Re: [ft] Freetype performance question?

[David Turner]

Michael Sander a écrit :
> Most users of freetype on a X system first use the freetype2 engine to 
> render the font to some in-memory bitmap and then use XPutImage to 
> render the image to the screen.
I wouldn't say "most users", since the typical scheme used to display 
anti-aliased text doesn't need
composition of in-memory bitmaps and XPutImage. Instead, it is the 
following:

- the client generates (monochrome or anti-aliased) glyph images for the 
characters it needs. it
 supposedly does so only *once*  per character / size / transform, and 
lazily (i.e., on-demand)

- the client sends these glyph bitmap images to the X Server through the 
XRender extension.
 The X Server globally caches them.

- when the client needs to draw some text, it simply sends the positions 
and glyph 'indices' needed
 to the X Server, the latter retrieves the bitmaps from its global 
cache, and performs the composition
 on the target drawable.

This is much more network efficient than having to send the glyph 
bitmaps each time, and the
composition can be trivially hardware-accellerated by the X Server.

> Does anyone have any performance benchmarks compared to simple X 
> rendering (i.e. XDrawString). I am especially concerned over slow X 
> connections. If one is rendering a full screen of text using freetype 
> on a slow X connection they have to effectively transfer a full screen 
> bitmap image.
To get back to your point, if you use some sort of glyph bitmap cache, 
your performance will probably
be dominated by the composition code that you'll be using to draw the 
glyphs into your in-memory
bitmap, as well as the network transfer of the result. The performance 
of FreeType will be totally
irrelevant.

It will be much slower than a simple XDrawString, because this one can 
be hardware accellerated by
the X Server. However, note that XDrawString is incapable of performing 
anti-aliased text rendering.

Hope this helps,

- David Turner
- The FreeType Project  (www.freetype.org)


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