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Panning and zooming


At times, your image buffer might be larger than the display, or have details that are too fine or too small to see. You can associate panning and zooming effects with the display to view specific parts of the image. Panning displaces an image horizontally and/or vertically on the display; panning is also known as scrolling. Zooming replicates or subsamples the pixels of an image upon display by a specified factor.

You can explicitly specify the amount to pan and zoom; alternatively, for windowed and exclusive displays, you can allow a user to pan and zoom the display interactively.

Explicitly specifying the amount to pan and zoom

Regardless of the display type, you can explicitly specify the amount to pan and zoom the display.

To pan displayed images, use MdispPan() and specify the required X- and Y-panning offsets in image pixels. To center images in the display, use MdispControl() with M_CENTER_DISPLAY instead.

To zoom displayed images in the horizontal and/or vertical direction with floating-point precision, use MdispZoom() with the required X- and Y-zoom factors; to reduce the size of the displayed images, specify a zoom factor less than 1.

The specified zoom factors also affect the panning offsets since these offsets are specified in image pixels. For example, if the X-zoom factor is set to 4, panning by an X-offset of one image pixel results in panning by 4 pixels in the horizontal direction on the display.

Instead of explicitly specifying a zoom factor, you can automatically scale the displayed images to fit the display, using MdispControl() with M_SCALE_DISPLAY. M_SCALE_DISPLAY ensures that the image aspect ratio is maintained. If you use M_SCALE_DISPLAY, the explicitly specified zoom factors (MdispZoom()) and pan settings (MdispPan()) will be overwritten.

For an example of zooming, refer to mimconvolve.cpp, where the display is zoomed by a factor of 2 to better demonstrate the result of an image processing operation (spatial filtering operation).

Panning and zooming the display interactively

With windowed and exclusive displays, it is possible to pan and zoom images interactively, using either the mouse or the keyboard. You can use the mouse wheel to zoom in or out of the image; to do so, scroll the wheel up or down, respectively. In addition, you can pan the image by clicking on the image and dragging in the required direction.

You can also use the keyboard to pan and zoom the display interactively. To do so, you can use the keys associated with the MIL display.

+

Increases the X- and Y-zoom factors.

-

Decreases the X- and Y-zoom factors.

Page-up

Scrolls the image buffer up to the previous display section.

Page-down

Scrolls the image buffer down to the next display section.

Up arrow

Scrolls the image buffer up to the previous row.

Down arrow

Scrolls the image buffer down to the next row.

Left arrow

Scrolls the image buffer left by one pixel.

Right arrow

Scrolls the image buffer right by one pixel.

Ctrl-Up arrow

Scrolls the image buffer up to the previous display section.

Ctrl-Down arrow

Scrolls the image buffer down to the next display section.

Ctrl-Left arrow

Scrolls the image buffer left to the previous display section.

Ctrl-Right arrow

Scrolls the image buffer right to the next display section.

Note that the window must have the focus for the keys to work. To give focus to an exclusive display, you must permit the mouse cursor to move over the display, using MdispControl() with M_RESTRICT_CURSOR.

By default, mouse and keyboard use is enabled for MIL windowed and exclusive displays and disabled for user-defined windowed displays. To enable or disable mouse use in the display, use MdispControl() with M_MOUSE_USE. You can also set whether the mouse cursor changes shape if interactive panning or zooming is possible using MdispControl() with M_MOUSE_CURSOR_CHANGE. To enable or disable keyboard use, use MdispControl() with M_KEYBOARD_USE.