"IMHO": Getting Into GIMP

Wow. I wish I saw this a couple of weeks back. I just bought Elements 6.0 for the Mac and noticed that GIMP will let you use some photoshop plug ins (the only reason I bought Elements). It's amazing what you find on here. Thanks for putting all this info up as well.

Note to self: Eddie, you have to use the search button sometimes ... tsk... tsk....

I knew there were GIMP enthusiasts on Flickr, but didn't know where all of them congregate.

"GIMP Users" seems to be the largest, and is located here.

But, apparently, there are 2,400 Flikr groups devoted to GIMP.

fmajor wrote: There's also a flickr Group dedicated to/for GIMP and it's also quite helpful.

I am a Flickr idiot. How can I find the GIMP group?

fmajor wrote: CPU: Intel i7 920 RAM: Corsair 6GB 1600Mhz GPU: eVGA GeForce GTX285 SC Edition SYS HDD: WD Velociraptor 300GB 10,000RPM Storage HDD: WD Caviar Black 1TB 7,200RPM PSU: Corsair 850TX Case: CoolerMaster 690

Wow, similar to my new system. Although I used the previous gen quad cpu, since the i7 wasn't out yet. I fretted over getting a solid state drive, but went with the raptor in the end. Next time around I'm sure we will be booting from SSD. It is really a pleasure doing image processing with this system!

--

Anyway, thanks for creating this thread! I thought I was going be so so helpful by posting a link to alpha_in_exile's great thread

Edited by redmalloc - 09 May 2009 at 22:44

Thanks SO much for this!

I am usually pretty inclined to learn new software, but photo-editing (and add to that video-editing too) is proving to be a stumbling block for me.

In fact, i just compiled a new PC to handle my perceived graphic editing needs:

CPU: Intel i7 920
RAM: Corsair 6GB 1600Mhz
GPU: eVGA GeForce GTX285 SC Edition
SYS HDD: WD Velociraptor 300GB 10,000RPM
Storage HDD: WD Caviar Black 1TB 7,200RPM
PSU: Corsair 850TX
Case: CoolerMaster 690

IF there was a place for GIMP tips here on my favorite (only?) photo website, that would be excellent.

Thanks for putting in the time and effort to make this for us, Matthew|alpha_in_exile!

Maybe it's also worth to put the various sharphening techniques mentioned into one big article as those methods aren't really GIMP specific, then you may get more attention and feedback, too - sometime ago we did start these case study writings, looks like this would be a nice follow-up

[EDIT]: Of course credit goes to Matthew|alpha_in_exile and not Mr. T

Edited by Micholand - 09 May 2009 at 21:25

Sharpening: Luminance Sharpening using L*A*B color

There's a decent tutorial on the GIMP website for "smart sharpening" using HSV decomposition, but it has a couple of issues. One, it was written for an older iteration of GIMP, and two, there are a couple of steps that are incompletely described. The HSV approach also sharpened parts of my image that I did not want sharpened. So I worked up a L*A*B approach.

HSV = Hue, Saturation, Value
L*A*B = Luminance, color channel "A," and color channel "B" (IIRC)

In a noisy image, even a slightly noisy image, the simple application of USM can cause sharpening of the noise, which can make the whole image appear noisier. Setting a high threshold in USM can sometimes help, but then you may fail to sharpen things you do want to sharpen. Or you can sharpen small selections, or sharpen using masks. But by separating out the luminance channel, it is possible to sharpen in just the luminance channel, thus preventing any sharpening of color/chroma noise.

Here is the photo to be sharpened:



There's very little that's in focus, in this shot, so it is a little difficult to sharpen just the in-focus parts and not any of the OOF parts. USM with a high threshold works moderately well, but I found that a judicious use of luminance sharpening through L*A*B could work even better.

The first step is to "decompose" the image into its L*A*B components.



You'll be given a dialogue box with a number of options. Choose LAB. Leave the 'decompose as layers' box checked, as you will want to work on the L(uminance) layer only. Leave the second box, 'foreground as registration color' unchecked.



Now, bring up the Layers dock (CTRL + L) or (ver. 2.6.x) under menus Windows-->Dockable Dialogs-->Layers, and click on the 'eye' symbols next to channels A and B, if you like, to make them invisible.



In order to sharpen only the edges in the image, we need to mask off everything that is not a hard edge. First, duplicate the "L" layer, and name the layer "sharpening" or something to differentiate it.



Then employ the Edge-detect filter (Filters-->Edge-detect-->Edge) on the sharpening layer only.



