Support DCT Filter

[aagubanov]

This is the enhancement request. The filter DCT (Discrete Cosine Transform) is not supported, and as a result, some embedded JPG pictures cannot be extracted.

[fnatzke]

@aagubanov , @EliotJones,

Do have this well underway.

The DCT decode itself is well developed (for 1 of 4 modes) however post decode there is a large variation and complexity yet to be addressed including

1. translating to (around 8 or so) “final” colorspaces 2.(sub)sampling
2. stretch
3. masks (alpha/transparency)

Beyond implementation will be testing.

The test matrix is large with a large number of options and combinations. Beyond testing from “cold” hand build test PDFs from scratch is finding “in the wild” examples of “good” and “bad” implementations.

Relates to #484 ; image export from provided PDF starts with DCT filter before usage of ColorSeparation colorspace.

ColorSeparation colorspace itself makes use of a “Tint function” which can be implemented in 4 modes: 0 Sampled function 2 Exponential interpolation function 3 Stitching function 4 PostScript calculator function

These are also well underway however testing these again will be significant.

Have found 11 (public) “in the wild” example PDFs using separation colorspace [it's rare].

DCT (Discrete Cosine Transform) based on ITU-T81 4.5 has four distinct modes of operation with various coding processes:

1. sequential DCT-based,
2. progressive DCT-based,
3. lossless, and
4. hierarchical. Currently only mode 1 is implemented. PDF spec calls for mode 2 support (but unlikely to be needed in practice for most PDFs). Supports 8-bit grayscale and YCbCr images. Translation to RGB colorspace done. Other colorspaces require work. Supports restart markers.

Adobe Technical Note TN.5116 details additional decode handling (from inside a PDF) including support for App14 "Adobe" Application Segment hint for colorspace transform support. The default is to use the YCC-to-RGB [color]transform. Byte 11 signals color translations of: // 0 = CMYK
// 1== YCCK

8 bit only (16bit or others require down/up sampling to 8 bit; yet to be implemented).

After all post image processing implemented final step will be translating (Device Independent Bitmap) to PNG for final export from library.

So coming but not soon.

[BobLd]

@fnatzke I've implemented the 4 function types in https://github.com/UglyToad/PdfPig/pull/557 and the separation colorspace now loads the actual function

Also, you can find a lot of strange pdf (I'm pretty sure you'll find PDFs using separation colorspace) here https://github.com/pdf-association/pdf-corpora#safedocs-issue-tracker-corpus

[fnatzke]

Well that's a surprise. Think I've recovered. Is there a way we can coordinate contributions?

Published your link and the URLS of PDFs I've found (so 30,000 ~ 50GB) in https://github.com/UglyToad/PdfPig/discussions/302.

For the Urls I've found provided a command line to download in the discussion. Hope it can help someone.

[BobLd]

@fnatzke I'm going to create a discussion where we can coordinate

[EliotJones]

@BobLd do you happen to know the current state of this, is DCT support now complete?

[BobLd]

@EliotJones as far as I know it's not. Not sure if @fnatzke is still working on that or not