Poppler

Bug #80921
Comment #27

Comment 27 for bug 80921

Revision history for this message

In freedesktop.org Bugzilla #3307, Éric Piel (pieleric) wrote on 2008-10-12:

#27

Ok, I've looked at the bug a bit deeper. To me it seems there are two different problems:
* poppler_page_render()
This actually works a bit: if I pass a surface which has font options set to subpixel antialiasing, some fonts are correctly antialiased. For instance, I've modified poppler-glib-demo, and either forcing the font options of an cairo_image surface or asking to render directly on the xlib surface leads to some font being antialiased with subpixel. (I'll come back to this later on.)

* poppler_page_render_selection_to_pixbuf()
This is the place which comment #2 "fixed". A cairo image surface has default font options set to simple antialiasing. That's perfectly fine, after all no one knows what is going to be done with this pixbuf. The problem is that some applications uses this function and later display the pixbuf on the screen. So I see three possible solutions, depending on what is the purpose of the function (I don't know enough poppler to decide):
- it's supposed to be a generic function, for rendering anywhere approximately correctly and easily used: then don't modify the function, update the API description to let know the developers that the antialiasing of the fonts is fixed to grey levels.
- it's supposed to be a generic function, but can be complexified: add a second function which has another parameter for font_options. Therefore the developers can decide which antialiasing to use (for instance by copying the values of X).
- it's supposed to be specific to glib on a screen: Add inside the function a read to the default values in glib and setup the font options accordingly.

Problem with some fonts:
I've noted that in the PDFs some fonts, do get correctly displayed following the subpixel antialiasing, while some don't. At first look, it seems that type1 embedded are correctly displayed, but TrueType embedded are not (always grey). This is a major problem.

Also, I've noticed that in poppler/CairoFontEngine.cc, the load of fonts is forced with FT_LOAD_NO_HINTING, I wonder if it's a good idea. There is no comment explaining why this is so...

* poppler_page_render_selection_to_pixbuf()
This is the place which comment #2 "fixed". A cairo image surface has default font options set to simple antialiasing. That's perfectly fine, after all no one knows what is going to be done with this pixbuf. The problem is that some applications uses this function and later display the pixbuf on the screen. So I see three possible solutions, depending on what is the purpose of the function (I don't know enough poppler to decide):
 - it's supposed to be a generic function, for rendering anywhere approximately correctly and easily used: then don't modify the function, update the API description to let know the developers that the antialiasing of the fonts is fixed to grey levels.
 - it's supposed to be a generic function, but can be complexified: add a second function which has another parameter for font_options. Therefore the developers can decide which antialiasing to use (for instance by copying the  values of X).
 - it's supposed to be specific to glib on a screen: Add inside the function a read to the default values in glib and setup the font options accordingly.

Also, I've noticed that in poppler/CairoFontEngine.cc, the load of fonts is forced with FT_LOAD_NO_HINTING, I wonder if it's a good idea. There is no comment explaining why this is so...