[cairo] Discussion: LCD Filtering API

[Behdad Esfahbod]

Yeah, ARGB32 in the input doesn't make sense.  Now lets stop designing and
writing code over email. :)

behdad

Lance Hepler wrote:
> Hi Behdad,
> 
> Haha! Good thing. I've already started working on it. And that's about
> the way I've set it up.
> One question I have is whether I'll need to handle ARGB32 surfaces, as
> the current code simply handles 3x (width/height) CAIRO_FORMAT_A8
> surfaces. Should I just assume we'll only need it for the A8 case and
> assert so in the function? Expanding the code to filter ARGB32 surfaces
> would be ... tricky.
> 
> Lance
> 
> On Sat, Jan 31, 2009 at 10:35 PM, Behdad Esfahbod <behdad at behdad.org
> <mailto:behdad at behdad.org>> wrote:
> 
>     Hi Lance,
> 
>     The filtering should go into a file of its own for now.
>      "cairo-lcd-filter.c"
>     works for me.  The public API will look like what was committed and
>     reverted.
>      The main function in that file will take a cairo image surface and a
>     filtering mode and subpixel order, and returns a new surface that
>     has the
>     filtering results...
> 
>     The rest of the work will be done in cairo-scaled-font.c.  Note that
>     I need to
>     do some code reshuffling before this can be hooked up.
> 
>     Thanks,
>     behdad
> 
>     Lance Hepler wrote:
>     > Hi Owen,
>     >
>     > It seems that Behdad is not interested in disturbing the ability to do
>     > native-style rendering. He's interested in making it possible to do
>     > rasterization in cairo so that animation of text can be done well.
>     This
>     > is an excellent point.
>     >
>     > I don't see any issues in that at all. The question is, how/where do I
>     > want to put a new file/funtions that contains the lcd filtering code
>     > that I've ripped from freetype? And how do we want this API to look,
>     > even if we keep it private for cairo until we're happy with it?
>     >
>     > I'm perfectly fine ripping out the sections of freetype that do the
>     > filtering (I'm sorry D. Turner!), making another file (seems best,
>     since
>     > we want this filtering available for all backends, not just freetype)
>     > for doing lcd filtering in the cairo src directory, and hooking the
>     > calls in cairo-ft-font.c to use this code to filter instead of
>     Freetype.
>     > Looking over ftlcdfil.c it doesn't seem to be too difficult to adjust
>     > the code to work within a cairo-only context. As long as we keep
>     pulling
>     > in the preferences from fontconfig, there shouldn't be any
>     regressions,
>     > and I don't foresee tremendous difficulty in making this
>     adjustment. Of
>     > course, I haven't looked at the formats we're using to keep the other
>     > backends bitmapped glyphs, so maybe I'm just naive.
>     >
>     > Anyways, doing this now will give us the immediate benefit of having
>     > these improved filtering methods finally upstream, which would be of
>     > benefit to the unnamed distros that are currently using one version of
>     > the 10301 patch or another. That, and I'm sure users of those distros
>     > that are not using the 10301 patches will benefit from/be grateful for
>     > the improved font rendering. Really, I just want to go the route that
>     > will upstream these filtering abilities as soon as possible.
>     >
>     > I am really interested and anxious to see the putative improvements in
>     > font rendering Behdad has planned (subpixel positioning for
>     > unhinted/slightly-hinted fonts might be really nice! (they aren't
>     really
>     > affected by the lsb/rsb deltas)).
>     >
>     > Nicolaus (Lance)
>     >
>     > On Thu, Jan 29, 2009 at 11:48 AM, Owen Taylor <otaylor at redhat.com
>     <mailto:otaylor at redhat.com>
>     > <mailto:otaylor at redhat.com <mailto:otaylor at redhat.com>>> wrote:
>     >
>     >     On Thu, 2009-01-29 at 01:39 -0500, Behdad Esfahbod wrote:
>     >     > Hi Lance,
>     >     >
>     >     > I agree with most of your positions.  However, my long term plan
>     >     is to move
>     >     > glyph rasterization away from FreeType and into cairo, and that
>     >     includes
>     >     > subpixel rendering.  I think the API should wait till that
>     >     happens.  In that
>     >     > case Carl's remaining issues will be moot as we can do the same
>     >     filtering on
>     >     > all platforms and it will not be FreeType-specific anymore.
>     >
>     >     One thing to be concerned about is that matching the system
>     rendering.
>     >     Whether text looks good is not just a matter of whether it
>     looks good in
>     >     isolation, but whether it matches other text on the screen in
>     contrast
>     >     and other characteristics.
>     >
>     >     For example, if you take some text rendered on the Mac, and
>     drop it onto
>     >     a Windows desktop, the typical reaction will be that it is
>     fuzzy. Taking
>     >     text the other way may produce complaints about letter shapes
>     or color
>     >     fringing.
>     >
>     >     - Owen
>     >
>     >
>     >
> 
>