[cairo] win32 bug when using HFONT with appropriate size

[Fred Beca]

I have noticed a bug in the win32 font backend. When setting the font on a
win32 surface with the code below and using the same size as the original
HFONT for drawing, the resulting font size is extremely small (text is
almost invisible).

// assuming my_hfont was created with -12 for LOGFONT.lfHeight
cairo_font_face_t*
font_face=cairo_win32_font_face_create_for_hfont(my_hfont);
cairo_set_font_face(ctx,font_face);
cairo_set_font_size(ctx,12);

The reason is that in _cairo_win32_font_face_scaled_font_create, the font
matrix is checked against -font_face->logfont.lfHeight whereas if you look
in _win32_scaled_font_get_scaled_hfont, the expected height should be
multiplied by WIN32_FONT_LOGICAL_SCALE (scaled HFONT created by cairo uses
logfont.lfHeight = -scaled_font->logical_size)

This means that using anything else than 1 for WIN32_FONT_LOGICAL_SCALE as
shown in the patch below will result in a downscaled font when using a
HFONT that was created with the right size.

However, if the check mentioned above is fixed, in order to have cairo use
the original HFONT, it should be created with a much larger size
(-WIN32_FONT_LOGICAL_SCALE*size). So this defeats the purpose of being able
to mix native win32 fonts with cairo because of this scaling factor (mixing
native and cairo code would require extra scaling everywhere).

So here is my question: is the scaling applied with
WIN32_FONT_LOGICAL_SCALE still useful? From what I have been able to
experience so far, setting it to 1 makes no difference, even with very
small fonts. And it seems this is actually used by many others.

Index: cairo-win32-private.h
===================================================================
--- cairo-win32-private.h (cairo-1.14.6)
+++ cairo-win32-private.h (working copy)
@@ -51,7 +51,7 @@
 #define SB_NONE 0
 #endif

-#define WIN32_FONT_LOGICAL_SCALE 32
+#define ç 1

 /* Surface DC flag values */
 enum {
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