Part III: Manipulating Images Analyzing Your Colors with Info Tools Before doing color correction on an image, it’s a good idea to understand the nature of that image in an analytical sense. The functions available in the Colors  Info submenu are well-suited to providing you with the raw data about your image that can facilitate that analysis. Histogram Probably the most useful of the informational tools is the histogram. As you may have noticed, the histogram is an integral part of many of GIMP’s color tools, including the Levels, Curves, and Threshold tools. It’s also the easiest way to see how the automated tools have modified the tonal qualities of your image. Because the histogram is so valuable, it’s actually one of GIMP’s dockable dialogs, briefly covered in Chapter 1. The Colors  Info  Histogram menu item is just a convenient shortcut that brings up the same dockable dialog you get by going to Windows  Dockable Dialogs  Histogram. As explained earlier in this chapter, a histogram is a chart that shows how many pixels in your image share a specific value for a given channel in your image. Figure 9-40 shows the Histogram for each of its available channels. FIGURE 9-40 The Histogram dialog can show statistical data for each of the channels in your image, including a combined view of all channels. Like in the Levels and Curves tools, you can switch which channel the histogram displays by clicking the Channel drop-down menu. You also still retain the ability to display the histogram 266 Chapter 9: Adjusting Colors on either a linear or logarithmic scale with the two buttons on the right side of the dialog. The Histogram dialog also features a set of triangle sliders beneath the actual histogram graph, but this does not modify anything in the image. It merely defines a range for giving you statistical information at the bottom of the dialog. The Histogram dialog displays information only for the active layer or the current selection of pixels within that layer. If you want to have the histogram show an analysis of all visible pixels, you first need to merge layers. Border Average Like the other color operations in the Info submenu, the Border Average operation (Colors  Info  Border Average) does not modify your image. However, it does change the foreground color used by your paint tools. Basically this tool traverses either the border of the active layer or your current selection and calculates the dominant color used for that perimeter. That color is assigned to your foreground color. To do this, the Border Average operation brings up a dialog, such as the one shown in Figure 9-41. FIGURE 9-41 The dialog for the Border Average operation This dialog features two primary controls:  Border Size — When the Border Average function runs, it creates a border around your active layer or selection and samples the pixels within that border. The Thickness value set here in the dialog determines how wide that border is. Larger Thickness values increase the number of sampled pixels that Border Average uses to pick your new foreground color.  Number of Colors — As the Border Average operation traverses the border it has created, it takes the colors it samples and organizes them into a series of buckets that it later uses to determine the dominant color along the border. The drop-down menu here dictates how many colors get included in each bucket. If you have a smaller Bucket Size, the Border Average operation yields more precise results about the nature of the colors in the selected border area. Colorcube Analysis As Figure 9-42 shows, the Colorcube Analysis function (Colors  Info  Colorcube Analysis) produces a histogram that looks remarkably similar to the Histogram dialog when set to the RGB pseudochannel. For the most part this is true; it just shows the information at a somewhat 267 Part III: Manipulating Images finer grain, using each of the six common hues in digital color (red, yellow, green, cyan, blue, magenta) and showing the results in an additive format where each color overlaps with the others to generate a new color. Where all of the colors overlap, you see white. FIGURE 9-42 The results of running Colors  Info  Colorcube Analysis on a color image Smooth Palette Unlike the other operations in the Info submenu, the Smooth Palette operation (Colors  Info  Smooth Palette) actually creates an image as output. Specifically speaking, it creates an image that looks very similar in appearance to the gradients in the Gradients dialog. Basically, it takes all of the colors in your image and generates a linear palette from them. The main purpose of this filter is to create custom input for the Flame filter (Filters  Render  Nature  Flame). However, it’s also a pretty convenient way to generate a map of the colors used in an image. Then you can use the strip that’s generated as your painting palette if you want to create an image with similar tonal qualities. Figure 9-43 shows the Smooth Palette operation’s dialog and an example palette that it created. FIGURE 9-43 On the left, the Smooth Palette dialog; on the right, a palette that this operation generated The Width and Height values in this dialog dictate the final size of the generated palette image. The Search Depth field stipulates the number of shades included in the palette. It has a range from 1 to 1024. If you choose to generate a palette with a Search Depth that’s greater than the default value of 50, it’s definitely a good idea to at least increase the width of the generated palette image. 