Canon Camera Hackers Manual page 63

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C H A P T E R 4 Teach Your Camera New Tricks
RAW sum. This function adds the pixel values of all marked RAW files.
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The effect is similar to taking an exposure with a longer exposure time.
For example, if you add 10 images, each exposed at 15 seconds, the re-
sulting image will almost look like an image taken with an exposure
time of 150 sec. This operation is very similar to the double exposures
taken with analog cameras.
The resulting image gets the same name as the last of the marked im-
ages, but the file name extension is set to .WAV in order to identify the
file as the result of the RAW merge operation.
RAW average. A similar operation, but it computes the average of the
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pixel values instead of their sum. Thus, 10 images exposed at 15 sec-
onds will produce an image with the exposure time of 15 seconds. The
main reason for using this operation is to reduce noise. Averaging im-
ages is the most effective method of noise reduction without losing
detail. Make sure that the single images are perfectly aligned and that
there are no moving objects. Mounting the camera on a sturdy tripod
and using the series function (section 4.6.2), an intervalometer script
(section 5.7.1), or a remote control (section 4.9) is a must. If you don't
have a tripod at hand, see section 4.3.6.
RAW develop. This function performs the same as the one discussed in
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section 4.5.5 but applies RAW development to all marked images.
Purge RAW. This function purges all orphaned RAW files (but not DNG
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files) that don't have a corresponding JPEG file from a selected folder.
When applied inside a folder, it purges only orphaned RAW files that are
not marked. Therefore, marking a file can protect it from the Purge op-
eration. Typically, the Purge operation is used after you have deleted
some images with the native camera function. The native function only
deletes JPEG images, not the corresponding RAW files. So, purging can
reclaim some memory. Warning: Purging cannot be undone. It is safest
to do a backup of the card before applying the Purge function.
Sub from marked. This function subtracts the currently highlighted im-
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age from all marked images. The resulting files get names composed
from the original file names, the Raw subtract prefix, and the Raw sub-
tract extension. Typically, image subtraction is used to detect move-
ments in a scene (for example, astronomers find new supernovas by
subtracting images). Again, use a sturdy tripod and a self-timer or re-
mote control (section 4.9) to trigger the shutter. If you unintentionally
introduce some shift between the images, relief images are more or less
the result.
DNG > CHDK RAW. The operations above all require true RAW files. But
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if your shots are DNG files, all is not lost. Using this function, you can
convert marked DNG files back to RAW.