[Okular-devel] [Bug 276199] New: subpixel rendering support for DjVu

[Jaan Vajakas]

https://bugs.kde.org/show_bug.cgi?id=276199

           Summary: subpixel rendering support for DjVu
           Product: okular
           Version: 0.12
          Platform: openSUSE RPMs
        OS/Version: Linux
            Status: UNCONFIRMED
          Severity: wishlist
          Priority: NOR
         Component: DjVu backend
        AssignedTo: okular-devel at kde.org
        ReportedBy: jaanvajakas at hot.ee


Version:           0.12 (using KDE 4.6.0) 
OS:                Linux

Currently djvu images are quite blurry when viewed at a small zoom level
(especially if the zoom level is not a unit fraction). It would be nice if
okular supported subpixel rendering like WinDjView-subpix does
(http://code.google.com/p/windjview-subpix/).

Looking at the code of WinDjView-subpix, it seems that there the problem is
solved by requesting an image at 100% resolution from libdjvu and then scaling
it (subpixel-consciously) to the appropriate size. So the fastest and simplest
approach to get it done would be to follow the same approach, copy-pasting code
from WinDjView-subpix (however, architecturally, there might be better choices,
so that one could avoid copying code between projects: on one hand, this
functionality could be included in libdjview, since this would be useful in
every DjVu viewing application; on the other hand, subpixel-conscious scaling
would probably also useful in some other raster formats that okular supports).

And then there should be some user interface option to enable/disable subpixel
rendering as well.


Reproducible: Always

Steps to Reproduce:
Open a DjVu document. Set zoom level to some level that is less than 100%.


Actual Results:  
The image is blurrier than it could be because the smoothing algorithm does not
care about subpixels. The blurriness is especially apparent if the zoom level a
non-unit fraction (i. e. a number <100% that is not 100%/n where n is an
integer), e. g. 18%.


Expected Results:  
DjView should use subpixel rendering (like WinDjView-subpix) to increase
sharpness.

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