268 Chapter 9: Adjusting Colors Color Filter Plug-ins The last set of items in the Colors menu is actual filter plug-ins that ship with GIMP and explic- itly work with color. For convenience and organization, they’ve been included with the Colors menu rather than cause undo clutter in the already lengthy Filters menu. Color to Alpha This filter performs the exact same function as can be found in Layer  Transparency  Color to Alpha. This feature is discussed more in depth in Chapter 6, but the thrust of it is that you use the dialog that appears (shown in Figure 9-44) to choose a color to be mapped to the alpha channel. The cool thing about this filter is that rather than do a binary transparency opera- tion where one specific color is transparent and all others are opaque, this filter measures the influence of the chosen color in each color channel and uses that to produce varying levels of transparency on the active layer or selection. FIGURE 9-44 The Color to Alpha dialog Colorify The Colorify filter is like a quick-and-dirty version of the Colorize tool. Rather than present you with a handful of sliders to adjust the hue, saturation, and luminosity of the overlay color for your active layer or selection, the Colorify filter presents you with a dialog (Figure 9-45) that has a preview window and seven preset colors to apply to your image: the six core component colors and white. If none of those colors are to your liking, you can click the Custom Color swatch and choose the color you want with the color picker that appears. Ultimately the effect is about the same as Colorize. You get an image that appears like you’re viewing it through colored glass. Filter Pack The Filter Pack filter is pretty interesting. It doesn’t really provide any facilities that you can’t get with any of GIMP’s standard color correction tools, but it does offer a cleaner interface to 269 Part III: Manipulating Images perform color-correction activities. When you call Filter Pack from Colors  Filter Pack, you get a dialog like the one in Figure 9-46. FIGURE 9-45 The Colorify filter’s dialog FIGURE 9-46 The Filter Pack dialog (left) and its various sub-windows for adjusting (in columns from left to right) Hue, Saturation, Value, and for performing advanced curves techniques The best way to approach the Filter Pack plug-in is to work down the left column of the dialog and then work down the right. So the first thing you want to set is what gets shown in the pre- view images at the top of the dialog. Just like the Rotate Colors operation, you have the choice of showing the Entire Image, your Selection Only (if you have a selection), or the Selection in Context. Both the Original and Current preview images at the top are updated to reflect your changes. 270 Chapter 9: Adjusting Colors After determining what you want to see in the previews, you can then decide which pixels you want to tweak first. The radio buttons under the Affected Range heading have the same effect as the Shadows, Midtones, and Highlights radio buttons in the Color Balance tool. Unlike a lot of other filters and tools that can work in RGB space or HSV space, the Filter Pack filter works exclusively in HSV. Therefore, the next set of radio buttons under the heading Select Pixels By is how you tell Filter Pack which channel you want to work on: Hue, Saturation, or Value (Luminosity). With all of the controls on the left side of the dialog set, you have your work environment for color correction established. Now you can really get into some color correcting. The way it works is pretty intuitive. On the right side of the dialog under the Windows heading, click the check box that corresponds with the channel that you want to modify. The first three options pop up sub-windows, each with an array of thumbnail previews of your image with various color adjust- ments applied to them. Click the image that most closely matches the final look you want. You can continue with this process until you’re satisfied with the final look presented. If you’re com- pletely satisfied, you can click OK and Filter Pack processes your color corrections to give you your final results. Figure 9-47 shows the secondary dialogs that appear when you enable the check boxes for each of the Hue, Saturation, and Value channels. FIGURE 9-47 From left to right, channel dialogs for Hue, Saturation, and Value If you’re not completely satisfied, you have two further options. For one, you may want to have more fine-grained control over the differences between the thumbnail choices that Filter Pack presents you with. You can compensate for that by adjusting the Roughness slider at the bottom of the Filter Pack dialog. Lower Roughness values reduce the amount of change between preview images. Your other option for fine-grained control is in the Filter Pack’s Advanced window. This window gives you even more control over the difference between the preview images presented to you 271 Part III: Manipulating Images in each of the filter’s sub-windows. This window is dominated by a Curves tool–like control for tweaking intermediate levels between the previews. You can also use the Preview Size slider on the right of this sub-window to increase or decrease the size of the previews so you can better see what Filter Pack is doing. Figure 9-48 shows the Advanced sub-window for the Filter Pack feature. FIGURE 9-48 The Advanced window for the Filter Pack gives you controls similar to the Curves tool. Hot The Hot filter is most valuable when you’re working on images that are destined for display on television in older standard definition (SD) formats like PAL or NTSC. The issue with older SD televisions is that historically they’re not particularly good at