Held: Tuesday, 11 February 2014

Back to Outline 13 - Transforming Colors. On to Outline 15 - Homogeneous Lists: Making and Manipulating Groups of Drawings .

Summary

Today we consider some basic control stuctures that will allow us to write more general image filters.

Related Pages

• Reading: Transforming Images
• Lab: Transforming Images
• EBoard (Source) (HTML) (PDF)

Overview

• Transforming images by transforming each pixel.
• Sequencing transformations with compose.
• Anonymous transformations.

Administrivia

• Wednesday's reading: Homogeneous Lists
• Lab writeup: Exercises 3 and 7a
  • Subject: CSC 151.02 Writeup 10: Transforming Images (YOUR NAME)
• I won't be able to hold a review session this week. Sorry.
• Don't forget that we generally can't see the effects of the pixel operations if the image is already visible.
• Don't forget that we generally can't refer to an image if you've closed it in Gimp.
• Both the graders and I remain a bit behind.
• Extra credit:
  • Thursday extra, Thursday @ 4:30 p.m., Sci. 3821, Red/Black trees.
  • CS Table Friday @ noon: Law, Order, and Computers
  • Royce Wolf Piano, Wednesday, Noon, Sebring-Lewis
  • Opera Iowa: The Elixir of Love, Thursday, 7:30, Roberts

Transforming images by transforming each pixel

• We saw in the color transformation lab that we can apply a transformation to each pixel, getting it and then setting it to a new value.
• This generalizes that strategy.
• (image-variant image color-transform) - build a new image by applying color-transform to every pixel.
• image-transform! image color-transform - change the pixels "in place"
• Interesting new concept: We can write procedures that take other procedures as parameters.

Sequencing transformations with compose

• When you need to do more than one transformation, you can sequence them with compose (also o).
• (image-variant image (o rgb-redder rgb-bluer))
• Note: This is a new approach. Traditionally, procedures appear immediately after parentheses, and they are doing the work. Now they are inputs to other procedures, and so do not have parentheses.

Anonymous transformations

• You don't even need to name those transformations.
• (image-variant image (lambda (rgb) ...))