the specified palette. It is always valid for type 4 (greyscale with alpha) and type 6 (truecolor with alpha) images.
Each frame inherits every property except the width and the height specified by any critical or ancillary chunks before the first IDAT in the file.
If the PNG oFFs chunk is present, it supplies offsets of the canvas that are in addition to the frame's "x_offset" and "y_offset".
If the PNG pHYs chunk is present, the APNG images and their "x_offset" and "y_offset" values must be scaled in the same way as the main image.
= Revisions to this Specification =
