CSC 1040 -- Computing with Images
Lab 7: Visual Perception
In this lab we will use Photoshop to create several optical illusions
and learn about visual perception.
Lecture slides: ppt pdf
Topics:
Basic physiology of the eye
The spectrum of visible light
Trichromacy in humans
Opponent model of vision
Optical illusions
Assignment:
1. Start with a 1000x1000 image
a. Fill it with a black background (use the paint
bucket to fill it with black).
b. Create a layer on top of the black and paint a circle of
the
color (red=123, green=46, blue=0). Think about how you would
describe this color.
c. Add a new layer and paint
it uniformly white (use the paint bucket again). Position the white
layer between the black (background) and the layer with the
circle.
d. Observe the color of the circle again.
You can go back and forth between the black and the white backgrounds
by hiding and unhiding the white layer. What do you notice?
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2. Start a new 1000x1000 image
a. Use the patern stamp to paint a background -
you can choose whatever you like here
b, in a new layer, using the rectangular marquee
tool,
select a rectanglular portion of the image; paint this rectangle red
(255,0,0)
c. Using the marquee tool, click and drag
to move the selectted area so that you have a rectangle adjacent and
below the first one; paint this one a dark red (127, 0,0)
d. Deselect (so that now you are working with the
entire image
e. In a nuew layer, create a sharp gradient from black to
transparent (note: NOT from black to white); then vary the opacity of
this layer down to 50%.
f. Double click on the
forground color to bring out, the color picker and, with the eyedropper
tool, examine the areas of this image. Write down your observations
here.
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3. Explore some more optical illlusions and write your observations here:
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References for further reading:
David Hubel, "Eye, Brain, Vision" available online at http://hubel.med.harvard.edu/index.html
-- online book - a good source of information about the eye and seeing
from a clinical perspective
David Briggs, "The dimensions of color" webpages http://www.huevaluechroma.com/
- a set of webpages that give a great overview of color as it is
perceived, from the artist's perspective
eChalk optical illusions http://www.echalk.co.uk/amusements/OpticalIllusions/illusions.aspx