The Perspective Correction module provides an image transform that corrects for
the effects of perspective. Perspective is created whenever a scene has
depth and will distort objects to appear larger the closer they are to the
camera source and smaller when further away. The Perspective Correction module
can undistort some of the perspective effects to provide a more calibrated
view for successive processing.
The effects of perspective can be readily seen when viewing a grid like surface
stretching out into the distance. Measuring the pixel size of any of the square
boxes will reveal that they shrink with distance. This can cause problems when
attempting to map out obstacles in from of the robot as their sizes cannot be
easily determined unless perspective is taken into account.
The Perspective Correction module works by effectively rotating the image
such that it appears that you are looking straight down at the scene instead
of at an acute angle. When viewing straight down at the scene square objects
will appear square and object sizes relative to each other are comparable.
Note that the Perspective Module is only working with the current 2D image and
is unable to perform a true 3D rotation due to the lack of data. However, despite
this limitation a perspective undistorted routine is a very valuable tool to have
for image processing.
1. Tilt - specify the amount of perspective correction to apply to the current image.
Note that the perspective correction is quite sensitive so the tilt number will
be very small typically around 0.001 to 0.003.
2. Transform X - select to apply the perspective transform to the horizontal
axis. Applying the transform to the horizontal axis will expand the image towards
the top to undo the horizontal shrinking of objects due to perspective.
3. Transform Y - select to apply the perspective transform to the vertical
axis. Applying the transform to the vertical axis will lengthen objects
that are shortened due to perspective.
4. Scale - specify a number greater than 1 to zoom into the image, specify a
number less than 1 to zoom out of the image.
5. Stretch X - manually stretches the image in the horizontal direction.
6. Stretch Y - manually stretches the image in the vertical direction.
7. Image Size - specifies how to crop the image dimensions based on the
result of the transform. Note that the transform can create very VERY large
images if a high tilt rate is specified. Use the Image size selections to automatically
crop the image to a workable size.
Maintain current size - centers the transformed image into the current image size
Scale to current size - automatically scales transformed image into current image size
Expand Width & Height - grows the image to fit the entire transform into view. Note
that this can cause delays in display while calculating such a big transformation.
Expand Width - only expand the image horizontally based on the resulting transformed
Expand Height - only expand the image vertically based on the resulting transformed
Expand to Stretch - expand the image size based on the stretch factors used in
relation to the current image size.
8. Pixel & Bilinear Resample - select the interpolation mode. Pixel is fastest but blocky. Bilinear is smooth but slower.
9. Grid Size - specify a grid size that will be superimposed over the current image
to help in alignment.
10. Fill Color - specify the color used to fill border areas that are out of bounds of the image. As
the image is warped in a non-square way there will be gaps between the image and the edge borders. The
fill color specifies what color is used to fill in these areas.
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