[cairo] win32 bug when using HFONT with appropriate size

[Adrian Johnson]

On 05/04/16 20:33, Fred Beca wrote:
> 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.

There were a couple of threads were the usefulness of
WIN32_FONT_LOGICAL_SCALE was discussed:

https://lists.freedesktop.org/archives/cairo/2007-January/009288.html
https://lists.freedesktop.org/archives/cairo/2007-February/009661.html

I don't think removing the logical scale is a good idea. We will need to
find a solution to make cairo_win32_font_face_create_for_hfont() work.

I suggest creating a bug and linking it to the 1.16 tracker (bug 84197).
I'll look at it next time I am working on the windows backend.

> 
> 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 {
> 
> 
>