XV: Image Manupilutator For X-Wondow System

xv is an interactive image manipulation program for the X- Window System. It can handle (display, convert from one to another, etc) almost all types image formats known to the X-Window system, including GIF, JPEG, TIFF, PBM, PGM, PPM, XPM, X11 bitmap etc. It can also generate PostScript files for printing purpose. It can also display PostScript files, but with very limited capability, if ghostscript (version 2.6 or above) has been installed. The main features of xv for the purpose of digital image processing include:

arbitrarily change size, rotate by any angle, flip the image, crop or pad images
adjust image brightness and contrast with a gamma correction function or an arbitrary function, and adjustment can be made to each individual Red, Green, and Blue color components
adjust global image saturation
perform global hue remapping
run a number of image-processing algorithms on any rectangular portion of the image
edit an image's colormap
reduce the number of colors in an image
dither in color and b/w

Here is the on-line manual of xv. Here is the official xv site.

In my opinion, xv is more than adequate for the final refining of the scanned image for the purpose of producing a faithful electronic copy of the image. In the following, some of the interesting (unique) features of xv are reviewed.

Fine Retouching

There are two tools provided by xv which is not obvious without resort to the manual. One is the pixel drawing: use the middle mouse button to pick a color, and use "Ctl middle button" to draw a pixel with desired color. Another is smearing the colors of a 3x3-pixel array. These two tools can be combined to do fine retouching of small areas. For large area, a cut-and-past operation is easier.

Contrast and Brightness Adjustment

This is very unique feature of xv which I find indispensable. Many image processing programs provide the "contrast and brightness" adjustments. Some give senseless values, and some better softwares give the Gamma value. In fact, all these values correspond to a mapping relation between the original image and the adjusted image. Such a relation can be visualized by a "tonal map" as shown to the right.

In this figure, the horizontal axis is the intensity (brightness) of the original image. and the vertical axis is the target image intensity. The straight line corresponds to the default (and unaltered) mapping. In general, the slope represents the contrast, and the height represent the brightness. Different Gamma values gives different but prescribed shapes of the curve.

The unique feature in xv is that the use can change the shape of the curve almost arbitrarily, by moving small square boxes (called handles) around. The curve changes accordingly, either as B-spline curve (a smooth curve) or zigzagged lines. When necessary, handles up to 16 can be added to the curve. This is an enormous freedom to manipulate and fine-tune the brightness of the picture. For instance, you can selectively increase the contrast of the shadow area and lower the contrast in the mid-tone area, and increase the contrast again in the highlight area.