As another step on my road to generating regular polygons in a
CoreImage kernel, I hunted up some code on the net to compute the
distance from a point to a line, and I now have the following kernel:
kernel vec4 drawLine(sampler image, float thickness, float Ax, float
Ay, float Bx, float By)
{
vec2 where = samplerCoord(image);
vec4 result = sample(image,where);
vec2 A, B, P;
// set up the endpoints
A.x = Ax;
A.y = Ay;
B.x =Bx;
B.y =By;
A *= samplerSize(image);
B *= samplerSize(image);
// Compute where the minimum is between A and B
float q = ((where.x-A.x)*(B.x-A.x) + (where.y-A.y)*(B.y-A.y)) /
(pow((B.x-A.x),2.0) + pow((B.y-A.y),2.0));
// Clamp the result between zero and one
q = q < 0.0 ? 0.0 : q;
q = q > 1.0 ? 1.0 : q;
// Compute the coordinates of the nearest point on the line segment
P.x = (1.0 - q)*A.x + q*B.x;
P.y = (1.0 -q)*A.y + q*B.y;
// Set Alpha to zero if we're outside the desired distance.
result.a *= distance(P,where) < thickness ? 1.0 : 0.0;
return result;
}
which generates line segments with nice, rounded end-caps:
I'm finding that my math skills were far rustier than I realized, so
if anyone would care to take a pass at optimizing this code, I'm all
ears. ;-) Also, I'd like to anti-alias the result. If anyone could
offer a suggestion of how to do so, I'd certainly appreciate it.
Here's the composition:
Attachment:
Circle kernel.qtz Description: application/quartzcomposer
-jcr
John C. Randolph <email@hidden> (408) 914-0013
Roaming Cocoa Engineer,
Available for your projects at great Expense and Inconvenience.
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