Paul,
You are correct in saying that XV will let you specify the resolution
of its PostScript files, however, this does not change the fundamental
point made in their manual. My printer is capable of printing a single
pixel in any one of four colours (five colours if you include NOT printing
the pixel). Photos don't look very good in anything less than twenty four
bit colour. According to XV's manual, they dither the PostScript colour
by generating a colour pixel that is four printer pixels square. This
means that at 300dpi, you are going to get approximately 72dpi in colour
pixels. This is not a limitation of XV, it is a limitation of how ink jet
and laser printers work. Maybe some day, someone will build a printer
that mixes primary colours, and prints each pixel at twenty four or thirty
bits.
Actually, my 720dpi printer should be able to manage 180dpi in colour
pixels. I am not sure if I have achieved this yet. XV has another
limitation. When you import a file whose resolution is in excess of the
computer screen resolution, it reduces the number of pixels. If you use
xpcd to capture a 3072x2048 image, XV will convert to TIFF at 1024x768. I
should be able to print a 1536x1024 image, at 720(180)dpi, but I haven't
quite figured this out yet. This is one of my projects for the Christmas
holidays.
I have used hpcdtoppm to generate a `Floyd-Steinburg' dithered image at
720dpi, which I imported into a LaTeX file. I then compiled and printed
the file at 720dpi. This is moderately complicated to do, and it takes a
while on my 486DX266. The result is a black and white image that has
somewhat better detail than that of a photo in a newpaper or most
magazines. The photos were printed 4-1/8" by 2-3/4" or 105x70mm. This is
not very big, and it represents the maximum resolution of the PhotoCD
image.
Howard Gibson
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