Good morning everybody. I'm sorry I didn't stop by lately, I just wasn't up to interacting with people. I asked my wife to update everyone so I guess I don't need to. I didn't want to get back in the world but my wife, with her usual wisdom insisted that I get back into it. I still feel like I never want to laugh or joke again, like something inside me has been ripped out.

However, we don't come here to feel dismal, we come here to have fun and enjoy each others company. So on that note I reenter the Gimp freeway with a problem.
That problem is with gmic, specifically, what is on the preview screen in gmic isn't what the final result looks like. I've included two examples. The top image is what shows in the preview (I had to do a screen capture), the second is what happens when I apply it. This was done with the Psychedelic Glasswork Tiler but it does this with most of the filters. It's done this with every version of gimp including the latest one (1.5.6.0) The Psychedelic Glasswork Tiler is one of my favorite filters if only I could get it to work right.

Welcome back, maybe I can help a little with the gmic thing. The preview quite often doesn't match the final because gmic has no control over it (not just for this filter), gimp decides what gmic gets to see and that's basically a very small version of the whole picture.

So, what you might be able to do is resize your original image (maybe a bit smaller?) to get a similar result... I know that's not ideal but I'm struggling to think of another way around it! The settings do let you increase the complexity but of course it still won't match what you see in the preview. Maybe PC can help if he's around - it lists him as the author.

Well usually each author is working towards the end result not the preview, in fact most would expect to work on lots of different sizes (they won't be making it for one size). The problem is caused by two things:

1. The preview is always the same size and it usually isn't the same size as the original
2. Some filters look different when the input image size is different. Just how different depends on what you're doing - sometimes if you try to make it look the same you lose the bonus of having a bigger more detailed image.

So the only size which will correspond exactly with the preview will be what gimp provides, which I think is 400x400

Great to see you back here GMS.
Doesn't it render the image the same when you view preview in G'MIC at 100%?

Well leaving the zoom factor in the preview on the default does help, but it's still quite often not perfect.

gms9810: welcome back, brother! tell your wife thanks for coming in to give us an update.

this preview thing is very annoying. i find that free experimentation with the preview window turned off is often the best - that way i can value the result in itself, regardless of what i expected it to be from the preview.

Welcome back gms ! What I can suggest first is to leave the preview zoom factor unchanged. This is really important. Most of the filters won't estimate an accurate preview when the zoom factor of the preview if modified. This can be understood easily, because as Garagecoder said, the G'MIC filters are often designed to :

- Either work on a 'local' scale : on each pixel, some math formulas are applied which depend only on the value of the pixel as well as its direct neighbors. For these filters, the 'most accurate' zoom factor for the preview is 1:1, and this is exactly what they do. Selecting one of these filters will display a very local portion of your image in the preview window. For instance, the 'Enhancement / Smooth [Anisotropic]' is one of these filters.

- Either work on a 'global scale', which means its algorithm probably uses arguments that are related to the global geometry of the image (percentage of the width for instance, or min/max values of the entire image). In this case, the most accurate preview thumbnail will be a resized version of the whole image. Selecting this kind of filter will usually display a global view of your entire image in the preview window. For instance, the filters in 'Arrays & Frames / ...' use this mode.

- Some filters work only on a pixel per pixel basis, and in these rare cases, the filter preview can be accurate in almost any scale, and you can safely change the zooming factor and still get an accurate preview. But very few filters allow that. For instance, filters for color mixing do that.

You have to know that when someone codes a new filter for the G'MIC plugin, he can choose what default zooming factor is the most accurate for his filter. If he doesn't do that, then the latter case will apply, and the preview window will use the latest zoom factor set. Sometimes, it is probably possible that some developers simply forget to set this parameter, which doesn't help
(after checking, it appears that PhotoComiX, the author of the Psychedelic Glasswork filter forgot to set this default zoom factor for the preview, that may explain parts of the problem. In this case, I can do something, but generally, the filter developer is the only one that can fix this).

Also, sometimes, there are filters that are working *both* on a local and global scale, and in this case, generating an accurate preview from a thumbnail is almost impossible. Because, either the thumbnail will be global view of the image, and accurate local informations will be missing, either it will be a small 1:1 portion of the image, and accurate global features will be missing (final width or height for instance). For these filters, nothing can really be easily done to make them rendering an accurate preview.

That's it. So, the final words would be :

- G'MIC filter developers, don't forget to set your default zoom factor for the preview window, when writing a new filter for the plug-in !
- G'MIC users, never touch the preview zoom factor button

GMS Welcome back. I myself have these hiatus periods where I just don't have time to get in here. Sometimes for a few months... But I always return sooner or later. I actually love being in this forum and the people in it who help each other out. In fact I''ve snuck back in again in the pst few days, not (sure for how long)...

Thanks, everybody. Ok, use smaller images and don't touch the zoom. The explanation made sense.