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Arc Radius Illusion

This exercise illustrates a few techniques for drawing with Image, and makes an illusion where the arc radii of two arcs from the same circle appear to be different. (You might wish to add drawing tools to your copy of ImageJ.

• Make a new Image with the File -> New menu. I am using a size smaller than the default for printing in these instructions.
  • (Make sure the Image Window button rather than the Text Window is selected. Does not apply for ImageJ.
    • The new image will have the color of the eraser tool.. Change the color of this tool (the background color) by clicking on it, then clicking on the eye dropper tool (the cursor will change to this tool), then clicking on the desired color in any image window, or in the LUT window. Note that white and black are at the bottom of the window.
    • In vanilla ImageJ, there is no eraser tool, and no paintbrush tool.  You can fill selections or draw outlines of selections using Edit/Fill, Edit/Fill Outside, and Edit/Draw. In ImageJ, the new image has black, white, a ramp, or what is in the ImageJ's clipboard, depending on the selection you make in the Edit / New dialog. In this case, select white.
  • In NIH Image, the background color of the new blank image can be changed by:
    • Making the paintbrush the desired color
      • Click on paintbrush tool
      • Click on dropper tool .
      • Click on desired color in LUT window. (On my PC, clicking on the black bar turns the paint brush white and clicking on the white bar turns the paint brush black. Drawing is done with the correct color (the color clicked on, not the paintbrush color.)
    • Clicking on the paint can tool
    • Clicking anywhere in the image. Since there are no lines to limit the painting, the color will spread throughout the image.
  • In ImageJ, you can change the background color by:
    • Selecting the color with the color picker
    • Selecting the background using Edit / Selection / Select All, then Edit / Fill.
    • Note that despite what the color picker shows, the actual color of the new image depends on the type of the image, which you can change. For example, to make the background green, use Image / Type / RGB Color, then use Edit / Fill.
• Draw a large circle in the window.
  • Select one of the 'fatter' line widths in the tools window:   In ImageJ, select a line width of 5 in the dialog from Edit / Options / Line Width.
  • Make the paintbrush black (see above) if the background is white. Make the color picker black.
  • Select the oval selection tool:
  • With the shift key held down, click near the upper left of the window (holding down the shift key to force the oval to be a circle) and drag to the lower right corner.
  • Let up on the mouse. See the 'marching ants' (yellow) selection circle (with small circular handles which you can drag)..
  • Paint the circle in the window using the Edit -> Draw Boundary menu (Edit / Draw)..
  • Stop the marching ants (cancel the selection - clear the yellow circle) by clicking on the image.

• Click on the rectangle selection tool and then select a small arc from the circle:

• Edit -> Copy Selection menu (Edit / Copy)..
• Edit -> Paste menu, to paste a copy of the selected piece of arc on top of itself. This piece can now be moved around the image by clicking and dragging it.
• Drag the selection to somewhere near the center, then click outside the selection to stop the marching ants. (Similar step done in background level illusion exercise.) The piece is now 'written' into the image.

• To make the illusion (that the short arc was cut from a much larger circle) more compelling, erase the left part of the circle.
  • NIH Image, Scion Image
  • Make sure the eraser tool is the background color by
    • Clicking on the erase tool.
    • Clicking on the background color in the LUT window (note that the cursor is an eye dropper in the LUT window, but a square (within which the background color will be painted when the mouse button is clicked) in the image window.
  • Click on the erase tool and erase with it, or...
  • Click on the rectangle selection tool , select the left half of the image.
  • Use the Edit -> Cut Selection menu, or hit the delete key (Mac only).
  • ImageJ
    • Make sure that the background color in the color picker is white (or the the background color of your image).
    • Click on the rectangle selection tool , select the left half of the image.
    • Edit / Clear

Note - the drawing is not anti-aliased (it looks jaggy).

• Using the Process -> Smooth menu (with the option key held down for more smoothing) helps a little, but also makes one feel that your eyeglass perscription needs changing.