avcodec/pngdec: Fix APNG_DISPOSE_OP_BACKGROUND

APNG works with a single reference frame and an output frame.
According to the spec, decoding APNG works by decoding
the current IDAT/fdAT chunks (which decodes to a rectangular
subregion of the whole image region), followed by either
overwriting the region of the output frame with the newly
decoded data or by blending the newly decoded data with
the data from the reference frame onto the current subregion
of the output frame. The remainder of the output frame
is just copied from the reference frame.
Then the reference frame might be left untouched
(APNG_DISPOSE_OP_PREVIOUS), it might be replaced by the output
frame (APNG_DISPOSE_OP_NONE) or the rectangular subregion
corresponding to the just decoded frame has to be reset
to black (APNG_DISPOSE_OP_BACKGROUND).

The latter case is not handled correctly by our decoder:
It only performs resetting the rectangle in the reference frame
when decoding the next frame; and since commit
b593abda6c642cb0c3959752dd235c2faf66837f it does not reset
the reference frame permanently, but only temporarily (i.e.
it only affects decoding the frame after the frame with
APNG_DISPOSE_OP_BACKGROUND). This is a problem if the
frame after the APNG_DISPOSE_OP_BACKGROUND frame uses
APNG_DISPOSE_OP_PREVIOUS, because then the frame after
the APNG_DISPOSE_OP_PREVIOUS frame has an incorrect reference
frame. (If it is not followed by an APNG_DISPOSE_OP_PREVIOUS
frame, the decoder only keeps a reference to the output frame,
which is ok.)

This commit fixes this by being much closer to the spec
than the earlier code: Resetting the background is no longer
postponed until the next frame; instead it is applied to
the reference frame.

Fixes ticket #9602.

(For multithreaded decoding it was actually already broken
since commit 5663301560d77486c7f7c03c1aa5f542fab23c24.)

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

0 files changed, 0 insertions, 0 deletions