Questions and postings pertaining to the usage of ImageMagick regardless of the interface. This includes the command-line utilities, as well as the C and C++ APIs. Usage questions are like "How do I use ImageMagick to create drop shadows?".
Bingo! For downsampling, Jinc-windowed Sinc 3-lobes sucks. The really really close (on paper) standard Lanczos does not suffer from the tone drift from hell. P.S. No, that's not it. They all suffer the same way, and what's happening is that moire is changing our perception of tone. Where there are moire rings, we perceive things as being darker.
-filter Cosine (Cosine-windowed Sinc) -resize is just the same as Ginseng is just the same as Lanczos, basically.
In the case of this fly test, the window function that does the best out of Jinc, Sinc and Cosine, is Cosine, by a minute amount (Welch too), strictly in terms of moire reduction.
That is, Cosine-windowed Sinc 3-lobe comes out on top for downsampling.
I'm going to have to have a second look at Cosine-windowed Sinc 3-lobe for upsampling, because in quantitative tests soon to be published by my student Adam Turcotte it's arguably the best 3-lobe method.
Last edited by NicolasRobidoux on 2012-09-20T17:19:13-07:00, edited 2 times in total.
OK: Looking at the backpack results, it's easy to see what's going on:
Ginseng has noticeably less second halo than Cosine and Welch, and about the same as Lanczos. So, it's a trade-off between a minute amount of additional antialiasing and a significant increase in the second halo.
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I think I'll stick to Ginseng.
What is a Cub-grange filter? What makes it special? And why this name? Is it related to Lagrange in some way?
This is something Henry HO put together. If I understand, when the image is "natural", it mimicks the EWA Lagrange when downsampling. My guess is that it is
@Anthony: Our eyes have different resolution with colour and intensity, so in principle it makes sense to do something different with each.
Whether this means applying the filter to linear intensity but not linear hue or whatever, or applying a different filter to each component, I don't know.
NicolasRobidoux wrote:@Anthony:
Ginseng on the fly through LAB is a disaster: The eyes turn to rust.
Perhaps without a perceptual Lightness. but a linear intensity?
Make need a bit of mix and matching of color channels from 'Gray' and 'Lab' L channel to get this right.
I am also pretty close to bringing linear light -resize Mitchell near the top of the list when downsampling, with linear light EWA Robidoux close behind if one wants a bit more sharpness.
The moire pattern of the Fly is my second concern.
My first concern is the change in overall brightness (i.e., de-focus my eyes and watch the compound eyes). Yet, unless we have a well-tuned monitor, it is impossible to detect judge whether there is any real brightness change.
@Henry:
This is why I may, at some point, use image difference metrics applied to the result of blurring the image slightly (like CMC 1:1).
But now, it's clear that moire is a problem, whether or not it affects the perceptual brightness.
