Hints on Photoshoppe

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Lecture 21
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Hints on Photoshoppe

• Your axes need to have replacechildren set
  before you do anything else to them
• The moment you invoke image or imagesc, the
  axes properties will be lost if you havent set
  replacechildren

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Setting Axis Size

• Your axes display a region of the plane
• image and imagesc decide what that region should
  be
• They generally make it larger than you want
• Fix with
• set(ltaxeshandlegt, XLim, xmin xmax)
• set(ltaxeshandlegt, YLim, ymin ymax)

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Image positioning

• 3 x 2 image displayed in plane
• Each pixel occupiesa small square
• Pixel (i,j) centeredon location (i,j)

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Zooming

• Theres a matlab procedure, zoom that will help
  you with zooming

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Getting a mouse click

• function PhotoAxes_ButtonDownFcn(hObject, ...
• pts get(hObject, 'CurrentPoint')
• pts is a 2x3 matrix
• pts
• x1 y1 z1
• x2 y2 z2
• We only want x and y.
• For non-rotated axes, x1 x2, and y1 y2.
• So
• x pts(1,1) y pts(1,2)

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Making the project easy

• Any code you copy from the handout
• step through it in the debugger
• understand every line of code
• read the documentation

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Fun and Games

• Genetic Images
• Due to Karl Sims, ca 1989
• Treat each point in an image as having (x,y)
  coordinates
• Color is a function sometimes complicated! of
  x and y

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Color function

• Described by a tree structure
• Arithmetic example
• (3(57)) (16/4)

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Arithmetic

• The internal nodes areoperations
• Leaf nodes are numbers
• Leaf nodes could alsobe variables like x or y
• Now value depends on x and y.

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For images

• Leaf nodes are color triples (rgb).
• Operations are on a per-color basis, so that
  (r1, g1, b1) (r2, g2, b2)
  (r1r2, g1g2, b1b2)
• All operations are binary (take two arguments)
  like , -, , /

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What about cosine?

• For things that normally take one argument, like
  sine and cosine, we define two argument
  operation by adding
• 4 2sine 3 (in analogy with 4 3) means
• sin(4 3)

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Operations producing numbers

• Sometimes its nice to have an operation
  producing just numbers
• Examples
• (r, g, b) -gt max(r, g, b)
• (r,g,b) -gt sum(r,g,b)
• (r,g,b) -gt if (r gt 0.5) then g, else b

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casting

• If an operation produces just ONE number
• Well copy that number 3 times to get an RGB value

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Evaluation

• If we have a tree, representing an expression
  like this
• Evaluate it recursively
• value(tree) DIFF(value(lefthalf),
  value(righthalf))
• In general, you evaluate left, eval right, and
  apply the operator at the node.
• If node is a number, the value IS that number.

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Matlab code

• function y evaltree(t)
• if (t.number)
• y t.value
• else
• op t.operation
• left evaltree(t.leftchild)
• right evaltree(t.rightchild)
• y apply(op, left, right)
• end

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