Manipulating Images

Part III: Manipulating Images

choices are actually a bit of misnomer. All of the blurs are Gaussian blurs, but the Selective

option is a specific type of Gaussian blur that pays attention to edges. The default Gaussian

blur without edge protection generally gives good results, but in images where you start

seeing glow around edges, you may want to play with one of the other three options.

Dark/Bright Mask Grayscale — These options control which of the three images you

want to use as the layer mask for the dark and bright image. Typically you’ll want to stick

with using the defaults here, but if you need to squeeze out some extra detail from some

parts, changing these values can help.

Dark Takes Precedence — When Exposure Blend finishes, you’ll have three layers, two

with masks, as shown in Figure 12-16. By default, the bright exposure layer is at the top of

the stack. However, this may yield results that are brighter than you want. You can manu￾ally move the dark exposure layer up or you can send it straight to the top here by enabling

this check box.

Auto-Trim Mask Histograms — Enabling this option is basically the same as opening

the Levels dialog and clicking the Auto button on each of the exposure masks.

Scale Largest Image Dimension to — Type in an image size here in pixels, such as

800x600, and the tone-mapped image that comes out of Exposure Blend is proportion￾ally scaled to be no larger than those dimensions. This is useful if you want to test some

settings before applying them to full-sized images.

FIGURE 12-15

The Exposure Blend dialog

Once you have all the settings adjusted the way you like, click the OK button and Exposure

Blend will do its magic. If the source files you’re using are large, this step might take a while,

especially if you’re running GIMP on an older computer. When it’s complete, though, you have

an image window with your processed results in it. The cool thing is that Exposure Blend doesn’t

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give you a flattened image as your result. You actually get three layers, as shown in Figure 12-

16. The bottom layer is the normal exposure and the two layers above it are the bright and dark

exposures, masked to reveal content that’s not apparent in the normal exposure.

FIGURE 12-16

When you run Exposure Blend, the result is an image with three layers. The normal exposure image

is at the bottom and the other two are masked above it.

What makes this so cool is that now you can manually tweak things with all of the tools avail￾able to you in GIMP to get more customized results. You can reorder the exposure layers, tweak

the masks in the Channels dockable dialog, or paint directly on each layer. If you want to re-run

Exposure Blend on your images to regenerate your masks or adjust their blur, you don’t have go

through the process of selecting your images all over again. Instead, choose Filters Exposure

Blend Reset Blend Details. This brings up a dialog like the one shown in Figure 12-17, which

allows you to adjust all of the settings without needing to reselect your exposure images.

FIGURE 12-17

The Reset Blend Details dialog allows you to adjust tone mapping settings without reselecting your

exposure images.

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Part III: Manipulating Images

Because the process of bracketing requires that you take multiple photos, there’s a chance that all

of those exposures will be slightly offset from one another, especially if you take the photos with￾out a tripod. Fortunately, Exposure Blend also includes features to help you align these images.

Choose Filters Exposure Blend Align Exposures and you’ll find four options:

Bright — This sets the bright exposure layer to the difference blending mode to help you

accurately align the bright exposure layer with the normal exposure layer. Use the arrow

keys on your keyboard with the Move tool (M) to move the layer one pixel at a time to

nudge it into place.

Dark — This does the same as the Bright menu option, but for the dark exposure layer.

Off — Choose this option after you’ve aligned your layers to set everything back to the

resulting tone-mapped image.

Trim Image to Overlap Area — If you adjust your layers to get them to line up, parts of

the exposure layers no longer cover the normal exposure layer. You can recognize this as

a strip along the perimeter of you image that looks markedly darker or brighter than the

rest of the image. To rectify this, you can crop these protruding bits. This menu option in

Exposure Blend does this automatically for you.

Figure 12-18 shows the difference between an image that’s been tone mapped with Exposure

Blend and the original, normal exposure image.

FIGURE 12-18

On the left, a digital photograph created with a single exposure; on the right, the same image tone

mapped with Exposure Blend

And with that, you’ve got a nicely tone-mapped image from a set of three bracketed pho￾tographs. It’s important to note here that you’re not technically saving an HDR image. If you

save in GIMP’s native XCF format, the layers and their masks give you high dynamic range

data, but it’s not really an HDR format. And if you flatten the layers to save to another format

like PNG or JPEG, the result is not HDR; it’s a regular image that’s been tone mapped with

bracketed photos. It’s a subtle distinction, but it’s definitely worth keeping in mind. To have an

HDR image, GIMP would have to support more than 8 bits per channel and be able to save to

an HDR-aware format such as OpenEXR. There is not currently support for this in GIMP, but

hopefully future versions that have tighter integration with GIMP’s new imaging library, GEGL,

will add this support.

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Chapter 12: Enhancing Photos

Summary

This chapter covered the use of GIMP’s painting tools that are extremely useful in photo retouch￾ing work. You saw how the Blur/Sharpen tool could be used to direct viewers to specific parts of

your image and how the Smudge tool can drastically modify an image by extending it and soft￾ening its appearance. You saw how the Dodge/Burn tool relates to the traditional photographic

printing techniques, but with the added flexibility of the digital medium. Then you got to play

with GIMP’s Clone, Perspective Clone, and Healing tools, allowing you to fix small parts of an

image or change an image drastically. At the end of this chapter you saw how the Exposure

Blend plug-in can be used to get a larger dynamic range in your image by taking advantage of

bracketed photos.

Next up, all of GIMP’s included filters. Sweet!

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Part IV

Exploring Filters

and Effects

IN THIS PART

Chapter 13

Implementing Blur,

Enhancement, and Distortion

Filters

Chapter 14

Using Image Creation Filters

Chapter 15

Using Compositing Filters

Chapter 16

Enhancing Images with Artistic

Filters

Chapter 17

Working with Specialized

Filters

Chapter 18

Batch Processing with

Automating Filters

Chapter 19

Using GIMP Animation

Package

Chapter 20

Working with Video-Specific

Functions in GIMP