A tile is basically a strip with a user-defined width. With that in
mind, adding support for them is quite straightforward. As a lot the
common code was named after 'strips', to avoid future confusion I
renamed everything that interact with either strips or tiles to a
global term: 'segment'.
Note that tiled images are supposed to always have a 'TileOffsets' tag
instead of 'StripOffset'. However, this doesn't seem to be enforced by
encoders, so we support having either of them indifferently.
The test case was generated with the following Python script:
import pyvips
img = pyvips.Image.new_from_file('deflate.tiff')
img.write_to_file('tiled.tiff',
compression=pyvips.ForeignTiffCompression.DEFLATE,
tile=True, tile_width=64, tile_height=64)
This variable stores the number of rows from the beginning of the image,
contrary to `row` that stores the number of rows relative to the start
of the current segment.
The first marker is always white in CCITT streams, so lines starting
with a black pixel encodes a symbol meaning 0 white pixels. Then, the
decoding would proceed with a black symbol. We used to set the symbol's
color based on `column == 0`, which is wrong in this situation.
The `read_tag()` function is not mandated to keep the reading head at a
meaningful position, so we also need to align the pointer after the last
tag. This solves a bug where reading the last field of an IFD, which is
placed after the tags, was incorrect.
Every TIFF containers is composed of a main IFD. Some entries of this
one can be a pointer to a sub-IFD. We are now capable of exploring these
underlying structures. Note that we don't do anything with them yet.
JPEGStream::byte_offset() now returns an offset relative to the start
of the stream, instead of relative to the buffered part.
No behavior change except if JPEG_DEBUG is set.
bab2113ec1 made read_whitespace() return ErrorOr, which makes this
easy to do.
(7cafd7d177, which added the fixmes, landed slightly after bab2113ec1,
so not quite sure why it wasn't like this immediately. Maybe commit
order got changed during review; both commits were in #17831.)
No behavior change.
We always store CMYK data as YCCK, for two reasons:
1. If we ever want to do subsampling, then doing 2111 or
2112 makes sense with YCCK, while it doesn't make sense
if we store CMYK directly.
2. It forces us to write a color transform header. With a color
transform header, everyone agrees that the CMYK channels should
be stored inverted, while without it behavior between decoders
is inconsistent. (We could write an explicit color transform header
for CMYK too though, but with YCCK it's harder to forget since the
output will look wrong everywhere without it.)
initialize_mcu() grows a full CMYKBitmap override. Some of the
macroblock traversal could probably shared with some kind of
for_all_macroblocks() type function in the future, but the color
conversion math is different enough that this should be a separate
function.
Other than that, we pass around a mode parameter and make a few fuctions
write 4 instead of 3 channels, and that's it.
We use the luminance quantization and huffman tables for the K
channel.
JPEGs can store a `restart_interval`, which controls how many
minimum coded units (MCUs) apart the stream state resets.
This can be used for error correction, decoding parts of a jpeg
in parallel, etc.
We tried to use
u32 i = vcursor * context.mblock_meta.hpadded_count + hcursor;
i % (context.dc_restart_interval *
context.sampling_factors.vertical *
context.sampling_factors.horizontal) == 0
to check if we hit a multiple of an MCU.
`hcursor` is the horizontal offset into 8x8 blocks, vcursor the
vertical offset, and hpadded_count stores how many 8x8 blocks
we have per row, padded to a multiple of the sampling factor.
This isn't quite right if hcursor isn't divisible by both
the vertical and horizontal sampling factor. Tweak things so
that they work.
Also rename `i` to `number_of_mcus_decoded_so_far` since that
what it is, at least now.
For the test case, I converted an existing image to a ppm:
Build/lagom/bin/image -o out.ppm \
Tests/LibGfx/test-inputs/jpg/12-bit.jpg
Then I resized it to 102x77px in Photoshop and saved it again.
Then I turned it into a jpeg like so:
path/to/cjpeg \
-outfile Tests/LibGfx/test-inputs/jpg/odd-restart.jpg \
-sample 2x2,1x1,1x1 -quality 5 -restart 3B out.ppm
The trick here is to:
a) Pick a size that's not divisible by the data size width (8),
and that when rounded to a block size (13) still isn't divisible
by the subsample factor -- done by picking a width of 102.
b) Pick a huffman table that doesn't happen to contain the bit
pattern for a restart marker, so that reading a restart marker
from the bitstream as data causes a failure (-quality 5 happens
to do this)
c) Pick a restart interval where we fail to skip it if our calculation
is off (-restart 3B)
Together with #22987, fixes#22780.
Non-interleaved files always have an MCU of one data unit.
(A "data unit" is an 8x8 tile of pixels, and an "MCU" is a
"minium coded unit", e.g. 2x2 data units for luminance and
1 data unit each for Cr and Cb for a YCrCb image with
4:2:0 subsampling.)
For the test case, I converted an existing image to a ppm:
Build/lagom/bin/image -o out.ppm \
Tests/LibGfx/test-inputs/jpg/12-bit.jpg
Then I converted it to grayscale and saved it as a pgm in Photoshop.
Then I turned it into a weird jpeg like so:
path/to/cjpeg \
-outfile Tests/LibGfx/test-inputs/jpg/grayscale_mcu.jpg \
-sample 2x2 -restart 3 out.pgm
Makes 3 of the 5 jpegs failing to decode at #22780 go.
That's all this function reads from Component.
Also rename from validate_luma_and_modify_context() to
validate_sampling_factors_and_modify_context().
No behavior change.
These are written by `mutool extract` for CMYK images.
They don't contain color profiles so they're not super convenient.
But `image` can convert them (*) to sRGB as long as you use it with
`--assign-color-profile` pointing to some CMYK icc profile of your
choice.
*: Once #22922 is merged.
.pam is a "portrable arbitrarymap" as documented at
https://netpbm.sourceforge.net/doc/pam.html
It's very similar to .pbm, .pgm, and .ppm, so this uses the
PortableImageMapLoader framework. The header is slightly different,
so this has a custom header parsing function.
Also, .pam only exixts in binary form, so the ascii form support
becomes optional.
When decoding a CMYK image and asked for a normal `frame()`, the decoder
would convert the CMYK bitmap into an RGB bitmap. Calling `cmyk_frame()`
after that point will provoke a null-dereference.
We previously were considering Float and Doubles as non-supported types.
But this was done in a sneaky way, by letting them hit the default case
in the `read_type` method. So, when I ported this function to the
generator we started to make this types flow into the system without a
proper support there. Since 3124c161, we would have crashes on images
containing tags with a floating point value.
A lot of images format use Exif to store there metadata. As Exif is
based on the TIFF structure, the TIFF decoder can, without modification
be able to decode the metadata. We only need a new API to explicitly
mention that we only need the metadata.
Support for JPEGs embedded in TIFF images was introduced with TIFF 6.0.
However, this implementation had major issues. It was so problematic
that they decided to reimplement it from scratch in 1995, three years
later. The two incarnations are obviously incompatible.
For more details see:
https://www.awaresystems.be/imaging/tiff/specification/TIFFTechNote2.txt
When present, the alpha channel is also affected by the horizontal
differencing predictor.
The test case was generated with GIMP with the following steps:
- Open an RGB image
- Add a transparency layer
- Export as TIFF with the LZW compression scheme
This tag is required by the specification, but some encoders (at least
Krita) don't write it for images with a single strip.
The test file was generated by opening deflate.tiff in Krita and saving
it with the DEFLATE compression.
