In Photos dot Org » Gimp Tutorials

The Curves Tool

Donncha O Caoimh — Mon, 15 Sep 2008 08:35:00 +0000

The curves tool is a very basic tool that can be used to improve photos with a few clicks of the mouse. It is used to change the brightness and contrast of an image. It can also modify the separate Red, Green and Blue channels of an image too. The Curves Tool has a histogram to represent the shadow, midtone and highlight detail in the image. In the GIMP, you access it by right clicking on an image and go to Colors->Curves.

This is the second article in my GIMP for Photographers series, but as usual, all of this applies to Photoshop, or any other image application with a Curves Tool. The first tutorial was on The Levels Tool, and worth a read if you missed it!

Here’s an image I shot at the Lord Mayor’s Picnic in Fitzgerald’s Park a few months ago, and the Curve Tool below it. Notice the histogram? The photo is fairly well exposed, but some highlights are “clipped”, as the histogram hits right hand side without sloping off.

Brightening Curve

It’s easy to brighten an image. Just drag points on the line up.

Darkening Curve

Now, let’s darken the image by dragging points down.

Contrast Curve

A classic use of the Curves Tool is to increase contrast in an image. You do this by darkening the shadows, and brightening the highlights. The curve looks sort of like an “S” when you do this. Don’t go overboard on this though, because it’s easy to lose detail in either direction.

If for some reason your image has too much contrast, a quick inverted S curve will solve that problem,

Wacky Colours

You can select any of the Red, Green and Blue channels and do strange things to your photos. Here’s what happens when you play with the Red Channel.

And here’s what happens when you change multiple colour channels in different ways.

Colour Picker

After you have opened the Curves Tool, click anywhere in the image. Notice how a vertical line goes up and down the histogram/line? That vertical line is the colour of the pixel where you clicked. That can be useful if you’re trying to modify a particular part of a photo. This is what you get when you click on the black coat on the left of the image above.

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Children at play

Donncha O Caoimh — Wed, 06 Jun 2007 11:49:02 +0000

Children at play

A sign in Fermoy, Co. Cork warns motorists to watch out for kids playing on the road.

I’ve seen so many people speed through built up areas these signs mean absolutely nothing to them.

Technique:
1. Original image was flat and plain. Background sky was monotonous so I ran it through auto-levels which brought out the colours.

2. Then I duplicated that layer and blurred it using Gaussian Blur with a radius of 25px (original image is 3504px wide). By adding a layer mask I was able to rub out some of the blurred layer to expose the sharp original below. Opacity was set to 41% to reduce the blur effect.

3. Finally an overlay layer was added and circular gradients drawn on with a low opacity. This darkens the sky and sign slightly in patches.

All manipulation done in the GIMP but will work just as well in Photoshop or other application.

Was that useful? Want more?

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Adam and his baby soother

Donncha O Caoimh — Sat, 26 May 2007 14:09:44 +0000

Adam and his baby soother

Adam is 5 weeks old today! Last Wednesday we saw his first smile, or perhaps it was trapped wind because he was just after feeding but it was a beautiful sight to behold. He has smiled since too, and was especially cheerful this morning!

Adam loves his baby soother though. He sucks on it like Maggie out of the Simpsons does. Non-stop and with feeling!
If a soother or dummy isn’t nearby and it’s available, a hair dryer works wonders for relaxing him. The sound soothes him and he can go from screaming the house down to docile and a gurgling baby in seconds when the hair dryer is switched on. It’s magic.

Technique: This is a high key portrait created using two layers.
1. Original image is the bottom layer.
2. Duplicate that layer.
3. Convert top layer to b/w using channel mixer.
4. Apply Gaussian blur to the top layer. I used a 75 pixel wide blur.
5. Change mode of top layer to screen.
6. Add white layer mask to top layer and reveal eyes from original image by painting over them with a black brush. Use an opacity of 10%.
7. Merge layers and save. Layers should be separate when resizing so you can unsharp mask the bottom layer.

