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VISITview tutorial

June, 2002

Using overlays

This page will show you how to make use of overlays. An overlay is another image that is placed on top of the background image. Usually, much of this image contains a color level that has been assigned to "transparent", so that the underlying (background) image can show through. An excellent example of this is putting radar data on top of a satellite image. The radar image's background color, for example: black (red=0, green=0, blue=0), is identified as the tranparency color, and so when the radar image is overlaid on top of the background (satellite) image, the satellite image will show through to the student everywhere there are no echoes.

You may use more than one overlay on a page of your lesson. Keep in mind that each overlay, however small the file on disk is, will still take up memory equivalent to the image dimension (width x height) times 4 bytes on the student's computer.

Overlay images are brought into your lesson just like images for Pages and Portals. We'll not detail the process again, so if you need review of that, please click here to go back to the second page of this tutorial.

  1. To work with Overlays, you must first bring the images into your lesson. First, click on the View Overlays button.
  2. Browse Directories and locate the sample images overlay1.jpg, overlay2.jpg, and overlay3.jpg.
  3. Put these three files into your lesson as a single overlay with 3 frames (similar to what you did for the portals -- remember you will have to switch to View Overlay Frames to put the 2nd and 3rd file into the lesson (as frames 2 and 3 of the overlay).
  4. When you have put them into your lesson, the screen should look like this:

    Don't forget to type in a label for the overlay!!

  5. Click the View Pages button to return to Page View.
  6. Select Page #2 (by clicking on it's thumbnail).
  7. Click on the Select Overlays button, and a window will pop up that shows the first frame of Page 2 and looks like this:

  8. Note the following controls:
  9. First, click on your overlay (named in the List box)
  10. You will see the first from of your overlay (should say "Overlay 1" and have a white background).
  11. Locate the text input box next to the "Transparency value.." label, and type into that box: xFFFFFF then press the "enter" key. Your display will change to look like this:

    We have just changed the value for the 'transparent color' from black (x000000) to white (xFFFFFF). Those of you familiar with editting HTML, will find this nomenclature similar to what is used there. For everyone:

  12. Each time you change the transparency value, the display changes immediately. NOTE: You may only have a single transparency value for your entire lesson!
  13. For some images (like the JPEG ones in this example), you may need a range of values. For example: xd0d0d0 , xFFFFFF -- try using this and see how it improves the "not quite white" pixels in our overlay image.

  14. When you're done, click on the Save & Quit button
  15. Back in the main window, you'll notice that above the thumbnail for Page 3, it now says: Page 2 (3)po indicating that this page has both a portal and an overlay.

When you're ready to learn about saving your lesson, please click here.
To return to the VISITview Home Page, please click here.