To determine the palette of colors we use the median cut algorithm.
While being a correct implementation, enhancements are obviously
existing on both the median cut algorithm and the encoding side.
This is useful to find the best matching color palette from an existing
bitmap. It can be used in PixelPaint but also in encoders of old image
formats that only support indexed colors e.g. GIF.
For example, for 7z7c.gif, we now store one 500x500 frame and then
a 94x78 frame at (196, 208) and a 91x78 frame at (198, 208).
This reduces how much data we have to store.
We currently store all pixels in the rect with changed pixels.
We could in the future store pixels that are equal in that rect
as transparent pixels. When inputs are gif files, this would
guaranteee that new frames only have at most 256 distinct colors
(since GIFs require that), which would help a future color indexing
transform. For now, we don't do that though.
The API I'm adding here is a bit ugly:
* WebPs can only store x/y offsets that are a multiple of 2. This
currently leaks into the AnimationWriter base class.
(Since we potentially have to make a webp frame 1 pixel wider
and higher due to this, it's possible to have a frame that has
<= 256 colors in a gif input but > 256 colors in the webp,
if we do the technique above.)
* Every client writing animations has to have logic to track
previous frames, decide which of the two functions to call, etc.
This also adds an opt-out flag to `animation`, because:
1. Some clients apparently assume the size of the last VP8L
chunk is the size of the image
(see https://github.com/discord/lilliput/issues/159).
2. Having incremental frames is good for filesize and for
playing the animation start-to-end, but it makes it hard
to extract arbitrary frames (have to extract all frames
from start to target frame) -- but this is mean tto be a
delivery codec, not an editing codec. It's also more vulnerable to
corrupted bytes in the middle of the file -- but transport
protocols are good these days.
(It'd also be an idea to write a full frame every N frames.)
For https://giphy.com/gifs/XT9HMdwmpHqqOu1f1a (an 184K gif),
output webp size goes from 21M to 11M.
For 7z7c.gif (an 11K gif), output webp size goes from 2.1M to 775K.
(The webp image data still isn't compressed at all.)
Truncating the value is mathematically incorrect, this error made the
conversion to grayscale unstable. In other world, calling `to_grayscale`
on a gray value would return a different value. As an example,
`Color::from_string("#686868ff"sv).to_grayscale()` used to return
#676767ff.
We are often trying to click the image before it has finished loading.
This results in us trying to click a 0x0 rect. Instead, wait until the
image load event.
This fixes a flake with form-image-submission.html often seen on CI.
Two bugs:
1. Correctly set bits in VP8X header.
Turns out these were set in the wrong order.
2. Correctly set the `has_alpha` flag.
Also add a test for writing webp files with icc data. With the
additional checks in other commits in this PR, this test catches
the bug in WebPWriter.
Rearrange some existing functions to make it easier to write this test:
* Extract encode_bitmap() from get_roundtrip_bitmap().
encode_bitmap() allows passing extra_args that the test uses to pass
in ICC data.
* Extract expect_bitmaps_equal() from test_roundtrip()
If this turns out to be too strict in practice, we can replace it with
a `dbgln("VP8X and VP8L headers disagree about alpha; ignoring VP8X");`
instead.
ALso update catdog-alert-13-alpha-used-false.webp to not trigger this.
I had manually changed the VP8L alpha flag at offset 0x2a in
da48238fbd to clear it, but I hadn't changed the VP8X flag.
This changes the byte at offset 0x14 from 0x10 (has_alpha) to 0x00
(no alpha) as well, to match.
