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Can somebody tell me...

Aug 15, 2013, 05:41
Post: #1
Can somebody tell me...
Can someone tell me what I'm doing and why I shouldn't be doing it, when I do this:

I'll load a decent photo into Gimp, click Value Invert under Colors, get a bunch of weird colors which are, I presume, the opposite values for each pixel's color, then pull down Edit, hit Fade Value Invert, get a dialogue box, choose Overlay in the Mode menu then hit Fade. Sometimes I'll use the Opacity slider if it's too dark. Most of the time I'm really pleased with the result.

This seems to save me a bunch of fooling around in Curves to get where I'm trying to go, which is a more vibrant, interesting shot, but it seems too easy. What's happening? Does it introduce excessive noise or artifacts or something?

What's happening here, I don't understand it.

I tried to add a couple of examples but I don't know how. I chose a file then hit Add Attachment and nothing happened, so I'm obviously doing something wrong and haven't figured it out yet, but you can try it with a photo of your own and see what happens.
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Aug 15, 2013, 08:49 (This post was last modified: Aug 15, 2013 09:07 by rich2005.)
Post: #2
RE: Can somebody tell me...
Quote: Can someone tell me what I'm doing and why I shouldn't be doing it, when I do this:..

Do whatever you like, Are you happy with the end result?

Many of the colour operations are just different interfaces for the same thing.

You can use the color curves or you can use color levels, ends up with the same result just a different way of visualising the adjustments.

Since Gimp is an 8 bit editor this generally results in some loss of colour informtion, which can be a problem or you might never notice it on your masterpiece. Much is in the eye of the beholder.

this might (or might not) explain.
Top image is uneditied, straight from the camera. The circled bit on the curves is smooth, all the colour information is there.
Middle image - After applying value invert etc, the little spikes are places where some info is lost. Not much in this case, it can be far, far worse.
Bottom image - the photo tweaked in a different application (delaboratory) which uses totally different mathematics to Gimp When opened in gimp the curve is back to a nice regular shape. However, part of this is due to exporting back to a jpg and re-opening but generally better to use some sort of 16 bit application, Photivo works, delabratory is ia bit 'experimental' at the moment.

[Image: yVuXcXL.jpg]

One way of posting an image is use an external host. This is the one I use
Upload your image and they give you several options to include in your post. The image might vanish in 6 months time if no one looks at it, but WTH nobody ever looks at old topics anyway, why use 'search' when you can post a new topic.

** now answering questions**
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Aug 15, 2013, 20:16
Post: #3
RE: Can somebody tell me...
Thank you both for replying. I got a bunch of good stuff out of both posts:

What I was worried about was changes that would affect a print, changes I had happen before when I did stuff and didn't see it on the screen but did on the print. Now I know to compare the histogram before and after.

The Photivo site is a real find for me. I didn't even know there were open-source 16-bit photo editors! And since most of what I use Gimp for is photo stuff that's a big help. As is Imgur, see: (before and afters)

[Image: ng4MzBQs.jpg] [Image: vMRNashs.jpg]

Well, okay, the size is a little off and they're kinda squished together, but I'll figure that out... later.


I had a feeling that I was doing something like that, using Overlay on a mask, but I wasn't sure. I actually got the Overlay idea from a site you and Rich recommended when I first started with Gimp, Pat David's site. One of his tutorials has something about changing the mode to Overlay before merging two layers.

I can only presume that the "your favorite plot utility" is subtle mathwhiz humor, playing on the fact that it's so hard to decide which is one's favorite.

More seriously, I can't help but notice that the shape of the s-curve in the plot is very similar to what I've been using in Curves to pump up dull pics, but I didn't totally comprehend that what I was doing was working on the brights and darks and leaving the midtones alone. Now I do.

Also, using Log instead of Linear in the Histogram is a big help. Why didn't I think of that?

And, bumping up against the file size is exactly the problem I was having. How'd you know?

Thanks again to you both.
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Aug 16, 2013, 06:11
Post: #4
RE: Can somebody tell me...
Yes, precisely(about the curve).

I didn't state that very well. I should have said that the plot curve is similar to but the reverse of what I've been doing. Only that I fiddle with it until it's completely screwed up and I have to start over. At least now I know what I'm trying to do. Thanks.
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