With cairo's recent 1.6.4 release, we've hoped to reach the nirvana of applications that display and print documents with perfect fidelity. Unfortunately, reality isn't always as pleasant as we would like. I recently received a bug report that Firefox 3 (using cairo 1.6.4) resulted in a blurry mess when printing a very simple web page, (some text, a table, and an image). Exploring the details of this case reveals at least three independent problems that conspire to give the bad results.
Bug 1: Firefox+cairo uses image fallbacks for table borders
First, here's the simplest web page I was able to construct to show the problem, (nothing more than a single-cell table with a border): bug.html (122 bytes).
Using Firefox3 with cairo 1.6.4 on a Fedora9 system, I did a "print to file" and obtained the following PDF output: bug.pdf (14,465 bytes).
This output is still quite accurate and fairly usable. But we've already seen problem #1. Note that the file size has increased by a factor of 100 compared to the original HTML. The PDF does have more content, (firefox adds a header and footer for example), but nothing that explains such a large file. Instead, something about the way that firefox is expressing the table border is resulting in cairo putting fallback images into the resulting PDF file. So that's the first bug. I'll look closer at this, (probably with libcairowrap), and make a bug report to the mozilla folks if necessary.
Also, note that when cairo puts the fallback images into the PDF file it uses a "knockout group" to do so. This is a particular PDF construct that I'll discuss later.
Bug 2: Poppler+cairo expands knockout groups to full-page fallbacks
Next, we can use the poppler library, (with evince or a pdf2ps utility), to read the PDF file and use cairo to generate a PostScript file: bug.ps (138,067 bytes).
Notice that there has been another factor of 10 increase in the file size. Here, poppler has convinced cairo to generate a full-page fallback image rather than just the minimal fallback images present in the PDF file. This is due to the way poppler is handling the knockout group and really comes down to the difficulty of getting a single, desired result to pass through two systems with very different rendering models.
To explain a bit, (but ignoring many gory details), a PDF knockout
group can be a very complicated thing, so poppler has some fairly
sophisticated code to handle these. This support involves rendering
everything in the group twice and then using cairo's
compositing operators to properly combine them. Well, PostScript can't
do fancy compositing like
ADD, so of course cairo falls
back to image-based rendering for things. The irony here is that the
only reason cairo is using a knockout group in the original PDF file
is to prevent any compositing from happening, (the fallback image
needs to replace any "native" content that might appear below it). And
it turns out that painting an image without any compositing is the
only kind of image painting that PostScript knows how to do.
So, cairo is using an advanced feature of PDF to describe precisely the semantic that PostScript supports natively. The change we need is to fix poppler to recognize this case and ask for the simple thing from cairo's PostScript backend so that we don't get this full-page fallback explosion.
Bug 3: Cairo uses the wrong resolution for fallback images (in groups)
If it were only for those first two bugs, the intermediate file sizes would have been larger than normal, but the final result would have looked great and printed just fine. And in that case, I probably would have never even received a bug report.
But there's a third problem that is the most pernicious, because it results in the final result looking just awful. When cairo inserts the full-page fallback into the final PostScript file, it is inserting it at 300dpi, but it does that only after rendering it to an intermediate 72dpi image, which is then scaled up. That's why the final PostScript file appears so blurry and hard to read.
This third problem is the first I attempted to fix, (since it involves cairo alone), and I described my attempts in several posts to the cairo mailing list over the past couple of days, beginning here:
In that series of posts I coded a minimal test case in cairo's test suite for the resolution problem, and a patch that fixes that test case. But when I use a patched cairo for the PDF to PostScript conversion of the file described here, I end up with the following result: bug-patched-cairo.ps.
Here, there's still a giant, full-page fallback image, (this is expected since I haven't touched poppler yet). And the image is at least rendered at the correct resolution this time, (notice that the text that appears is much more sharp than in the previous PostScript file). However, the original HTML table is now entirely missing. So there's definitely something wrong with my patch.
I'll continue to chase these bugs down. The interesting thing about this chain is that it's only as strong as its weakest link. Fixing any of the individual problems here will make the end-to-end behavior be quite acceptable.
And I'll continue my quest to get high-quality display and print output from cairo-using applications. It can be a challenging goal, but it's also a lot of fun and very rewarding. Please feel free to jump in and help if you're interested.