displaying colors or values at full saturation or luminosity. So to compensate for that television editors and graphic artists have to work in ‘‘television-safe’’ colors, kind of like the Web-safe color palette. However, GIMP doesn’t natively limit you to working only in the NTSC or PAL color spaces. This is where the Hot filter comes in. Launch this filter from Colors  Hot and you get the dialog shown in Figure 9-49. The options in this dialog allow you to adjust your image’s histogram to fit within either the NTSC or PAL color spaces. Furthermore, it also gives you the chance to create a new layer in your image with the modified version of your image. This way you can compare it with the orig- inal and see if there are any glaring problems with the automated color conversion that this filter implements. Maximum RGB The Maximum RGB filter (Colors  Maximum RGB) has the relatively simple dialog box shown in Figure 9-50. This filter works by analyzing each pixel in your image and determining which of the three chan- nels (red, green, or blue) is the most dominant or least dominant for that pixel. Which one it chooses to display is determined by the radio buttons in the dialog: 272 Chapter 9: Adjusting Colors  Hold the Maximal Channels — The dominant channel is retained. The other two chan- nels are discarded.  Hold the Minimal Channels — The least dominant channel is retained and the other, more dominant channels are discarded. FIGURE 9-49 The Hot filter’s dialog FIGURE 9-50 The Maximum RGB filter dialog Regardless of which channels are retained and which are discarded per pixel, the end result is an image that consists entirely of the primary colors red, green, and blue. Retinex Typically speaking, the human eye is much better at seeing in low light conditions than the electric sensor of a camera. Therefore, cameras don’t always pick up the detail in low light that humans would find critical for correctly understanding what’s going on in the image. The Retinex filter is an attempt to enhance dark images in a way that’s more akin to the way the human eye works. The core of the filter is in the MultiScale Retinex with Color Restoration (MSRCR) algorithm. The term Retinex is a combined abbreviation of retina and cortex. It works well on underexposed photographs and is often used to show more detail in astronomical photographs and medical images like x-rays and MRIs. When you activate this filter by going to Colors  Retinex, you get a dialog like the one in Figure 9-51. 273 Part III: Manipulating Images FIGURE 9-51 The Retinex filter’s dialog The key to the MSRCR algorithm is that it attempts to replicate the behavior of the human eye. That being the case, many of the settings in this filter’s dialog are a bit obtuse and lean toward the technical side of things. Explanations of the inner workings of this algorithm are beyond the scope of this book, but if you’re interested, there’s a good reference paper online at www.dti.unimi.it/rizzi/papers/josa.pdf. Fortunately there aren’t too many controls, so this filter is pretty conducive to a healthy amount of fiddling and playing with the controls to get a good feeling for how they behave. The default values tend to yield acceptable results, but the following descriptions of each control setting should help you attain better results:  Level — The Level drop-down gives you three options that deal with the light intensity in the image.  Uniform — This is the default value. It treats both high and low intensity areas evenly and tends to yield the most natural results.  Low — The results of this option tend to look a bit like simple edge detection. The emphasis here is on the low-intensity details of the image at the expense of washing out the rest of it.  High — This option tends to emphasize the clearest part of the image, accentuat- ing contrast and enhancing the colors. If the results from the Uniform option appear muddy, this option should appear cleaner.  Scale — The lower this value is, the more rudimentary the filtering is in the results. At low values, you may still see details in the image, but there’s often a glow or color shift around features that reduces the clarity of the image overall. Typically, the optimal setting for this option is a value of 240 and therefore that’s also the default value. 274 Chapter 9: Adjusting Colors  Scale Division — The Retinex filter is an iterative filter, meaning it runs its core algorithm multiple times when you click OK. This value controls how many iterations are used. The default value of three gives good results. Setting the Scale Division too high not only increases processing time, but it also tends to make your final image a bit noisy.  Dynamic — Of the controls for this filter, this one is the most image-dependent. Higher values reduce the saturation and overall contrast of the results and lower values tend to blow out the details in the image. The biggest thing to know about the Retinex filter is that it can sometimes take a while to finish processing. This is particularly true on larger images. That’s something to keep in mind as you monkey around with the settings. Be careful when adjusting them (especially the Scale Division slider), or you may be waiting a while for GIMP to finish processing after you click OK. Figure 9-52 shows the results of the Retinex filter for each of the three Level options. FIGURE 9-52 From left to right, the original photograph and that same photograph with the Retinex filter applied using Uniform, Low, and High levels (Photo credit: cgtextures.com) Exporting Color Separations with the Separate+ Plug-in Earlier in this chapter