LXF114.tut_gimp.pdf

Tutorial Gimp

Gimp Open source image-editing

software you can get your teeth into

Gimp: Speedy colour fixes

Smile big and show your colours as Michael J Hammel

guides you through quick colour fixes for your photos.

supported in Gimp. Colour management is all about

matching colours from input devices like cameras and

scanners to output devices like monitors and printers. Colour

adjustment, on the other hand, is a very different subject. The

colour adjustments I’m talking about this month are very specific

to editing your images in Gimp: blue instead of red, green instead

of yellow. If Red Hat were to be bought by Novell, all those Fedoras

would need to be changed to green, right? Gimp makes these

kinds of colour edits a quick and painless process. But while

they’re easy to perform, they take practice to get just right.

Each colour correction is made up of three important steps.

The first is to make a proper selection. This may be as simple as a

rectangular outline or as complex as the union of multiple

selections. Once the selection is created the next step is to feather

it – this will cause your colour adjustments to blend seamlessly

into their surroundings. Feathering is not optional when editing

photographs. Without feathering, you simply won’t be satisfied

with the results.

The last step is the most important: choosing the right colour

tool. There are no cut and dried rules for which tool will produce

the best results. In some cases desaturating the selection will be

required, while in other cases you’re better off just applying a

colour change directly to the coloured selection.

Part 1 Costume change

Our

expert

Imagine you sell T-shirts to college students. You need to find

which colours the students prefer. Should you buy different

coloured shirts and show them around? Maybe, but before you

start pulling from your not-yet-earned profits how about doing

some market research for a lot less money using Gimp?

Start with a photo of the type of shirts you plan to sell. This

stock photo might just be for the ladies’ version of the shirt but it

will do as an example. We start, as always, with a good selection.

Because this shirt has a high colour contrast with the rest of

the photo it is a good candidate for use with the Foreground

Selection tool. However, the thin straps and shadowed areas on

the left-hand side make require some fine-tuning. Once again, I’ll

clean the selection up with the Quick Mask tool. The Foreground

Selection tool uses a multi-step process to make its selection.

After choosing the tool from the toolbox, the first step is to drag a

rough outline around the item to be selected. This is just like

outlining it with the Free Select (aka Lasso) tool. Once this has

been done, the unselected area is tinted blue.

To fine-tune the initial rough selection a paint stroke is dragged

through the untinted region and over just the shirt. The purpose of

this paint stroke is to tell the Foreground Selection tool about the

colours to keep in the selection. Multiple paint strokes improve on

Michael J

Hammel is a

contributor to the

Gimp project and

the author of three

books on the

subject, including

his latest, The

Artist’s Guide to

Gimp Effects.

If you photograph your own models, put them in front of a

solid-coloured background that doesn’t match their clothing.

Last month We got technical with colour management for printing.

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L ast month I talked about the colour management tools

Gimp Tutorial

Gimp: Speedy colour fixes

In tint mode the

selection looks

good, but finalizing

the selection

shows the left

side needs some

cleanup.

The colour of the tint used by Foreground Select can be

changed in the Tool Options dialog. You should make these

changes before you make your initial freehand selection.

After fixing the selection in Quick Mask mode, return to the

selection by clicking on the Quick Mask button again. Finally,

feather the selection by five pixels. You’re now ready to apply some

colour changes.

To recolour the shirt, first remove all colour content using

Colours > Desaturate. Choose the Lightness option (though any of

the three available options would suffice). To put the colour of your

choice back in, choose the Colourise dialog (Colours > Colourise)

and adjust the Hue level first. Fine-tuning of the colour occurs with

the Saturation and Lightness options.

the selection. You can adjust the size of the brush (which is shown

in the canvas as a circular outline) used for these paint strokes

using the Tool Options dialog.

The first stroke should use the default brush size. Start in the

upper-left strap and draw around the neckline, up into the other

strap, down the right-hand side of the shirt across the bottom, up

the left-hand side (just touching the shadow area, then across the

middle of the shirt). Because of the uniform colour tones in the

shirt this process creates a very accurate selection except for the

left-hand area of the shirt. Hit the Enter key to finalise the selection.

Switch to Quick Mask mode, zoom in and use a small brush on

the problem areas. Remember that painting with black removes

from the selection, while painting with white adds to it. You can

paint with other colours as well, but they just produce areas that

are partially selected, much the same way that the edges of a

selection are partially selected when you feather the selection.

However, you seldom use anything other than black or white when

working with Quick Mask.

You may need to select trickier areas by hand.

