Image Palettes
An image palette is a color mapping between image brightness and some number of colors. Mira uses 256 palette entries per channel to display a luminance image (i.e., an image not in RGB format). For such images, the image levels are set by the Transfer Function, then a palette of colors is assigned to each level of the transfer function. Since there are a discrete number of levels in the transfer function that must cover some range of image luminance, there is a range of luminance assigned to each palette color. By changing the color assignment to the palette indices, you can change the way the image is rendered.
Images are converted from pixel values to the screen display using two protocols: a Transfer Function for non-RGB images, and a Palette for all images. The transfer function converts image brightness values into a look-up table with 255 entries (255 per channel for RGB images). The palette then assigns a color or shade of gray to each entry in the lookup table. You can change both of these quantities from the Image Window:
Change the Transfer Function by opening
the Transfer Function Properties dialog from the
button on the Image Toolbar.
Change the Palette interactively by dragging the mouse pointer from inside the Colormap. You can change even more of the palette mapping using the Palette Properties dialog. Changing the palette is often called "stretching". You can see the origin of this term by watching the color curves inside the Palette Properties dialog.
Procedures for Changing the Image Palette
|
Contrast & Brightness |
Move the mouse cursor into the Colormap area. You will see it change to a mode cursor with a yin/yang shape upon entry. Mouse down to activate palette mode, then drag the cursor left/right to change brightness or up/down to change contrast. Release the mouse button when finished. |
|
Gamma & Brightness |
Move the mouse cursor into the Colormap area. You will see it change to a mode cursor with a yin/yang shape upon entry. Mouse down to activate palette mode, then drag the cursor left/right to change brightness or up/down to change contrast. Release the mouse button when finished. |
Adjusting the Palette, Palette Properties, Transfer Functions, Image Windows