General Preferences: Luminance
You can adjust the settings for Image>Collect Luminance here.
The more information collected, the larger the tracing files.
Five elements are always collected: mean luminance, standard deviation, minimum luminance, maximum luminance, and total number of pixels. In addition, you have the option to store the histogram and the sampled image.
Save Image Histogram
If the original image was a color image, the color pixels are converted to luminance values. The histogram describes the distribution of luminance values in the sampled area. Adding the histogram has a minor impact on the size of the tracing files.
The luminance histogram is calculated as (.299 * Red) + (.587 * Green) + (.114 * Blue) unless separate color channels are selected.
Save Image
Saving the image provides the most comprehensive information about the area sampled, but it can greatly increase the size of the tracing files if the contour is large. Save Image does not save the histogram, but gives future access to the histogram since the image itself if saved and the histogram can be generated again.
Saved Image Format
The images can be saved in two different formats.
Save as luminance image: Converts the original image to luminance values before storing the image. It uses 1/3 less memory than Save as original image when color images are used.