you saw how the Decompose operation could be used to take images from GIMP’s native RGB color space and spit out color plates for channels in a variety of other for- mats, including HSL, LAB, and CMYK (pronounced in the GIMP community as ‘‘schmuck’’). As part of that discussion, I recommended against using Decompose to produce your CMYK plates. The reason for that is because you typically want to go to CMYK only when you want to take an image to a printer using a traditional four-color process. That being the case, straight CMYK likely won’t cut it. The reason for that is because GIMP’s Decompose feature doesn’t take any color management profiles into account. When it comes to getting computer graphics off of a computer monitor and on to some other media, color management becomes very important. It’s through the color profiles used by color management systems (CMS) that hardware printers and 275 [...]... color and then uses the grayscale channel as a mask for that color By doing that and compositing the layers on the same image, you get a rough composite of what the final color image looks like Figure 9 -55 shows the color separations that Separate+ creates as well as the layer structure when you have Make CMYK Pseudo-Composite enabled FIGURE 9 -55 The CMYK color separations for a photograph and the layer... Character menus These menus are specifically related to the language that you’re writing in and 2 85 Part III: Manipulating Images they’re accessible at the very bottom of the Tool Options panel for the Text tool, under the label of Language Simply right-click in this text field and you get the exact same context menu that appears when right-clicking in the Text Editor FIGURE 10 -5 On the left, the Input Methods... this field, GIMP gives you a hanging indent That is, the first line of text aligns with the left of the frame while the rest of the paragraph is indented Figure 10-9 shows an example of what changing the values in this field does 289 Part III: Manipulating Images FIGURE 10-9 If you set the indentation value to 50 pixels, you get a normal indent like the one on the left If you set the value to 50 pixels,... on the text you enter in the image window The next option is the text’s Size By default, this is defined in pixels and you can adjust the value by typing it in or using the up/down tumbler buttons on the right of that number value If you want to describe the text size in different units, such as points, picas, inches, or any of the other units that GIMP recognizes, you can choose those units from the. .. colors throughout the design You can also explicitly add colors to the history by clicking the arrow button to the left of the history swatches And because this is a persistent history within GIMP, these swatches are available to you each time you run GIMP, regardless of whether you’re re-opening the same image file FIGURE 10-8 The Text Color dialog gives you just a little bit more than the Colors dockable... text in the editor first and then get it to fit in the frame once you’re done To enable the Text Editor window, go to the Text 284 Chapter 10: Working with Text tool’s Tool Options in the Toolbox and enable the Use Editor check box Upon doing so, you get a window like the one shown in Figure 10-4 FIGURE 10-4 GIMP s Text Editor window is another place where you enter the text that appears in the text... on the right, the Insert Unicode Control Character submenu Below the text entry area for the text editor is a Use Selected Font check box When you enable this option, whatever font you’ve chosen in the Text tool’s options appears in the text area The size of the font doesn’t change in the text area, but being able to see the font here certainly helps get your basic text layout visible from within the. .. Separate, as shown on the left here On the right is the Separate+ dialog called when you go to Image Separate Separate+ The real functionality you want to work with is at Image Separate Separate+ The two most important controls in this dialog are dictated by the buttons next to the Source and Destination Color Space labels These define the color space that you’re working with in GIMP and the color space that... and GIMP recognizes them Currently, though, GIMP allows you to control only letter spacing and not kerning Note You might notice that none of the spacing values have any units associated with them This is because they use the units that you stipulate at the top of the Text tool’s options panel for your font size Pay close attention to this, because GIMP doesn’t automatically convert units for you on these... than along the shape of a path Of course, the biggest disadvantage is that the text along the path is not directly editable as text You don’t have a ‘‘what you see is what you get,’’ or WYSIWYG editing experience here If you want to edit the content of your text, you need to use the Text tool on the original text layer and then repeat the previous process to flow the newly edited text along the path This . techniques The best way to approach the Filter Pack plug-in is to work down the left column of the dialog and then work down the right. So the first thing you want to set is what gets shown in the pre- view. dialog like the one in Figure 9 -51 . 273 Part III: Manipulating Images FIGURE 9 -51 The Retinex filter’s dialog The key to the MSRCR algorithm is that it attempts to replicate the behavior of the human eye just the active layer, the Separate+ functions are not found in the Colors menu. Instead, go to Image  Separate to see the Separate+ submenu, shown in Figure 9 -53 . FIGURE 9 -53 See the items in the