Hope that is of use!

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First Draft: Ready to go!

Donncha O Caoimh — Tue, 05 Dec 2006 18:05:24 +0000

John asked what did he original Ready to go! look like and I’ll oblige now. Showing what the original photo looks like is akin to showing what the first draft of a written essay or post reads like. Sometimes the image comes out perfectly in the camera but that’s rarely the case. At the very least light levels have to be balanced and if resizing for publication online then the resized image has to be sharpened.

Hover over the image below to see what the original shot looked like. Hopefully this will work for RSS readers but if it doesn’t, visit the blog and leave your mark here!

Notice how I rotated the image? I had to reconstruct the bumper on the right of the picture, as well as filling in the gaps at the other corners of the photo. Tree branches and leaves are easy enough, as is the relatively solid black texture of the tar on the road, but the bumper was difficult, and the shaded area of the building on the left presented me with a few extra minutes of clicking to get right.

Want to see more “First Draft” posts? I can’t promise to do many, but if you have a compelling reason why you’d like to see the original of a photo I’ll do my best to help!

Hosted on Flickr

PS. Bryan – you might recognise the CSS. I took it from the button of doom you did! Hope you don’t mind!

PPS. Treasa has posted a tutorial of how she worked on two photos with steps in Photoshop to get the desired effect. Nice!

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GIMP Tutorial Videos

Donncha O Caoimh — Wed, 21 Jun 2006 10:37:08 +0000

Jakub ‘jimmac’ Steiner has published several demos of the GIMP in action.
Subjects such as defining shortcuts, image templates, transformations and paths and more are covered. Use the mirrors, because I haven’t downloaded the videos myself yet!

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Simple steps to photo touch-up

Donncha O Caoimh — Wed, 19 Oct 2005 15:19:38 +0000

In this post I’m going to show you how to go post-process this image:
By the end, we’ll have an image that looks like this:

This tutorial was created using the GIMP, but it’s equally applicable to your favourite editing software as long as it has the same tools. Photoshop, and other editing software should work equally well.
The steps described here are worth practising, and will apply equally well to any portrait!

First of all, I came across this photo on Flickr through my contacts page. Here’s the original photo, and Ayhtnic kindly let me use her image.

After you load the image, the first thing to do is use Auto Levels from the Layer->Colors menu. This tool alone does wonders for most photos, especially if they’ve been captured as Jpeg straight from the camera.

The image is a little noisy so let’s clean it up a bit. Use Selective Gaussian Blur from the Filters->Blur menu. Use small values as we just want to smudge the noise away without losing too much detail. A radius of 3, and delta of 10 worked fairly well here.
Let’s brighten it a bit and add contrast. Use the Curves tool from Layers->Colors for this. The classic “S” shape always adds life to a photo.
Open the Layers dialog and duplicate the background layer.
Select the new layer (called “background copy” here) and use the Curves tool again to brighten this layer a lot.
With the same layer selected (the top one, the “background copy”), we’ll apply some blur. Open up the Gaussian Blur tool, it’s in Filters->Blur. Apply a blur of 5 pixels to the top layer. Don’t worry, we’re not finished!
We’re going to change the “mode” of the top layer now. With the top layer selected, click on the drop down box that says “Normal” and scroll down to “Soft Light”. You can also try other modes, they’ll make for interesting photos!
Notice how the image suddenly changed?
Even with the nice glowing effect, the image looks indistinct. Let’s sharpen the bottom layer. Select that layer in the Layers dialog and load the “Unsharp Mask” filter. This is in Filters->Enhance->Unsharp Mask.
Don’t apply too much sharpening. Make it subtle. The settings in the screenshot work well.

All that’s left is to save the image, save it with a quality setting of 92%. Don’t bother with higher as it’s practically impossible to see any difference in quality.

The finished photo

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