Re: ... aPNG (Animated PNG)

John Bowler wrote:

> GIF has two basic operations combined into one.  They are:
> 
> 1) SAVE the current frame buffer
> 2) RESTORE the previously saved frame buffer
> 
> The "save" occurs before a frame with "restore to previous" disposal, the
> "restore" occurs after the time delay specified for the frame.
> 
> If I remember MNG correctly the *only* thing this requires is to make a COPY
> of the current abstract image - APNG needs one abstract image (the size of
> the logical stream) and *at most* one copy of it.  The copy is only ever
> used to replace the current abstract image.

MNG does not have read access to the current frame buffer.  I remember
vehement arguments about that while designing MNG, and the end result
was to design the PAST chunk which can be read.  So a MNG that wants
to do restore-to-previous would maintain a PAST object and display it
repeately as each frame is built.  If a restore-to-previous comes along,
it can composite that image over the PAST object and display it, then
restore the PAST object to the screen after the appropriate delay time.

Glenn

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