[cairo-bugs] [Bug 25893] New: Mixed clipping using printing surfaces to windows metafiles

bugzilla-daemon at freedesktop.org bugzilla-daemon at freedesktop.org
Mon Jan 4 09:54:10 PST 2010


           Summary: Mixed clipping using printing surfaces to windows
           Product: cairo
           Version: 1.8.8
          Platform: x86 (IA32)
        OS/Version: Windows (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: win32 backend
        AssignedTo: cairo-bugs at cairographics.org
        ReportedBy: teknos at gmail.com
         QAContact: cairo-bugs at cairographics.org

Printing surfaces query the initial clip box of the device context upon
creation. This is used as the extent of the surface later on.
This mechanism causes problems, when the device context is a metafile DC. Some
drawing operations are automatically clipped to the surface extent (I have seen
font output or image fallback doing so), but some other drawing operations are
not (such as lines, for example). This is inconsistent and should not happen on
my view. I think clipping (even if based on extents) should apply equally to
all operations.

How to reproduce
HDC hdc = CreateEnhMetaFile(NULL, "test.emf", NULL, NULL);
surface = cairo_win32_printing_surface_create(hdc);
cr = cairo_create(surface);
cairo_move_to(cr, 0, 0);
cairo_line_to(cr, 10000, 10000); //bigger than screen
cairo_move_to(cr, 10000, 10000);
cairo_show_text(cr, "This will not be visible using a truetype font");
cairo_destroy (cr);
cairo_surface_destroy (surface);

My opinion
The problem is difficult to circumvent. Since most windows metafiles are
created using a copy of the screen device context as reference context, the
initial clip box is essentially the max size of the display device. It would be
nice to support metafiles created this way. I could not change this initial
clipping box from the device context before creating the cairo surface, so the
freshly created surface will always have the extent of the display device. An
alternative would be to use a different device context as reference DC when
creating the metafile - but I think obtaining a printing DC is complicated and
not so ubiquitous as not all systems have a printer installed.

I think cairo should somehow check if the device context is a windows metafile
and treat such printing surfaces as unbounded, if possible. If not then maybe
asking for explicit extent could be an alternative.

I think this mechanism is still there in 1.9.4.

An additional comment/very related feature request. All the above is relevant
for Enhanced Metafiles (EMF). Cairo does not seem work with DCs created from
old-format windows metafiles (WMF). I think this is partially to the fact that
those do not have an initial clipping region and when cairo queries the initial
clip in _cairo_win32_save_initial_clip(), the call fails and no surface is
created. There may be other GDI calls that do not work for old-format
metafiles, but if not then maybe this limitation should be lifted, so that
cairo can draw directly to old-format metafiles. (EMF to WMF conversion does
not work that fine, so having a direct WMF output is better. WMF output is
needed for OLE, since OLE unfortunately does not support EMF.)

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