Analyzing Image Information with the Histogram


A histogram analysis is a tool found in many imaging applications and camera LCD readouts. It is a graph that plots the brightness values of an image from the blackest to the whitest. By looking at a histogram, you can tell such things as where the pixels are concentrated in an image (in the shadows, midtones, or highlights), whether the overall tonal range is smooth, and whether the shadows or highlights are clipped. Knowing how to read a histogram is critical to making informed decisions about an image at every stage of production, from capture to output.

In Aperture you can view five types of information in the histogram: luminance, combined RGB channel data, and separate red, green, and blue channel information. Even though our Five Star Location images don't need heavy-duty image analysisthat's more important for final shotsthe histogram is an important resource, and we can use it to help us make some additional image adjustments.

1.

Select the SoBe_2005 4 of 50 image in the Browser.

This image, shot before sunrise, captures the ethereal quality of natural light at dawn combined with artificial light in the windows of the hotel. Because most of the image's pixel values are quite dark, they are concentrated on the left side of the luminance (default) histogram at the top of the Adjustments Inspector.

2.

Choose Histogram Options > RGB from the Adjustments Action pop-up menu.

The histogram is updated to show the RGB image data so that you can see not only the three individual color channels but also where their channel information overlaps.

Let's apply an automatic adjustment to make the image look as though it had been taken after sunrise rather than before, and then see how the histogram reflects that change.

3.

Click the Auto Exposure button.

Aperture brightens the image, and the values in the histogram shift to the right, toward highlights.

Comparing Images with the Histogram

Another productive way to use the histogram is to compare data in different images.

1.

Click Jackson Hole 9 of 23 and then Command-click Jackson Hole 11 of 23 in the Browser to select them both.

Since we previously adjusted the Jackson Hole 9 of 23 image, its histogram should be quite different from the histogram of Jackson Hole 11 of 23.

2.

Using the Adjustments Action pop-up menu, switch the Histogram view to Luminance. This will allow you to focus on the luminance values of the images.

3.

Alternately click the two images in the Viewer and notice the differences in the histograms. The Jackson Hole 11 of 23 image has far fewer bright values.

Jackson Hole 9 of 23 and its histogram

Jackson Hole 11 of 23 and its histogram

4.

Command-click Jackson Hole 9 of 23 to deselect it. Apply the Auto Levels Combined adjustment to the Jackson Hole 11 of 23 image. Compare the before and after histogram by selecting and deselecting the checkbox next to Levels.

The Auto Levels adjustment nudged the values in the image toward the brighter areas without significantly altering the image color, just as it's supposed to do.

The histogram is your main source for analyzing image data in Aperture. The secondary histogram in the Levels area of the Adjustments Inspector shows unadjusted values, so you can compare data. (Click the Levels disclosure triangle to reveal or hide the secondary histogram.) You will learn more about the Levels controls group and both histograms in Lessons 7 and 9.




Apple Pro Training Series(c) Aperture 1.5
Apple Pro Training Series: Aperture 1.5
ISBN: 0321496620
EAN: 2147483647
Year: 2007
Pages: 190

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