.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.
Type 2 <=> One-dimensional Group3, customized for TIFF
Type 3 <=> Two-dimensional Group3, uses the original 1D internally
Type 4 <=> Two-dimensional Group4
So let's clarify that this is not Group3 1D but the TIFF variant, which
is called `CCITTRLE` in libtiff. So let's stick with this name to avoid
confusion.
Images with a display mask ("stencil" as it's called in DPaint) add
an extra bitplane which acts as a mask. For now, at least skip it
properly. Later we should render masked pixels as transparent, but
this requires some refactoring.
SamplingFactors already has default initializers for its field,
so no need to have an explicit one for the first of the two fields.
No behavior change.
We now allow all subsampling factors where the subsampling factors
of follow-on components evenly decode the ones of the first component.
In practice, this allows YCCK 2111, CMYK 2112, and CMYK 2111.
Previously, we handled sampling factors as part of ycbcr_to_rgb().
That meant it worked ok for code paths that used YCbCr ("normal"
jpegs, and the YCC part of YCCK jpegs), but it didn't work for
example for the K channel in YCCK jpegs, nor for CMYK.
By making this a separate pass, it should now work for all cases.
It also makes it easier to support more subsampling arrangements
in the future, and to use something better than nearest neighbor
for upsampling subsampled blocks.
All the data is passed using the `Metadata` object, which has a
`main_tags` method. This method should be used when displaying only a
few main tags, for example to fill the property window of a file
manager. Another method returning the entire list of tags will be
implemented later on.
We refuse any image with a sample depth greater than 32, storing these
value as `u64` prevent any overflows. This is probably overkill as no
one in their right mind will use a 32 bits color table.
frame() still returns a regular RGB Bitmap (now lazily converted
from internal CMYK data), but JPEGImageDecoderPlugin now also
implements cmyk_frame().
