Creating Transparent Image Overlays

It is possible to create and display a transparent image using either IDL Direct Graphics or IDL Object Graphics. Creating a transparent image is useful in the warping process when you want to overlay a transparency of the warped image onto the reference image (the image in which Xo, Yo control points were selected). The method used to create and display the transparent image depends on whether the resulting image is being displayed with Direct Graphics or Object Graphics.

Displaying Image Transparencies Using Direct Graphics

Creating a transparent overlay in Direct Graphics requires devising a mask to alter the array of the image that is to be displayed as a transparency. The mask retains only the pixel values that will appear in the transparent overlay. The base image and the transparent warped image can then be displayed as a blended image in a Direct Graphics window.

With Direct Graphics displays, only a single color table can be applied to the blended image in a display window. For an example of creating a blended image, combining a warped image and a base image, see Warping Images Using Direct Graphics.

Note
For precise control over the color tables associated with the reference image and the warped image transparency, consider using Object Graphics.

Displaying Image Transparencies Using Object Graphics

In Object Graphics, a transparent image object is created by adding an alpha channel to the image array. The alpha channel is used to define the level of transparency in an image object. For an example, see Defining Transparency in Image Objects and Warping Image Objects (Object Programming).