The Edge filter attempts to filter out anything that is not a strong edge, making non-edged sections black, and edges white; the image in the preview window will show you what elements are being detected as edges. Adjust the "Amount" parameter until you are satisfied that only the edges you want to sharpen are being emphasized.



The layer will now be black with white highlights. For my image, the layer looks like this:



You should be able to see not only the points on each spike, but also lots of much smaller details or points that were captured by the edge filter. I don't want all of that stuff sharpened, though. So it is recommended, but not absolutely necessary with all photos, to adjust the Levels (menu: Colors-->Levels), moving the black point up and the white point down, to make your edges whiter and your non-edges (or the detail you don't want sharpened) darker.



As you can see, I moved the black point up to 11 and the white point down to 170. Remember this is just your sharpening layer, not your final image. Notice that the many small white points, which were present after I first applied the edge filter, have now faded to black, so I have a nice black background and, in isolation, bright white edges.

Once you've isolated the edges you want to sharpen, then apply a slight gaussian blur (Filters-->Blur-->Gaussian Blur) to widen the edges a bit.



(sorry I didn't get part of the image in the preview window!)

The "smart sharpening" tutorial from the GIMP website suggests again adjusting the levels after the application of gaussian blur. The blurring does darken the 'halo' around your edges, and you want them fairly white for the next step, so it is recommended to again raise your black point and lower your white point as needed.

Now, go to your "L" layer (bring up your Layers dialog with CTRL+L and select the L layer), and add a layer mask (either by right-clicking on the L layer, or by going to the menus: Layer-->Mask-->Add Layer Mask).

Here I've done it by right-clicking and selecting from the menu:


Mask type: choose White (full opacity)


Your layers will now look like this:



Next, go back to your sharpening layer, select all (CTRL+A or menus: Edit-->Select All) then copy (CTRL+C or Edit-->Copy).

Now return you your L layer, and, specifically, click on your layer mask (the small window/rectangle to the right on your L layer)...



..."paste" the image into the mask. Anchor the pasted image by clicking outside your canvas, or by right-clicking on the layer mask in the Layers dialog box and selecting "anchor layer" like this:

[ IMG]http://i8.photobucket.com/albums/a43/xj-xile/LABUSMdemoanchorlayer.jpg[/IMG ]

Once anchored, your layer mask should look like your sharpening layer, in the Layers dialog box.



Once your sharpening layer has been copied into the layer mask on the L layer, you should delete the "sharpening layer." Later, when we recompose the image, you can have only three layers.

The next part is great -- right-click on the layer mask and choose "Mask to Selection" (or menus: Layer-->Mask-->Mask to Selection).



This will select -- on the L layer -- only the white edges from your layer mask. It is now safe to delete the layer mask (right-click & choose 'delete layer mask' or menus: Layer-->Mask-->Delete Layer Mask). Only the mask!

[ IMG]http://i8.photobucket.com/albums/a43/xj-xile/LABUSMdemodelmask.jpg[/IMG ]

What you are left with is the L layer with 'marching ants' showing the selected edges. This is where you apply USM, which will only affect the selected edges and only affect the luminance channel.

[it seems I forgot a screen capture of the image with marching ants - to be supplied tomorrow]

As the old GIMP "smart sharpening" tutorial states, you will find that you can use a much stronger application of USM without introducing artefacts. I used settings of: Radius 0.6, Amount 1.88, Threshold 4. These are still very conservative settings; I based them on the type of image I'm editing, and the thought that I may print this image very large. If I print it small, though, I'll probably want to do more sharpening (maybe just USM + Fade).

When you've finished sharpening, you must recompose the image (menus: Colors-->Components-->Recompose). This will return you to an RGB color version of your image (in the same window as your original image).

[also forgot to upload the screenshots of the Recompose process]

Below are some 100% crops for comparison.





(image on left the font is hard to read -- USM settings were R 0.5, Amt 0.30, T 20 for first pass, R 0.5, Amt 0.74, T 20 for the second pass)



USM faded = USM applied with settings R 0.5, Amt 1.36, T 20 and faded to 80%. It seems to equal or even better the L*A*B version for clarity of detail, which may be my fault -- a function of the conservative USM settings I used in the L*A*B version, or too-low threshold, or some other fault -- or it may be simply a superior method.

[I'll have to update this post in stages by editing the post over & over again... stay tuned!] [I have yet to test the luminance sharpen plugins, and I think I set too low a threshold for the USM in the L*A*B version, or I skipped a step (conversion to greyscale of the sharpening layer) so may have darkened the image somewhat, looking at the comparison crops]

Edited by Turerkan - 09 May 2009 at 20:49