Contents

Inversion Illusion

(Intensity and Up-Down)

We interpret two dimensional images of three dimensional objects using shading as a visual cue, assuming light to come from above.

• Load Sample Images / Filter Samples / scaled 06 2000.tiff, a SEM image of a ductile fracture.
• (This exercise might be repeated with the more easily interpreted Sample Images / Filter Samples / face.tiff, which does not show the same illusion, or with the Sample Images / Illusions / Mountains.tiff, where the illusion is so strong that the image is difficult to interpret when inverted.

• Note the 'hole' in the center.
• Try inverting the pixel values (this is not the operation to give the illusion, but might be done by mistake, so do it anyway) using the Edit -> Invert menu.

The 'hole', might still appear to be a hole, although this 'negative' image is difficult to interpret.

• Invoke the Edit -> Invert menu again to restore the image to its original form. (Or use the File -> Revert to Saved menu or Edit / Undo).
• Invert the image using the Edit -> Flip Vertical menu Process / Rotate / Flip Vertically.

The 'hole' doesn't look much like a hole any more (at least to me), although I still can see all the other little 'holes' correctly.