Apple Shake 4 - Manual (Page 633)

Chapter 23 Color Correction 633
Setting Up the PixelAnalyzer Node
Attach a PixelAnalyzer node to the problem image. It will eventually be used as a source
of color values by expressions placed within the parameters of color-correction nodes.
The color-correction nodes are not attached to the PixelAnalyzer node; instead, they’re
branched off of the source image. Three different examples show three different color-
correction nodes in use—different situations may require different approaches,
depending on the image.
Note: To accurately analyze changes in brightness, the PixelAnalyzer node’s analysis
area should be positioned over the brightest area of the image.
Method 1: Using an Add Node
Attach an Add node to the image, then enter the following expression into its red,
green, and blue channels:
(PixelAnalyzer1.area1AverageRed@@1)-PixelAnalyzer1.area1AverageRed
(PixelAnalyzer1.area1AverageGreen@@1)-PixelAnalyzer1.area1AverageGreen
(PixelAnalyzer1.area1AverageBlue@@1)-PixelAnalyzer1.area1AverageBlue
The first part of the expression takes the first frame of the image as the base value
(specified by @@1). The average channel values from all other frames are compared to
frame 1. For every frame, the current channel value is subtracted from that of frame 1.
For example, if at frame 1 the average red value is .5, and at frame 10 the average red
value is .6, the above expression subtracts .1 from frame 10 to arrive at .5 again.
Note: To avoid problems when analyzing images with a lot of noise or grain, use the
PixelAnalyzer node’s Average value parameters.
In one possible scenario, examining the resulting image with the PlotScanLine viewer
script might reveal that the midtones are OK, but that the darks are creeping down.
This might indicate that the change in brightness is not occurring because of addition,
but perhaps is a result of multiplication.
Method 2: Using a Brightness Node
If the Add node didn’t provide satisfactory results, a Brightness node might have a
better effect. Attach a Brightness node, then enter the following expression into the
value parameter:
(PixelAnalyzer1.area1AverageRed@@1)/PixelAnalyzer1.area1AverageRed
The expression uses the same basic approach as in Method 1, except that the color
value from frame 1 is divided by the current frame’s color value. Since Brightness is a
multiplier, this makes an adjustment based on the difference. As a result, if at frame 1
the average red value is .5, and at frame 10 the average red value is .6, the above
expression multiplies frame 10 by .83333 (.5/.6) to arrive at .5 again.