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position of highest alpha pixel
Hi, I have to a new project that would require the user to import a graphic ( from a 3rd party ) and I would like to know if it is possible to have the position of a text layer appear 50 pixels ( for example ) above the top of this graphic. It could be a png, ai, etc.
I would probably auto resize it with an expression I have used before.
Is there a way to use sampleimage to look for the highest ( in regards to position ) alpha pixel ?
replied 3 years, 1 month ago 3 Members · 10 Replies
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10 Replies
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Hi Adam,
there was a thread about this a few years ago:
https://creativecow.net/forums/reply/2435715/#.ZC0d9tachSs.link
it samples lines/columns from all four sides until it hits one that contains alpha information.
It can be made faster with a binary search, but for precision while sampling and averaging out, you would again want the project to be at least 16bpc
Reply To: Is there a way to know the width and height of a layer similar to sourceRectAtTime?
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Here it is with a faster binary (powers of 2) approach, only searching the top edge, up to pixel accuracy:
posterizeTime(0);
const l = thisComp.layer("Logo");
const w = l.width;
const h = l.height;
const heightLog2 = Math.ceil(Math.log(h)/Math.log(2));
const threshold = 1; // on average 1 pixel in a row is opaque to count as the topEdge;
var topEdge = 0;
for(i=heightLog2; i>-1; i--) {
let partition = 2 ** i;
let s = l.sampleImage([w / 2, topEdge+partition/2], [w / 2, partition/2], false, time);
if (s[3]<=threshold/(w*partition)) topEdge+=partition;
}
l.toComp([w/2, topEdge - 50]); -
Amazing Filip, thanks so much.
is there a way to have the x value constant ? I tried to replace the values with a number but it didnt’t work. I only need the height position to change.
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the lastline could be something like:
[960, l.toComp([w/2, topEdge])[1] -50 ]; -
my error, I think it was an anchor point issue on my end,
thanks again Filip, this is a great expression
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oh yes this works, I was entering that value in the wrong place.
Thanks Filip !!!! ( for the 3rd time )
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Filip, just for the math, could we do the same to find the bottom pixel and place something underneath ?
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I have tried this but Im not sure it’s the most accurate way;
[960, (h-l.toComp([w/2, topEdge])[1]+50) ];
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ok this finds the bottom in case anyone needs it; ( thanks Filip )
posterizeTime(0);
const l = thisComp.layer(“TITRE_SHOW”);
const w = l.width;
const h = l.height;
const heightLog2 = Math.ceil(Math.log(h)/Math.log(2));
const threshold = 1; // on average 1 pixel in a row is opaque to count as the bottomEdge;
var bottomEdge = h;
for(i=heightLog2; i>-1; i–) { let partition = 2 ** i; let s = l.sampleImage([w / 2, bottomEdge-partition/2], [w / 2, partition/2], false, time); if (s[3]<=threshold/(w*partition)) bottomEdge-=partition; }
[960, l.toComp([w/2, bottomEdge])[1]+50];
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Thank you, Filip!
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