May 27, 2004

Contents

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.
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• Click on the rectangle selection tool.
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• Edit -> Copy Selection Edit / Copy or <control>-c
• Edit -> Paste  Edit / Paste or <control>-v
• 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.)
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