User manual

Small Vision System User Manual 13
It is important for stereo processing to have the left camera image appear as the left image in the
smallv program. Once the video input is displayed, you can check this by pointing the cameras along
your line of sight. The right camera appears on your right side, and the right image on the smallv
display should show this image. You can cover one camera with your hand, and observe which displayed
image goes dark. With dual framegrabbers or RG input, the solution to having the wrong camera inputs
is to simply switch the inputs, or to use the swap button in smallv, which interchanges the images in
memory.
Under line interlace mode (Figure 2-3), the first horizontal line of a video field is from the left
camera, the second from the right, the third from the left, and so on, making a single video stream. The
SVS software de-interlaces the video stream, reconstructing the left and right images in memory, at half
the original vertical resolution. Because of the variation in how framegrabbers determine which is the
first line of a field, the SVS software will sometimes switch the left and right fields during de-interlacing.
The swap button switches the left and right fields during deinterlacing.
2.1.5 Frame Size
The SVS libraries as delivered can work with frame sizes up to 1288 by 1032. In fact, the SVS
algorithms can work with arbitrarily sized frames.
A subset of frame sizes are supported for video input in smallv; the following table summarizes
them. Most framegrabbers support hardware interpolation and scaling, so that bus traffic is minimized by
working with smaller frames. The exception is the Meteor RGB, which passes a full field to memory,
where it is decimated by the SVS software.
Video frame size is selected with the Size drop list in the Source area. Video sizes can be
changed at any time with most framegrabbers; however, the Video for Windows interface is unstable and
may crash if frame sizes are changed during acquisition. It is always best to stop video acquisition before
changing frame sizes.
2.1.6 Image Sampling
The sampling for analog framegrabbers is implicit in the frame size. For example, if the camera
image size is 320x240, and the requested frame size is 160x120, then the full image is scaled down by the
framegrabber, usually using interpolation to produce a smooth image.
With the MEGA-D digital interface, the user has full control over the sampling method, and the
Sample and Size controls combine to produce the final result. For example, if the sampling mode is x1
(no subsampling), then and image size of 320x240 produces a subwindow within the full image.
(Subwindowing is not available from analog framegrabbers supported by SVS.) The placement of the
subwindow can be changed in real time under program control, using the dialog from the video… menu.
There are several sampling modes. Decimation samples the image by removing pixels, e.g., “x2
dec” means that every second pixel in a line is removed, and every other line is removed. Binning
samples the image by averaging over a block of four pixels, to produce the same result. Binning produces
smoother images with less noise, but it is slower than decimation, which is done by the stereo hardware.
Video Format Frame Sizes
Line interlace 160x120
320x120
Dual framegrabber 640x240
and RG component 320x240
320x120
160x120
1394 (digital) interface 1280x960, 640x480, all
others above
Table 2-2 Frame sizes available for video input
in smallv.