The Histogram PaletteA histogram is a simple bar chart that plots the tonal levels from 0 to 255 along the horizontal axis, and the number of pixels at each level along the vertical axis (see Figure 6-6). If there are lots of pixels in shadow areas, the bars are concentrated on the left; the reverse is true with "high-key" images, where most of the information is in the highlights. The height of the bars is arbitrarythey're simply comparative indicators of how many pixels have a given tonal value. Figure 6-6. Histogram paletteEndpointsWith a quick look at the histogram, you can see if the image has suffered clipping in the highlights or shadows (see Figure 6-7). If there's a spike at either end of the histogram, the highlight or shadow values are almost certainly clippedwe say "almost" because there are some images that really do have a very large number of pure white or solid black areas. But they're pretty rare. Figure 6-7. Highlight and shadow clippingFigure 6-8. Limited dynamic rangeHow Much Information Is Present?The overall appearance of the histogram also gives you a quick, rough-and-ready picture of the integrity of your image data (see Figure 6-9). A good capture uses the entire tonal range, and has a histogram with smooth contours. The actual location of the peaks and valleys depends entirely on the image content, but if the histogram shows obvious spikes, you're probably dealing with a noisy capture device. If it shows a comb-like appearance, it's likely that the image has already been manipulatedperhaps by your scanning software or camera firmware. Figure 6-9. A previously manipulated imageHistograms Are GeneralizationsOnce you've edited an image, the histogram may look pretty ugly. This is normal; in fact, it's almost inevitable. The histogram is a guide, not a rule. Histograms are most useful for evaluating images before you start to edit. A histogram can show clipped endpoints and missing levels, but a good-looking histogram isn't necessarily the sign of a good image. And many good-looking images have ugly histograms.Fixing the histogram doesn't mean you've fixed the image. If you want a nice-looking histogram, the Gaussian Blur filter with a 100-pixel radius will give you one, but there won't be much left of the image! The Histogram is just a handy way of looking at the data so that you can see how it relates to the image appearance. The image is what matterstake what useful information you can from the histogram, but concentrate on the image.Tip: Don't Forget to Refresh the HistogramFor performance reasons, while you work on an image, the Histogram palette shows you values based on the antialiased screen display of your image. This view can hide posterization, giving you an unrealistically rosy picture of your data. Photoshop warns you of this by displaying a warning icon when the histogram is showing you an approximation of your data. To see what's really going on, click the Refresh button in the Histogram palette. (See Figure 6-10.) Figure 6-10. Histogram warning |