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Tutorial Gimp

Part 2 Blue-eyed girl

Try going through this process but without feathering

the selection – you’ll see how artificial the effects are.

Why are we changing her eye colour? Because we can!

remove all saturation (all the colour) and replace it with blue. The

first step is to desature the selection using the Desaturate dialog

(Colours > Desaturate). I chose to desaturate using the Lightness

setting, though for this image at least, there is no visual difference

between the settings.

After desaturating, open the Foreground colour dialog by

clicking on the Foreground colour box in the Toolbox and select an

appropriate blue for the foreground colour. I set the HTML

Notation field to 153664. To colour the eyes, choose the Bucket Fill

tool from the Toolbox. In the Tool Options set the Mode to Dodge

and the Affected Area to Fill Whole Selection, then click inside the

canvas. The blue colour will be blended with the desaturated eyes.

The feathering will let this colouring fade toward the edges so you

don’t get any abrupt colour loss.

The choice of a Dodge blend mode for the Bucket Fill is

completely through experimentation. This process is extremely

simple, so repeated experiments with different shades of blue and

different blend modes is fast and simple. Remember: for projects

like this, Ctrl+Z (or Edit > Undo) is your friend.

Experiment with

the replacement

eye colour until

you find the one

that works best.

This next example is similar to the first, in that I’ll remove colour

from a pair of lovely brown eyes before tinting them blue. The

stock photo is small, so I’m zooming in on the eyes. Zooming in

causes some pixellation, resulting in a slightly blocky appearance

in these screenshots, but it doesn’t make working on the image

any more difficult.

Start with the Free Select tool and draw an outline around the

left eye. I enable the Add mode in the Tool options, then draw a

second outline around the right eye. This is then converted to a

Quick Mask to clean up the outlines before converting it back to a

selection, just as we did in the first example. The selection is then

feathered by five pixels. Since we’re changing the brown eyes to

blue, reducing saturation won’t be enough. In fact, we want to

The blend mode and colour used is very important. Review some photos from stock

websites for the colour of eyes you need and sample the colour from those images.

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Gimp Tutorial

Part 3 Paint job

Initial selections aren’t complete – they leave many small

gaps. But these well-fed selections are ready for a little growth.

Keeping with the commercial theme, let’s now imagine you’re a

painter and you need to offer colour options to a home owner.

Throwing swathes of paint on the home probably won’t be an

option they’ll accept, so you choose to do it the Gimp way: select,

feather, colour.

Because of the snow sticking to the side of the house and

the bushes in front of it, the most suitable selection tool is the

Fuzzy Select tool. For this image the Tool Options Threshold is set

to 35 and the Add mode enabled. There are multiple blue walls to

include, each of which is bounded by a solid white border, so click

on each wall one at a time to add it to the selection.

Each wall selection is incomplete. There are small areas of

unselected wall we need to include in the final selection. This is

done by growing the selection a small amount. Open the Grow

Selection dialog and set the amount to one pixel, which is

sufficient for an image of this size. The selection still requires a few

light touches with the Quick Mask to unselect some of the bushes

in front of the house and add a few shadowed areas on the walls

that have not yet been included. The final selection is created

using the Airbrush tool instead of the Paintbrush when fine-tuning

the Quick Mask. This is so that the edges of the bushes and, most

especially, the barely visible tree branches are blended smoothly

with the colour changes. Remember that reflected light from the

house will slightly colour these items, so soft-edged selections are

required around them. Finally, the selection is feathered by five

pixels. Growing the selection gets most of the walls but it also

picks up some of the bushes, so for accuracy, Quick Mask must

be used for final adjustments.

To colour the selection we can use any of the previous

methods, but I found that using the Curves dialog provided the

best results. To turn the house a dark red, move the Blue and

Green curves down on the left-hand side while moving the Red

curve up on the left and down on the right. It may be hard to see in

this image, but snow was falling when the photo was taken and the

falling snow remained white but tinted red where it passed in front

of the house while the picture was snapped. LXF

Growing the

selection gets

most of the walls

but it also picks

up some of the

bushes – use

Quick Mask for

final adjustments.

Tutorial images

This month’s stock imagery can be found on

BigStockPhoto.com:

1 Costume Change

Beauty_Young_Sweaty_Woman_In_B_3535508

2 Blue Eyed Girl Spa_Girl___1892998

3 Paint Job House_292318

The snow remains white as it falls, tinted only slightly as it

passes in front of the house.

Next month Paths, layers and blurs combine to make a wine bottle. Glug!

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