Adobe Photoshop CS4 - Manual (Page 163)

156 USING PHOTOSHOP CS4
Color and tonal adjustments
Last updated 1/10/2010
Cache Level Shows the current image cache used to create the histogram. When the cache level is higher than 1, the
histogram is displayed faster. In this case, the histogram is derived from a representative sampling of pixels in the
image (based on the magnification). The original image is cache level 1. At each level above level 1, four adjacent pixels
are averaged to arrive at a single pixel value. So, each level is half the dimensions (has 1/4 the number of pixels) of the
lower level. When Photoshop makes a quick approximation, it can use one of the upper levels. Click the Uncached
Refresh button to redraw the histogram using the actual image layer.
View the histogram for a multilayered document
1 Choose Expanded View from the Histogram panel menu.
2 Choose a setting from the Source menu. (The Source menu is not available for single-layered documents.)
Entire Image Displays a histogram of the entire image, including all layers.
Selected Layer Displays a histogram of the layer that’s selected in the Layers panel.
Adjustment Composite Displays a histogram of an adjustment layer selected in the Layers panel, including all the
layers below the adjustment layer.
Preview histogram adjustments
You can preview the effect on the histogram of any color and tonal adjustments.
❖ Select the Preview option in the dialog boxes of any color or tonal adjustment command.
When Preview is selected, the Histogram panel shows how the adjustment affects the histogram.
Note: When making adjustments using the Adjustments panel, changes are automatically reflected in the Histogram
panel.
Preview of histogram adjustment in the Histogram panel
A. Original histogram B. Adjusted histogram C. Shadows D. Midtones E. Highlights
Refresh the histogram display
When a histogram is read from a cache instead of the current state of the document, the Cached Data Warning icon
appears in the Histogram panel. Histograms based on the image cache are displayed faster and are based on a
representative sampling of pixels in the image. You can set the maximum cache level (from 2 to 8) in the Performance
preference.
Note: A higher cache level setting will increase the redraw speed for large, multi-layer files, but requires additional usage
of system RAM. If RAM is limited or you work mainly with smaller images, use lower cache level settings
❖ To refresh the histogram so that it displays all of the pixels of the original image in its current state, do one of the
following:
• Double-click anywhere in the histogram.
• Click the Cached Data Warning icon .
• Click the Uncached Refresh button .
A
B
C D E