Since GIF only supports one format (up to 8 bits per pixel, with a colormap), there will be no file size difference between a Full Color and a Greyscale image. A B/W Dithered image, on the other hand, will be considerably smaller.
If you are currently in 24-bit mode , and you are saving in any color mode other than B/W Dithered , the currently selected 24->8 conversion algorithm will be used to generate an 8-bit version of the current image, and that image will be written. (See "The 24/8 Bit Menu" for more info.)
Full Color images are written in a 24-bit RGB format, and Greyscale images are written in an 8-bit greyscale format. B/W Dithered images should not be used, as they will probably wind up being larger than Greyscale versions of the same images, due to the way JPEG works. Note: You cannot write a Reduced Color JPEG file. Trust me, given the method that JPEG uses to compress, it's not in your best interest to save Reduced Color JPEG files. If you attempt to do so, a Full Color JPEG file will be saved.
When you save in the JPEG format, the dialog box shown above will pop up and ask you for a quality setting and a smoothing value. '75%' is the default quality value, and really, it's a fine choice. You shouldn't have to change it under normal circumstances. The lower you set the quality, the higher the compression ratio will be (i.e., the JPEG file will be smaller). Note that setting the quality setting to '100%' will not result in 'lossless JPEG' (i.e., the original and the JPEG'd images will not be exactly the same, just very close).
The smoothing value is used to 'blur' images before saving them. It's often a good idea to blur GIF (and other 8-bit color) images before saving them, as you'll get better compression that way, and it also may partially undo the dithering that been done to many 8-bit images. On the downside, you'll also get somewhat blurred images. Something you have to decide for yourself.
Note: The JPEG support in xv is optional. While it is normally enabled, it is possible that it may not be enabled on your system (due to problems compiling the JPEG library, or something). If this is the case, you won't have a JPEG selection in the Format menu. Please feel free to complain to whomever built the binary that you're using.
When you save in the TIFF format, the dialog box shown above will pop up and ask you which type of image compression it should use. None, LZW, and PackBits compression types are available for use with all the Color modes. In addition, there are two B/W Dithered-only algorithms, CCITT Group3 and CCITT Group4 .
Note: The TIFF support in xv is optional. While it is normally enabled, it is possible that it may not be enabled on your system (due to problems compiling the TIFF library, or something). If this is the case, you won't have a TIFF selection in the Format menu. Please feel free to complain to whomever built the binary that you're using.
xv writes Encapsulated PostScript, so you can incorporate xv-generated PostScript into many desktop-publishing programs. xv also prepends some color-to-greyscale code, so even if your printer doesn't support color, you can still print 'color' PostScript images. These images will be three times larger (in file size) than their greyscale counterparts, so it's a good idea to save Greyscale PostScript, unless you know you may be printing the file on a color printer at some point.
Also, you should probably never need to generate B/W Dithered PostScript, as every PostScript printer I've ever heard of can print greyscale images. The only valid cases I can think of are: A) doing it for a special effect, and B) doing it to generate a much smaller (roughly 1/8th the size) PostScript file.
Note: When you try to save a PostScript file, the xv postscript window will pop up to let you specify how you want the image printed. (See "The PostScript Window" for details.)
In the raw variation of the PBM formats, the header information is written in plain ASCII text, and the image data is written as binary data. This is the more popular of the two dialects of PBM, as it produces considerably smaller image files.
Note that xv-created PBM files (both raw and ascii variants) may break some PBM readers that do not correctly parse comments. If your PBM reader cannot parse comments, you can easily edit the PBM file and remove the comment lines. A comment is everything from a "#" character to the end of the line.
If you are currently in 8-bit Mode, and you select Full Color, Reduced Color, or Greyscale, xv will write out an uncompressed 4- or 8-bits per pixel BMP file, based on the number of different colors in the current image.
If you are in 24-bit Mode and you select Full Color, the program will write out an uncompressed 24-bits per pixel image.
If you are in 24-bit Mode and you select Greyscale, an uncompressed 8-bit per pixel BMP file will be written.
If you select B/W Dither, a 1-bit per pixel BMP file will be written.
Full Color images are saved in the 3-plane, 1-band, PM_C format. Greyscale and B/W Dithered images are both saved in the 1-plane, 1-band, PM_C format. As such, there is no size advantage to saving in the B/W Dithered format.