Images and Canvas

  1. Tcl images are created with the image create command:

    Syntax: image create type ?name? ?option value ...?
    Create a new image.
    type The type of image. May be photo for multicolor images, or bitmap for bitmap images.
    ?name? An optional name for this image. If not provided, Tcl will assign a name.
    ?option value ...? Keyword/Value pairs that define the image object. Options include -height and -width, etc.

    You can load an image from a disk file with the file option:

    image create photo -file $filePath

    Images can be displayed in a Tcl canvas widget. The canvas widget is created with the canvas command:

    Syntax: canvas name options
    Create a new canvas widget.

    name The name for this canvas, starting with a "."
    options Key/value pairs such as: -height 200 -width 300

    Download this image and display it in a Tcl canvas.


  2. We can manipulate images once we've created them. Every time Tcl creates an image, it also creates a command that can be used to manipulate the image.

    The red/green/blue values for a pixel can be extracted from an image with the get subcommand:

    Syntax: imageName get xLoc yLoc
    Retrieve RGB values from an image
    imageName The name of the image as returned by image create
    xloc yloc The x and y location for the pixel

    A pixel can be assigned to a particular color with the put subcommand.

    Syntax: imageName put colorList -to x1 y1 ?x2 y2?
    Set pixel values in an image
    imageName The name of the image as returned by image create
    colorList A list of colors to assign to the pixels in Tcl color format.
    -to x1 y1 ?x2 y2? The X/Y location for a pixel, or a rectangular area for a set of pixels.

    You can put named colors like red, blue etc just by using the color name.

    $myImage put red -to 0 0 10 10

    draws a red square in an image.

    Write a script that creates a canvas and image, displays the image on the canvas and then draws a set of squares in the image.

    The results should look like this:


  3. In order to use the put subcommand with an RGB color value, we need to use the format command to construct the color:

    Syntax: format formatString ?data1? ?data2? ...
    The format command returns a new string based on a format definition and a set of data values.

    formatReturns a new string, formatted as defined by the formatString string.
    data#Data to substitute into the format string.

    The format command supports many options to describe exactly how a number should be formatted. For simple colors, the format


    is good.

    This tells the format command to format the numbers as 2 digit hex values with a leading 0 for values less the 0x10.

    Modify the previous code to draw a set of gray boxes instead of red boxes. The result should look like this:


  4. With the ability to get pixels from an image, and put pixels into an image, we can make a pixel-by-pixel copy script.

    Write a script that makes a canvas and 2 images. One image will contain the mandala.gif the other will start blank, but we'll copy the pixels from the first image into it.

    You'll need a set of nested for loops for this, one to step through the X locations of the pixels, and one to step over through Y locations.


  5. Modify the previous script to swap the red and green parts of the image. This will create a second image in which the red parts are green and the green parts are red. Cyan areas will become purple, and purple areas will become blue-green.

    The result should look like this:


  6. We can do arithmetic operations on pixels while we are copying them.

    Modify Exercise 4 to use apply a bit mask to each pixel so that only the top 4 bits of the image are displayed.

    The results should look like this:

    The right hand image looks crisper and sharper than the left hand image.



Copyright Clif Flynt 2009