May 27, 2004
Background Level Illusion
The preceived gray level of an object depends strongly on the background
gray level. This has implications when using a gray level scale to read
intensity values from a printed image (rather than using the Info window,
and reading the pixel values on a digital image.)
- Make a new window with a ramp in it. Open
the ready-made ramp in sample images / ramps / ramp-550x400.tif.
- File -> New
- Edit -> Select All
- Edit -> Draw Scale
- Click on pencil tool.
- Click on the rectangle selection tool.
- Edit -> Copy Selection Edit / Copy
- Edit -> Paste Edit / Paste
- Now, by clicking and holding down the mouse button somewhere within the
rectangle, the rectangle can be dragged over the image, apparently (but not
actually) changing in brightness as it is moved .
- The rectangle can be pasted in various places by dragging it there, then
using the Edit -> Paste Selection menu
or just <control>-v. (Similar step done
in arc illusion exercise.)