The gpu can only handle a certain amount of instructions in one go.
More than that, and you'll have performance problems at best, and at
worst you'll hit driver bugs.
Loops in core image are compiler candy, they are entirely unrolled
before upload to the gpu.
Sent from my iPhone
On Aug 8, 2009, at 6:19 PM, James Walker <[email protected]> wrote:
I haven't been able to find much on performance or limitations of
Core Image kernels beyond the statement that you can't use variable
loop limits. I thought I could just use fixed limits bigger than
needed, like so:
kernel vec4 blur(sampler image, float radius)
{
vec2 basePos = destCoord();
int i, j;
vec4 sum = vec4(0.0);
float radSq = radius * radius;
vec2 delta;
for (i = -10; i <= 10; ++i)
{
delta.x = float(i);
for (j = -10; j <= 10; ++j)
{
delta.y = float(j);
sum += ((dot(delta, delta) < radSq)?
sample( image, basePos + delta ) :
vec4(0));
}
}
sum.rgb /= sum.a;
sum.a = 1.0;
return sum;
}
As is, this works, albeit more slowly than I would like. But what I
don't understand is that if I change those loop limits from 10s to
20s, it doesn't just take 4 times longer, weird stuff happens. Like
the whole screen flickers, and Quartz Composer hangs, and then the
whole machine hangs requiring a forced restart. What's up with that?
By the way, someone will doubtless notice that I am essentially
doing a disk blur here, and point out that Apple ships a disk blur
filter. What I ultimately want to do is more complicated, this is
just a learning exercise along the way.
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