Edge Overlay - Canon Camera Hackers Manual

4.7

Edge overlay

The CHDK Edge Overlay function is able to extract the edges from an image
as soon as you half-press the shutter button and overlay the resulting dia-
gram with the current content of the display. This allows viewing the previ-
ous shot and the current shot in context, making it easy to register images
and apply techniques such as onion skinning
Edge overlay has applications in panorama photography, stereo pho-
tography (with a single camera), time-lapse photography, bracketing
photography, and more.
Of course, it is possible to choose the color of the edge overlay (menu
entry Edge overlay color) or to show the edge overlay in Replay mode. A
threshold value (Edge overlay threshold) determines which brightness dif-
ferences are regarded as edge. The lower this value, the more and wider
edge lines you get; higher values create fewer and finer lines.
With the menu entry Lock Edge Overlay, you can temporarily freeze an
existing edge diagram so that it is not replaced by a new one when you
half-press the shutter button. Typically, you would use this option when you
want to register several images with one and the same reference image.
Edge overlays can be saved (Save Edge Overlay) to the folder CHDK/EDGE/
on the memory card. This allows you to use them at a later time. The func-
tion Load Edge Overlay opens the file browser (section 4.10.1), allowing you
to select an edge overlay file.
For example, if you want to photograph a tree at different times
throughout the year, you could save an edge diagram of the first picture
and reload it when you return to the scene. By registering the diagram with
the image in the display, you can be sure that the tree will always have the
same position in all images.
The zoom level is saved with the edge overlay as well, so when you re-
load an edge overlay (Load Edge Overlay), the camera is optionally set to the
stored zoom level if option Load+Set Zoom is enabled.
7 The term onion skinning refers to a technique of creating animated car-
toons by overlaying individual frames drawn on onionskin paper.
7
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4.7 Edge overlay
Figure 4-40
Edge overlay during a panorama shot.
When the first image is taken, the
CHDK extracts the edges of the
subject. Then the camera is turned.
Because the first image is still visible as
a "ghost", the photographer gets the
opportunity to register the next shot
with the previous one.
Figure 4-41
The Edge overlay submenu
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