The Histogram Palette

A histogram is a graph depicting the intensity of different values in your image. Photoshop's Histogram palette is capable of showing the brightness values as well as the combined color and individual color values in an image. Keep it up on-screen and you're never more than a glance away from knowing the distribution of levels in your image at any given time.

Choose Window Histogram to display the Histogram palette. By default, the palette opens in its Compact View, featuring only a graph that looks like a mountain range. That mountain range is in fact the histogram, which is a bar graph of all the colors in the image. The histogram comes to us from the world of statistics, where it measures a fixed number of relative values. As it just so happens, the same holds true for Photoshop. Your typical scanned image or digital photograph is an 8 bits per channel, full-color RGB file that contains 256 brightness values per channel, ranging from black to white. By default, the Histogram palette shows a composite view of all color channels. So it follows that the histogram contains precisely256 vertical bars, each one pixel wide, so the entire histogram invariably measures 256 pixels wide, regardless of your screen resolution.

As illustrated in Figure 12-8, the histogram starts at black on the left-hand side and progresses through increasingly lighter colors, finally ending at white on the right. The numerical value for black is 0, and the value for white is 255, giving you a total of 256 possible whole-number variations. The height of each bar in the graph tells you the relative number of pixels inside the selection or layer associated with that brightness value. This means you can use the histogram to gauge the distribution of darks and lights in your image. Mind you, there's no need to count pixels; the histogram is just there to give you a rough sense of what's going on. And with enough experience, it becomes an invaluable tool, serving as a kind of reality check for the colors that you see on-screen.

image from book
Figure 12-8: Looking a lot like a bar graph, the histogram describes exactly 256 brightness levels in a single channel or throughout the composite image, beginning at black and ending at white. This image has a pronounced spike in dark colors.

If you aren't familiar with reading histograms, Figure 12-9 serves as a quick primer. Here a series of black-to-white gradients are each accompanied by its histogram reading. First up is Photoshop's standard black-to-white gradient (stored as the preset Black, White). The histogram shows how the brightness values are largely evenly distributed, with Gaussian emphasis on the blacks and whites. The next example shows the effect of drawing a black-to-transparent gradient over the top of the first gradation. As the histogram shows, this (not surprisingly) shifts the balance of colors to the dark end of the spectrum. A white-to-transparent gradient results in more light colors, and a gray-to-transparent gradient shifts the colors to the middle.

image from book
Figure 12-9: A series of four gradients with their brightness values graphed inside histograms. A black-to-white gradient (top) has a disproportionate number of darks and lights, and colors to taper away naturally. Adding black (second), white (third), or gray (last) shifts the distribution of colors to dark, light, or medium shades.

If Figure 12-9 seems simple, good ” it's supposed to be simple. Doubtless, a histogram is a complex statistical tool that would take you hours to measure, plot, and draw by hand. Fortunately, it's Photoshop's job to do this kind of menial grunt work for us. And so far we've only seen the Histogram palette in its compact view; when you open up the palette menu and choose Expanded View or All Channels View, as shown in Figure 12-10, that's when the real grunting begins. You'll get access to the following options:

image from book
Figure 12-10: The Histogram fully expanded, displaying all channels
  • Channel: The Channel pop-up menu lets you determine the types of values displayed by the Histogram palette. By default it's set to the color mode in which you're working. If you're editing an image in Grayscale mode, this menu is unavailable.

    From the Channel menu, you can choose to display the default combined tonal levels or any of the color channels individually. Choose Show Channels in Color from the palette menu to display the individual channels in the colors they represent. Any additional channels you have created can also be selected from the Channel pop-up menu. Choose Luminosity to display a histogram of only the brightness levels in your image. Lastly, you can call up an overlapping composite graph of all of the color channels by choosing Colors.

  • Uncached Refresh: Whenever you make an adjustment to an image that affects its brightness or color intensity levels, the histogram redraws itself to compensate. It saves time doing this by analyzing the already existing cache of the image and guessing how the change you've made affects the graph. The Histogram palette uses the image cache to automatically regenerate unless you click the Uncached Refresh button in the upper-right corner, which tells the palette to redraw the graph based on the current image. Photoshop provides a number of other ways ” one might even say too many ways ” to refresh the histogram. You can also click the triangular cached data warning icon that appears in the top-right corner of the histogram whenever you're viewing a graph created from the cache. Additionally, you can accomplishthe same thing by doubling-clicking on the histogram itself. Finally, you can choose the Uncached Refresh option from the palette menu.

