[pdftex] pdftex compression -- proposed addition to manual

[siep at elvenkind.com]

Note: I waited a while with my reaction to the post below, since it
would have been better if my comment would have come from someone more
knowledgable about pdftex compression. I don't recall such a comment
having been posted so here comes:

jpeg compression should NOT be used as an all-purpose compression format
for image data. If the image consists of flat areas and discrete colors
(eg. screenshots, diagrams) then lossless compressed formats such as png
are quite efficient, whereas jpeg compression would introduce artefacts.
The use of jpeg should be limited to photographic and comparable images.

Siep Kroonenberg

On 14 Aug, Ben Crowell wrote:
> I'd like to suggest adding the following to the pdftex manual.
> Those of you who read comp.text.tex will notice that in a moment
> of brain-deadness, I posted it there earleir, when really I meant to
> post it here. Oops!
> 
> In addition to the information I wrote up below, I'm curious
> whether anyone can shed any light on the following:
> - AFAICT, there is no native support for PNG images in PDF format.
> 	What does pdftex do with PNG images? Decompress them?
> 	Convert them to JPEG? If it converts them to JPEG, what
> 	compression does it use?
> - Does pdftex have any support for CCITT or JBIG2?
> 
> 		Ben Crowell
> 
> ---------------------------------------------------------------------------
> 	proposed addition to manual
> ---------------------------------------------------------------------------
> 
> If you want to make PDF files with their compression tuned up
> perfectly for your purposes, then you'll need to understand some of
> the technical details about the PDF format below. If you want
> to skip the complexities and just produce reasonably well compressed
> output files, there are two main things you should know.
> First, you should check that compress_level is set to 9.
> Second, you should prepare bitmapped graphics input files
> in a compressed format --- typically JPEG --- that makes an appropriate
> tradeoff between compression and image quality; pdftex retains
> the compression of the input image, but doesn't do much further
> compression.
> 
> PDF format has some generic lossless compression capabilities.
> Old versions of the format only allowed the LZW compression
> algorithm, which is patent-encumbered. Newer versions also allow
> the use of the Flate algorithm. Because of the patent issues,
> pdftex only supports Flate. If your compress_level is set
> appropriately, pdftex will use Flate compression. Flate
> compression does a good job of compressing text and
> line art.
> 
> For bitmapped images, however, Flate compression isn't enough
> to produce good compression. If your input images are
> uncompressed, Flate will compress them somewhat, but not
> as much as a lossless compression algorithm designed for
> images. If your input images are in a compressed format
> such as JPEG, Flate
> compression does not produce very much improvement.
> PDF format therefore allows the use of several
> different compression schemes for images: JPEG,
> CCITT, and JBIG2. CCITT and JBIG2 are meant for black
> and white text. JPEG is a more general-purpose
> lossy-compression format for greyscale and color images.
> If you use a JPEG file as an input, pdftex simply copies
> it to the output, without changing its resolution or
> applying any further compression. (Flate compression will
> be applied if you've set compress_level appropriately,
> but it has very little effect.)
> 
> A typical method of working with compressed images would
> be to maintain all your original images in a lossless
> format such as PNG, and produce JPEG versions as inputs
> to pdftex. You can tune up the resolution and compression
> level of the JPEG versions to achieve the desired tradeoff
> between compression and image quality in your output file.
> 
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