A question about halftone screens

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davidh
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Joined: June 9th, 2009, 2:16 am

A question about halftone screens

Post by davidh »

There are three Spiral halftone screens, but none of them creates any spiralling effect. I wanted to look at them in the detail to see how they are built but have not found their files. I suppose they are somehow embedded. Are they?
I do not need a spiral effect I just try to understand the halftoning principles better.
jsachs
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Joined: January 22nd, 2009, 11:03 pm

Re: A question about halftone screens

Post by jsachs »

The spiral refers to the order in which the halftone cells are filled as the brightness of the underlying image gradually increases.

This is the built-in threshold table for the 10 x 10 spiral:

97, 89, 81, 69, 61, 62, 70, 82, 90, 98,
96, 77, 53, 45, 33, 34, 46, 54, 78, 91,
88, 60, 41, 25, 17, 18, 26, 42, 55, 83,
76, 52, 32, 13, 5, 6, 14, 27, 47, 71,
68, 40, 24, 12, 1, 2, 7, 19, 35, 63,
67, 39, 23, 11, 4, 3, 8, 20, 36, 64,
75, 51, 31, 16, 10, 9, 15, 28, 48, 72,
87, 59, 44, 30, 22, 21, 29, 43, 56, 84,
95, 80, 58, 50, 38, 37, 49, 57, 79, 92,
100, 94, 86, 74, 66, 65, 73, 85, 93, 99

It spirals out from the center, based on the distance from the center.
Jonathan Sachs
Digital Light & Color
davidh
Posts: 835
Joined: June 9th, 2009, 2:16 am

Re: A question about halftone screens

Post by davidh »

So the "spiral" in the name has got nothing to do with, or does not guarantee, any spiral effect in the output image. It is just a way dots are arranged based on a threshold table composed of numbers organized into a specific pattern.

How is it when a custom halftone screen image is loaded instead of in-built threshold table? Is the image internally converted into some kind of threshold table?

It might be interesting to see how the threshold table for the 10 x 10 spiral (or any other) would translate into a screen image, because if the texture image diameters are smaller then the input image, there are visible seams in the output image. However, the built-in halftoning methods based on the threshold table create seamless outputs.
jsachs
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Joined: January 22nd, 2009, 11:03 pm

Re: A question about halftone screens

Post by jsachs »

The image is the threshold table. The matrix in the earlier posting is normalized to run from 0 .. 255 when used as a threshold table.

The basic halftoning algorithm is extremely simple -- the threshold image is tiled to cover the entire input image. If an input pixel is darker than the threshold image, a black dot is output. Otherwise a white dot. The error diffusion algorithm is different.

If you are going to use the Texture transformation to generate custom threshold images (which is how the mezzotint textures were created), you need to run the texture output through the Equalize transformation to make all threshold values appear in roughly equal numbers.
Jonathan Sachs
Digital Light & Color
davidh
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Joined: June 9th, 2009, 2:16 am

Re: A question about halftone screens

Post by davidh »

..to use the Texture transformation to generate custom threshold images (which is how the mezzotint textures were created), you need to run the texture output through the Equalize transformation to make all threshold values appear in roughly equal numbers.

This is a very valuable advice, thanks. Even though Equalize effect seem to be hardly noticeable in the texture image, the final effect on the output image can be striking (depending on the texture, of course). I especially like how the output comes out a bit more contrasty revealing at the same time detail hidden behind dots. Really very interesting.
I think this tip deserves its place in the Help.
jsachs
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Joined: January 22nd, 2009, 11:03 pm

Re: A question about halftone screens

Post by jsachs »

It already mentions this in the help file.

If you are interested in the technical aspects of digital halftoning, there are several good books on the topic.

Digital Halftoning by Robert Ulichney
Modern Digital Halftoning by Daniel L. Lau, Gonzalo R. Arce

Unfortunately, technical books have gotten very expensive over the years.
Jonathan Sachs
Digital Light & Color
davidh
Posts: 835
Joined: June 9th, 2009, 2:16 am

Re: A question about halftone screens

Post by davidh »

Thanks for the tip. I will give it a try.

You are right about the Help. Equalize is mentioned under Halftone. I was looking in Texture at Tips and Other uses for Texture.
davidh
Posts: 835
Joined: June 9th, 2009, 2:16 am

Re: A question about halftone screens

Post by davidh »

Going back to the books to give some feedback. I only confirm that technical book can be very expensive. Even so, not being much of a mathematician I must admit that after reading a few reviews, something like Halftoning for Dummies, if anything like this existed, which I doubt, would be the right text for me to start with. :)

Fortunately PWP Texture transformation is a field intriguing enough for me to explore and stick to for some time ahead.
davidh
Posts: 835
Joined: June 9th, 2009, 2:16 am

Re: A question about halftone screens

Post by davidh »

One more question:
if the screen is larger then the input image, is it cropped or resized to the input image dimensions in pixels?
Thanks.
jsachs
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Joined: January 22nd, 2009, 11:03 pm

Re: A question about halftone screens

Post by jsachs »

Cropped
Jonathan Sachs
Digital Light & Color
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