  • Source: In a multilayered image, you can tell the Histogram palette to display values for the Entire Image or just the Selected Layer via the Source pop-up menu. If the image contains any adjustment layers, select one of them and choose Adjustment Composite from the Source pop-up menu to display a histogram of the layers being affected by that adjustment layer.

Beneath the Source menu in the expanded view you'll find statistical information about your image or layer, which you can toggle on or off by choosing Show Statistics from the palette menu. The details in the left column are pretty arcane. The Pixels value tells you how many pixels were used to calculate the current histogram, so if you refresh the histogram using one of the plethora of ways described above, you'll see the Pixels value update to the total number of pixels in your selection or image. The statistics on the right side update as you move your cursor within the histogram itself; you can also click and drag to create a selection within the histogram if you want to see the statistics applied to a range of values. You'll probably never need any of this statistical info , but it's here if you want it.

One of the great strengths of the Histogram palette is the fact that it updates on the fly, even while you're working within one of the color adjustment dialog boxes (but only if you have the Preview check box selected). Even better, the palette doesn't simply replace the original histogram with the new one: it keeps the original histogram visible behind the adjusted histogram in a grayed-out version. It's almost as if you're viewing the Ghost of Histograms Past, as you can see in Figure 12-10.

Tip  

When you select a portion of an image, the Histogram palette only displays intensity levels and statistics for the selected area.

Although many of the other palettes become frozen when you're working inside a dialog box, you never lose access to the options inside the Histogram palette. Well, almost never. You can't access the Histogram palette options when the Filter Gallery window is open.

Starting with Photoshop CS, you saw greatly expanded support of 16-bit color. This support continues in Photoshop CS2, and is further expanded with support for 32-bit color. In case you're a little hazy on what all this means, remember that everything inside your computer comes down to 1s and 0s. Think of a pixel 8 bits long as having 8 open slots, each of which could be filled with either a 0 or 1. There are 256 possible combinations ” 00000000, 00000001, 00000010, 00000011, 00000100, and so on up to 11111111. So if a color channel can contain 256 luminosity values, then the three channels in an 8-bit/channel RGB image combine for a whopping16,777,216 possible colors. With a 16-bit/channel image, there are 65,536 possible values per channel, combining for 281,462,092,005,375 different colors. A 32-bit image will exceed the average calculator's ability to display digits. Kind of puts your box of 64 Crayolas to shame, doesn't it?

Although 281.5 trillion different colors sounds impressive in theory, theory is about as far as it goes. Your monitor can't display more than 16.8 million colors, andyou'd already need a 17-megapixel digital camera to be able to theoretically capture every possible color inside one 8-bit/channel image. And the Histogram palette only shows 256 levels of brightness, no matter the mode. So what's all the big fuss about 16-bit color? Why did Adobe bother to expand its support, now letting you work in 16-bit mode with layers, text, channels, shapes , painting tools, and three times as many filters as before? Well, even though you can't actually see all those trillions of colors, they do provide a lot of wiggle room when performing drastic color adjustments to images.

In Figure 12-11, an 8-bit/channel image appears. Image Duplicate was used to open a separate copy of the image, and then Image Mode 16 Bits/Channel was chosen to set the copy to 16-bit mode. Next, Ctrl+L ( z +L on the Mac) was pressed and the black slider dragged in the Output Levels of the Levels dialog box to 246 for both images, essentially washing them out to white. (Much more on Levels to come in just a sec.) Finally, Image Adjustments Auto Color was chosen for each image, to attempt to restore the highlights and shadows. As you can see, the 8-bit example on the top did not fare well, with heavy banding quite evident, but the 16-bit example on the bottom still maintains its clarity. Now, no one's recommending you start working exclusively in 16-bit mode. File sizes are twice as large as 8-bit, and you'll also lose most of your file format options and filters. But many specialized fields like video prefer working with 16-bit/channel images, and if you're going to be performing drastic color manipulation, the option is available.

image from book
Figure 12-11: Levels drastically washed out both an 8-bit/channel and 16-bit/channel copy of an image.The 8-bit/channel copy shows extreme banding (top), but the 16-bit/channel copy is relatively undisturbed(bottom). The 8-bit histogramshows huge gaps between levels, while the 16-bit looks smoother.


Photoshop CS2 Bible
Photoshop CS2 Bible
ISBN: 0764589725
EAN: 2147483647
Year: 2003
Pages: 95

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net