Aids for Stereo with a single camera
SDM has two aids for taking and previewing stereo image-pairs taken with a single camera.
They are edge overlay mode and the rangefinder function for taking and stereo playback as grey or colour anaglyphs or side-by-side for playback viewing.
Edge Overlay Mode
For taking the images, 'Edge Overlay'(or Outline) mode allows you to determine if there is excessive deviation or perspective deviation.
To toggle this mode on/off, press FuncSet when in <ALT> mode until you hear a beep.
(a shorter press toggles all onscreen displays on/off, including edge overlay).
The menu options are in menu 'Stereo/Edge Overlay'.
You can choose to display the overlay with other OSD's or with no OSD's.
Point your camera at the subject and half-press the shutter.
A brightly coloured outline will appear around parts of the subject.
You only need to be able to detect a few reference points at the nearest and furthest distances.
Fully-press the shutter to capture the image, its outline will re-appear and it will say 'Frozen' on the display.
Move slightly to your right and line-up the nearest part of the subject with its outline from the previous shot.
The background should be no more than about 2mm (depending on screen size) from its previous position.
If it is, you have moved too far and the deviation is excessive.
Take your second image.
To preview your results, switch to playback mode, select the first (left) image and half-press the shutter.
Now select your second image and half-press the shutter again to see a monochrome anaglyph preview.
(users of the s2is, s3is, s5is and A550 should use the SetFunc button instead of half-press).
This is intended to show the accuracy of hand-held but may also be viewed in stereo with red/cyan glasses, especially on larger screens.
Alternatively, you could try connecting an external display via the AV socket.
(there is a problem with some of the latest supported cameras in that the AV and USB connections are combined in a single socket.
See here for a possible solution.)
This works on most cameras, but not all, and on some cameras the odd/even position of the images in your browsing sequence can prevent the creation of the anaglyph.
You can change the threshold value and colour for the edge outline from the menu if the default setting is not suitable for a particular situation.
If there is far too much overlay, increase the threshold level.
Visibility outdoors in bright sunlight and/or on small screens can be a problem and a DIY or commercial screen shade may be needed.
This mode is also useful for studio and outdoor macro shots (under suitable conditions) because you can instantly determine background deviation and perspective distortion due to change of viewpoint.
For close-ups, especially at the wide-angle setting, the slightest shift produces very different perspective views.
For hyper landscapes you could capture the image from the first viewpoint and then walk to a second viewpoint and line-up the subject with its outline and determine resulting stereo-depth.
It is also useful for stop-motion animation (a technique known as 'onion skinning').
You may save the edge overlay to a file for later reloading.
The file is located in CHDK/EDGES folder and as well as containing a snapshot of the overlay buffer, it also contains the zoom position.
When the overlay is reloaded the camera automatically zooms to the correct position.
You could visit a location a year later and setup the cameras to frame exactly the same view.
As SDM requires far less memory than CHDK, memory-usage should not cause a problem.
If it does, there is an option to release memory (after disabling edge overlay mode).
One of the OSD's displays the amount of free-memory available.
Single Camera Rangefinder
As the Canon cameras presumably attempt to measure distance by changing focus until image-contrast is maximised, this makes accurate ranging difficult.
The very great depth-of-field (DOF) of small-sensor cameras exacerbates the problem, especially at the wide-angle setting, not just because of the great DOF but also the great angle-of-view that includes subject from near to far all more-or-less in-focus.
SDM allows you to measure subject distance using the extreme telephoto setting.
You should set the autofocus area to the middle of the screen.
Make sure that you have set 'camera spacing' to correct value in 'Stereo' menu.
In <ALT> mode, browse to the 'SINGLE CAMERA' OSD and then exit <ALT> mode.
Zoom to maximum telephoto, aim the camera at the nearest part of the subject and half-press the shutter until the 'Near' value is updated.
Repeat for farthest part of subject until 'Far' is updated.
Zoom-back to the desired focal-length.
The 'Shift' value displayed is the amount you would move the camera in order to create the maximum stereo depth (that is acceptable to you) for those near/far distances.
Your maximum acceptable stereo-depth is defined by the maximum-acceptable deviation (defined as a fraction of image-width).
You set it as 'deviation factor' in the stereo menu.
If the near distance is coloured red, it means that you are too near to the subject if that amount of shift is used.
This is based on the 'Near-subject factor' that you have entered in the Stereo menu.
It determines the how much perspective distortion due to change of viewpoint you think is acceptable, even though deviation may be within limits.