Working with Images

Check which version of DrScheme you have. You can find the version number in the interactions window after you press execute.

Setting up Images under DrScheme 206

The image operators are built-in to version 206. You do not need to install anything in this case. The available operators are listed below.

Setting up Images under DrScheme 205

Download the images distribution to your computer.

Install the three .plt files in the distribution by selecting "install .plt file" under the File menu in DrScheme and selecting each file in turn.

Under the Language menu, choose "Add Teachpack" and select the images-teachpack.ss file from the distribution. (Do not use DrScheme's Open menu item for images-teachpack.ss.)

The teachpack adds the operations listed in the next section.

Image Operations

image=?    contract is   image image -> bool

Takes two images and returns true if the images are the same, false otherwise.

image-width    contract is   image -> num

Takes an image and returns its width in pixels.

image-height    contract is   image -> num

Takes an image and returns its height in pixels.

image+    contract is   image image -> image

Takes two images image and produces an image where the second is placed on the first. The top-left of the result image corresponds to the top-left of the given images.

offset-image+    contract is   image num num image -> image

Like image+, but with two number arguments inserted between the image arguments. The first number is the number of pixels right to shift the second image, and the secnd number is the number of pixels down to shift the second image.

offset-masked-image+    contract is   image num num image image -> image

Like offset-image+, but with an extra image argument inserted before the last argument. This image must be the same size as the last image, and pixels are copied from the last image to the result only where the extra image has non-white pixels.

solid-box    contract is   num num symbol -> image  

Takes two numbers and a symbol and produces an image. The first number determines the width of the result image and the second number determines the height. The symbol determines the color that fills the entire result image. See also the list of available colors.

outline-box    contract is   num num symbol -> image  

Like solid-box, except that only the border of the image uses the specified color, and the rest of the image is white.

solid-dot    contract is   num num symbol -> image  

Like solid-box, except that only a circle bounded by the image is filled with the specified color, and the corners of the image are white.

outline-dot    contract is   num num symbol -> image  

Like solid-dot, except that only the circle edge bounded by the image is draw with the specified color, and the rest of the image is white.

image-inside? : image image -> bool

Returns true if the second image appears somewhere in the first, false otherwise.

find-image : image image -> posn

Returns a posn reprsentating the location of the second image within the first. (If the second image does not appear in the first, an error is reported.) The result posn indicates the location of the second image's top-left corner relative to the first image's top left corner. The X part of the posn indicates the number of pixels to the right that the second image's corner is shifted, and the Y part indicates the number of pixels down that the second image's corner is shifted.

(define-struct color (red green blue))

Colors on a computer screen are represented by a combination of red, green, and blue light intensities. The intensity of each color varies between 0 and 255. For example, pure blue is (make-color 0 0 255), while (make-color 0 0 128) is a dim blue. Using the same intensity for red, blue, and green always produces a shade of gray; in particular, (make-color 0 0 0) is black, (make-color 255 255 255) is white, and (make-color 128 128 128) is a medium gray. Other combinations produce other colors. For example, (make-color 255 255 0) is bright yellow.

image->color-list : image -> list-of-color

Returns a list of color values representing the pixels of a given image. The first color corresponds to the top-left pixel, the second color corresponds to the pixel one step to the right, and so on --- left-to-right then top-to-bottom. The number of colors is in the list is the same as the image's width times its height.

color-list->image : list-of-color num num -> image

The opposite of image->color-list. This function needs the width and height of the image to construct, in addition to the pixel content.