[cairo] cairo+pixman profiling

[Nguyen Vu Hung]

2007/11/6, Jeff Muizelaar <jeff at infidigm.net>:
> On Tue, Nov 06, 2007 at 01:03:43PM +0900, Nguyen Vu Hung wrote:
> > 2007/11/6, Jeff Muizelaar <jeff at infidigm.net>:
> > > These are called when using the software rasterizer. The ideal solution
> > > to this problem is replacing the rasterizer. However, that is likely
> > > quite a bit of work. There has been discussion about possible
> > > implementations but no real conclusions were ever made.
> > >
> > You are talking about the problems of "software rasterizer".Do you
> > know any solutions for "hardware rasterizer"? Is that GPU makes
> > rasterization faster?
>
> cairo's software rasterizer is a bad software rasterizer. It is possible
> to do much better in software. This thread has a bunch of discussion
> about it:
> http://lists.cairographics.org/archives/cairo/2007-July/011092.html
Worth's post is worth reading:
http://lists.cairographics.org/archives/cairo/2007-July/011116.html
Summay: The best-known algorithm has complexity of O(Nlog N) and the
algorithm that cairo is using has the complexity of O(Nlog(N+k)). He
agreed that "many
things that could be improved here".

If what Worth said is right, i.e. O(Nlog N) is the best known algirthm
and cairo's complexity is O(Nlog(N+k)), then your "make perf" has
shown a - possibly - failed test data. I mean, it only holds in your
case, but not every cases.

Do you have a patch or code that prove what you were saying?



> > > > 2. Where unpremultiply_data is callled? What this function does?
> > >
> > > Probably in the png writing code. It does c*255/alpha on each component
> > > of a pixel.
> > Thank you. I don't think this bottleneck be be overcome because I need
> > to write PNG files ( with alpha channel ).
>
> It is possible to have a faster software implementation of
> unpremultiply. You can probably at least double the speed. The basic
> trick involves multiplying by the inverse instead of dividing. You can
> either computer the inverse at runtime or use a lookup table. I have
> some code for this, but I haven't touched it for a while so I'd need to
> look it over if I was to post it to the list.

Do you have a patch or reference code?

The "unpremultiply_data" function convert ARGB -> RGBA. It just
changes the position of alpha channel to the right ( after pixman's
format ). This takes up 4.23% running time of my application. I wonder
is there any room for improving this? ( see below code )

--
Each sample counts as 0.01 seconds.
  %   cumulative   self              self     total
 time   seconds   seconds    calls   s/call   s/call  name
 4.23    832.34    56.22                             unpremultiply_data

/* Unpremultiplies data and converts native endian ARGB => RGBA bytes */
static void
unpremultiply_data (png_structp png, png_row_infop row_info, png_bytep data)
{
    int i;

    for (i = 0; i < row_info->rowbytes; i += 4) {
        uint8_t *b = &data[i];
        uint32_t pixel;
        uint8_t  alpha;

	memcpy (&pixel, b, sizeof (uint32_t));
	alpha = (pixel & 0xff000000) >> 24;
        if (alpha == 0) {
	    b[0] = b[1] = b[2] = b[3] = 0;
	} else {
            b[0] = (((pixel & 0xff0000) >> 16) * 255 + alpha / 2) / alpha;
            b[1] = (((pixel & 0x00ff00) >>  8) * 255 + alpha / 2) / alpha;
            b[2] = (((pixel & 0x0000ff) >>  0) * 255 + alpha / 2) / alpha;
	    b[3] = alpha;
	}
    }
}

-- 
Best Regards,
Nguyen Hung Vu
vuhung16plus{remove}@gmail.dot.com
An inquisitive look at Harajuku
http://www.flickr.com/photos/vuhung/sets/72157600109218238/