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Chapter 8: Image Overlays

Image overlays often facilitate the interpretation and analysis of closely-related images. JHV supports image overlays for 8-bit raster images, allowing the user to select a base image, overlay that with one or more images, and adjust transparency and opacity to optimize the resulting composite image.
8.1 Opening the Image Overlay Window
8.2 Overlay Options

The discussion in this chapter uses the sample file samples/layers8bit.hdf located in the JHV installation directory.


8.1 Opening the Image Overlay Window

To start overlaying images, first set Layer in the main window View menu, then select a folder with multiple 8-bit raster images, such as the folder 8-bit Raster Image in the file samples/layers8bit.hdf. The image overlay window will appear, with the overlay panel on the left and a control panel on the right.


Figure 8.1 The image overlay window

8.2 Overlay Options

JHV provides options to choose the base layer and top layer image(s), to render a selected background color transparent, and to adjust image opacity.

Image List
Image layers are selected from the image list at the top of the control panel. The images are laid down in the order of selection: the first image selected will be placed at the bottom as the base layer, or base image; other images will be laid one over another in sequential order; the last image selected will be the top image in the overlaid stack. Figure 8.1 illustrates a high and low air pressure map (image #2 in the 8-bit Raster Image folder in layers8bit.hdf) overlaid on a United States map (image #0 in the same folder and file).

Transparency
Image overlays require that all images above the base image have a transparent background. Any upper image lacking transparency will obscure the images below it in the overlay.

To make an image transparent, select the image from the image list and click on the image to select the background color to be rendered transparent; the selected background color will appear in the Background panel. Upon pressing the Apply button, the selected background will become transparent and lower images in the stack will become visible.

In Figure 8.2, the left side illsutrates the high and low air pressure map (image #2) with white background overlaid on the United States map (image #0). Since the pressure map's white background has not yet been made transparent, it still obscures the U.S. map. The right side of the figure illustrates the same image layers, but after rendering the pressure map's white background transparent.


Figure 8.2 Overlaid image stack before and after rendering the background of the top image transparent

Opacity
In many cases, the non-transparent portions of two or more images may overlap. JHV provides a tool for adjusting the upper image's opacity so that overlapping details are all visible.

Slide the opacity slidebar, labeled Opaque in the image overlay window control panel, and press Apply to change the opacity of an image. You can repeat the process to fine-tune the opacity. An image layer with 0% opacity is completely transparent, while a layer with 100% opacity is completely opaque. Opacity can be set independently for each layer.

Figure 8.3 illustrates a cloud cover image (image #4 in the above folder and file) overlaid on the U.S. map (image #0) with 50% opacity.


Figure 8.3 An example of image opacity


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