[cairo] Re: [Mesa3d-dev] Points in Cairo Glitz paper

Jon Smirl jonsmirl at yahoo.com
Wed Apr 21 16:10:51 PDT 2004

Can multi-texture do this? I believe it is described more in the original Cairo

If not this is something that should be discussed at the conference.

--- Brian Paul <brian.paul at tungstengraphics.com> wrote:
> Jon Smirl wrote:
> > I'm reading the Cairo Glitz paper and somethings caught my eye. Is there a
> > better way to do this in OpenGL that doesn't involve intermediate buffers?
> Or
> > could this be fixed by changing the compisiting process?
> > 
> > Here's a description of the image compositing process.....
> > 
> > To composite one surface onto another with OpenGL, texturing of a
> rectangular
> > polygon is used. This means that the source surfaces must be available as
> > textures. The default method for making a surface available as a texture is
> to
> > have the graphics hardware copy the pixel data from a drawable into a
> texture
> > image. As this is a relatively slow operation, glitz does its best to
> minimize
> > the area and number of occasions for this copy operation. On some hardware,
> a
> > feature called render-texture is available that allows glitz to completely
> > eliminate the need for this copy operation and instead use a pbuffer
> directly as
> > a texture. 
> > 
> > The optional mask surface that can be provided to the general composite
> > operation creates some additional complications. The source surfaces must
> first
> > be composited onto the mask using the Porter-Duff in-operator and the result
> > must then be composited onto the destination. The default method for
> handling
> > this is to create an intermediate off-screen surface, which can be used for
> > compositing using the in-operator. This surface can then be composited onto
> the
> > destination with an arbitrary operator for the correct result. 
>  >
> > The best way to do this would be to perform compositing with a mask surface
> > directly without the creation of an intermediate surface. Even though the
> fixed
> > OpenGL pipeline does not seem to allow for such an operation, glitz is able
> to
> > do this on hardware that supports fragment programs. Fragment programs allow
> for
> > fragment level programmability in OpenGL, and in combination with
> > multi-texturing, glitz can perform composite operations with a mask surface
> very efficiently.
> I'd bet that multi-texture alone could be used to avoid computing the 
> intermediate result.  I don't think a fragment program is needed.
> -Brian
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Jon Smirl
jonsmirl at yahoo.com

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