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by NicolasRobidoux
2010-11-16T09:18:50-07:00
Forum: Developers
Topic: image size convention in resize.c
Replies: 37
Views: 78577

Re: image size convention in resize.c

Quick note: Some people recommend using unsharp mask with a very tight radius as the very last step of a chain of transformations applied to a resized (or warped) image. IMHO, a method which is a bit blurrier but very moderately aliased, like distort Lanczos, is a good candidate for this approach to...
by NicolasRobidoux
2010-11-16T07:04:54-07:00
Forum: Developers
Topic: image size convention in resize.c
Replies: 37
Views: 78577

Re: image size convention in resize.c

There is something somewhat miraculous with Jinc-Jinc 3-lobe Clamped EWA which does not happen, for example, with the 2-lobe version. Assume bounded data (-1 to 1, or 0-255 or whatever). Let "no-op" ("no operation") mean: filter only at the original pixel locations (that is: don't enlarge, don't shr...
by NicolasRobidoux
2010-11-15T14:06:04-07:00
Forum: Developers
Topic: Resizing artifacts with Lanczos
Replies: 23
Views: 62311

Re: Resizing artifacts with Lanczos

I hadn't given the "diagonal noise" issue much more thought than thinking it might be because there's more energy left to see in the higher diagonal frequencies after downsampling a 'noisy' image. (To quote a recent Disney movie) It's not noise, it's artifacts (assuming we are talking about the sam...
by NicolasRobidoux
2010-11-15T11:21:00-07:00
Forum: Developers
Topic: image size convention in resize.c
Replies: 37
Views: 78577

Re: image size convention in resize.c

Here are the results with http://web.cs.laurentian.ca/nrobidoux/misc/IM/rose_lanczos.png Lanczos resize (tensor Sinc-Sinc 3-lobe) http://web.cs.laurentian.ca/nrobidoux/misc/IM/rose_LanczosSharp.png LanczosSharp distort (Clamped EWA Jinc-Jinc 3-lobe with blur=0.9812505644269356) IMHO, the results sug...
by NicolasRobidoux
2010-11-15T09:47:04-07:00
Forum: Developers
Topic: Resizing artifacts with Lanczos
Replies: 23
Views: 62311

Re: Resizing artifacts with Lanczos

One last ;-) comment: Filters without negative lobes ("monotone" methods) will not be affected as much by color profile shenanigans.
by NicolasRobidoux
2010-11-15T09:34:01-07:00
Forum: Developers
Topic: Resizing artifacts with Lanczos
Replies: 23
Views: 62311

Re: Resizing artifacts with Lanczos

Maybe you'll find the following interesting. (Don Munsil is a video processing expert of some notoriety.) Some Don Munsil quotes from forum discussions with other experts: From http://www.freelists.org/post/opendtv/Math-of-oversampling,12 : > The electronic sinc-like low-pass filter can have a sharp...
by NicolasRobidoux
2010-11-15T09:27:22-07:00
Forum: Developers
Topic: Resizing artifacts with Lanczos
Replies: 23
Views: 62311

Re: Resizing artifacts with Lanczos

Anyway my idea was to try filtering with a proper 2D radial kernel slightly larger (as in higher frequency) than normal to avoid wasting the corners of the frequency response while allowing some aliasing energy to spill into the result, or some variation on that theme. 'Sounds an awful lot to me li...
by NicolasRobidoux
2010-11-14T23:16:34-07:00
Forum: Developers
Topic: image size convention in resize.c
Replies: 37
Views: 78577

Re: image size convention in resize.c

I just realized that there is not strict need for a flag that allows automatic switching, since this can be emulated by doing resizes one direction at a time using each convention separately. ( Oops! This only works when using orthogonal resizing, and then only if one controls carefully the intermed...
by NicolasRobidoux
2010-11-14T22:03:05-07:00
Forum: Developers
Topic: Resizing artifacts with Lanczos
Replies: 23
Views: 62311

Re: Resizing artifacts with Lanczos

anthony wrote: Apologies to Nicholas if I implied that it did.
Thanks for the heads up (I get easily confused, so no apologies needed).
by NicolasRobidoux
2010-11-14T19:12:15-07:00
Forum: Developers
Topic: Resizing artifacts with Lanczos
Replies: 23
Views: 62311

Re: Resizing artifacts with Lanczos

IMHO, "diagonal noise" is something which comes from using filters with negative lobes in a tensor (orthogonal) way: Basically, if l(x) is a function which alternates between positive and negative, l(x)l(y) will give you a checkerboard. With a (say) vertical line, you don't get the checkerboard, bec...
by NicolasRobidoux
2010-11-14T18:44:38-07:00
Forum: Developers
Topic: Resizing artifacts with Lanczos
Replies: 23
Views: 62311

Re: Resizing artifacts with Lanczos

Another problem I have had with IM Lanczos is the square fiter kernel: It gives a kind of "diagonal noise" look to textures with a lot of sharp content (eg leaves from a distance). If I get time some day I'll experiment with round kernels of varying sizes. Please try the brand new distort Lanczos a...
by NicolasRobidoux
2010-11-14T18:08:08-07:00
Forum: Users
Topic: Best way to upscale pixel art exactly 200%
Replies: 27
Views: 73122

Re: Best way to upscale pixel art exactly 200%

I also agree with giving a try to adaptive resize.
by NicolasRobidoux
2010-11-14T17:28:44-07:00
Forum: Users
Topic: Best way to upscale pixel art exactly 200%
Replies: 27
Views: 73122

Re: Best way to upscale pixel art exactly 200%

The suggestion of separating the alpha channel from the image and using a different filter on it is also a good one. The 'triangle, cubic, hermite or gaussian" suggestion above was made specifically because of what these filters do with sharp transitions (they are so-called "monotone" methods: no "n...
by NicolasRobidoux
2010-11-14T16:36:10-07:00
Forum: Users
Topic: Best way to upscale pixel art exactly 200%
Replies: 27
Views: 73122

Re: Best way to upscale pixel art exactly 200%

Sorry for the terse answer but here is anyway (more a tease than anything, and Anthony knows way more about all this than I): Given that it's 200% (not a very large ratio) and that, if I understood the description of your images, you have sharp transparency transitions, I would first try triangle (=...
by NicolasRobidoux
2010-11-14T11:17:36-07:00
Forum: Developers
Topic: Second even bigger improvement for Distorts [CLOSED]
Replies: 6
Views: 18540

Re: Second even bigger improvement for Distorts

I just changed the Lanczos2Sharp parameter value slightly in svn. When doing 'no-op" on a vertical stripes images, you immediately see that I got it just right this time. (It's about a 1% change in the blur value.)

I'm now done tweaking the blur values with the better optimization technique.