Show HN: Trigrad, a novel image compression with interesting results

Re:Show HN: Trigrad, a novel image compression with interest... by JosephRedfern on Saturday July 04, 2015 @06:17AM (#9829785)

How are the sample points selected? I get that they're weighted according to edge intensity, but what kind of distribution are you using in cases where there is no edge?

EDIT: I've read the code - it seems to be using random sampling. Still not entirely sure how a point can be placed at a place with absolutely no Sobel response - maybe it can't, which would make sense. My question arose after looking at: https://i.imgur.com/9YHOtQ0.png and then https://i.imgur.com/XRF7mz4.png. It looks like samples have been placed in regions with no response, but perhaps my eyes just can't see the edges.

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Re:Show HN: Trigrad, a novel image compression with interest... by ruarai on Saturday July 04, 2015 @06:38AM (#9829809)

This is an area that needs work, but basically there's the table of 'edge intensity' that gets multiplied by a constant baseChance variable for every pixel.

baseChance = 8 * samples / (width * height)

samples is the desired number of samples.

Again, this needs work. I'm pretty sure there's a way I can more accurately match the number of output samples to the desired number of samples.

Edit: And no, it can't produce a response if there's no edge (a value of zero). However, there's always some level of noise. This is true of the example you've shown - there's very light noise visible.

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Re:Show HN: Trigrad, a novel image compression with interest... by JosephRedfern on Saturday July 04, 2015 @07:17AM (#9829853)

I see - thanks for the explanation!

Have you tried using Canny Edge detection rather than Sobel? It might help a bit with the noise. I've just attempted it, but can't seem to get Canny working under Mono (I've not got a windows machine at the moment).

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