[prev in list] [next in list] [prev in thread] [next in thread] 

List:       xfree-render
Subject:    Re: [Render] [Fwd: X11 acceleration issues]
From:       Keith Packard <keithp () keithp ! com>
Date:       2000-08-28 20:25:51
[Download RAW message or body]


(please forward back to the SDL list)

    Can anyone with XFree86 4.x confirm this, is XCopyArea with a clip_mask
    accelerated?

"accelerated", yes, using the hardware for the clip-mask, no.  XF 4.0
still converts the clipmask into a list of rectangles.

    However it seems that he uses premultiplied alpha exclusively, which
    neither SDL nor OpenGL does. Can common video hardware use premultiplied
    alpha?

Actually, GL is quite happy with pre-mul alpha, many operations turn out 
to prefer it if you look more closely.  Most common video hardware 
accelerate only premul alpha; the essential difference is that alpha is 
computed the same way as the other components when generating the 
destination alpha values.

Jim Blinn makes a convincing argument that premultiplied alpha is the 
canonical representation, more natural for computation~\cite{blinn:1994}.

However, if you have images without premultiplied alpha, or with separate 
alpha, you can easily manipulate them with the extension as well, just use
the same pixmap with two separate pictures, one excluding the alpha, one 
providing only alpha and then stick them as the 'src' and 'mask' arguments
to the general operator:

	dst = (src IN mask) OP dst;

ref:
@article{blinn:1994,
 title          = "Compositing Theory",
 author         = "Jim Blinn",
 journal        = "IEEE Computer Graphics and Applications",
 year           = 1994,
 month          = "September",
 note           = "Republished in~\cite{blinn:1998}"    }
 
@book{blinn:1998,
 title          = "{Jim Blinn's Corner: Dirty Pixels}",
 author         = "Jim Blinn",
 year           = 1998,
 publisher      = "Morgan Kaufmann",
 isbn           = "1-55860-455-3",      }


_______________________________________________
Render mailing list
Render@XFree86.Org
http://XFree86.Org/mailman/listinfo/render

[prev in list] [next in list] [prev in thread] [next in thread]