Hi, I recently had the clever idea to accelerate drawing of text in my
app by caching the output of the fl_draw in an image.  I used
Fl_Image_Surface, and it almost works... except the output has no
alpha channel.  Since the text is always black, I made a copy of the
image and inferred an alpha with `0xff - avg(r, g, b)` but the results
are noticeably thinner than the native antialiased fl_draw output.
The black is still entirely opaque as it should be, but the
transparent parts are more transparent.

Is the alpha channel non-linear in some way?  Or is `1 - avg(r, g, b)`
too simplistic?

And of course, is there some better way than manually copying the
image and inferring an alpha channel?  When the OS drew the text in
the first place, surely it was using alpha... is there any way to get
my hands on that?  I notice it uses fl_read_image(), which does have
an alpha argument... but it's just a hardcoded value.  Tracking down
the OS X implementation, I wind up at a CGContextRef, and it looks
like these may or may not have alpha info.  In fact,
Fl_Cocoa_Screen_Driver.cxx looks like it does actually use a depth=4,
but fl_read_image is confusing.  It looks like if its alpha arg is
non-zero and read_win_rectangle returns an image with alpha, then it
leaves the alpha intact, but copies it into another Fl_RGB_Image, so
we can get the array out, so we can make it into another Fl_RGB_Image,
in Fl_Quartz_Image_Surface_Driver::image... which always passes
alpha=0.

Regardless of the alpha situation, wouldn't it be faster and simpler
for fl_read_image to directly return the Fl_RGB_Image from the
relevant screen driver, instead of making multiple copies and fiddling
with the alpha in complicated ways?  I assume this arose due to
history, but maybe we could have a direct path and leave fl_read_image
for backward compatibility.

Alternately, I'm sure caching text drawing is a common thing to do...
has anyone done something like this, and can share how they did it?

I'm using fltk-1.4 on OS X.

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