paul@manray.sgi.com (Paul Haeberli) (12/15/89)
I have softwre tools that may be used on IRIS 4D workstations to
transform RGB images into color separations for 4 color printing.
If you would like a copy of this software to use on your IRIS,
please mail me a 1/4" streaming tape.
Paul Haeberli
Silicon Graphics Inc.
2011 North Shoreline Blvd
Mountain View, CA 94039
415-962-3665
paul@sgi.com
Here's a description of how the process works. . . .
Creating digital color separations
on the IRIS.
Version 0.5
Paul Haeberli
Silicon Graphics Inc.
This document describes the process we've been using to convert
RGB images to film for 4 color printing. We've been using this
software for about 3 years now for internal publications and
have processed aver 100 images. We assume you have a 4D
workstation that has the ability to display images in RGB mode.
Here's a brief overview of the process:
1. Calibrate your monitor.
2. Gamma correct the image to look right on your calibrated
monitor.
3. Increase the contrast of the image if it needs it.
4. Decide whether you need to do apply color correction to
the image. Possibly correct the image.
5. Color correct your image for printing.
6. The CT2T tape data is plotted at a constant 300 pixels per inch,
so we have to scale the image to the size we want it printed
7. If this looks good, Write out the CT2T tape.
8. Contact your printer and find out some specific information
about what kind of negatives they'll need.
9. Package the tape, and some other info and send this to
Kedie/Orent in Sunnyvale, CA.
10. Get the film and a Chromalin(TM) proof back from Kedie/Orent and
Give the film to your printer.
And here's the gory detail:
1. Calibrate your monitor.
For this process to work as expected, you must start out by calibrating
your monitor. This is very easy to do on your IRIS. Run the program
"gamcal". Make it cover about one quarter of the area of the screen.
This program displays a gamma calibration test pattern. Now run the
command "gamma 2.4". This sets the monitor gamma to 2.4.
Now adjust your brightness and contrast controls until you can see the
shade of the pattern smoothly taper off to black on the left. You
should also notice that on the left side of the pattern, the average
brightness of the boxes with horizontal stripes would be about the
same as the solid boxes in the same column. If things look confusing,
you may want to try another machine with a different monitor.
I've included a printed sample image with the software tape. To see
what this images looked like on the screen do:
ipaste images/sample.rgb
This shows the current state of the color calibration between a screen
image and a print image.
2. Gamma correct your image.
Now display your image using "ipaste". If your image is in a foreign file
format you may be able to convert it into an IRIS image with some of the
tools found in /usr/people/4Dgifts/iristools/imgtools. Check to see if
the midtones of the image look too dark or too light.
If you want to darken the image, use commands like:
gammawarp mypic.rgb dark.rgb 1.5
ipaste dark.rgb
If you want to lighten the image, use commands like:
gammawarp mypic.rgb light.rgb 0.7
ipaste light.rgb
The numeric argument is a power to raise the pixel values to. Values
greater than 1.0 will darken the image, while values less than 1.0 will
lighten the image.
3. Increase the contrast of the image if it needs it.
Use the program "hist" to display a histogram of the image. This shows the
distribution of pixel values in the image. Check to see where the
distribution tapers off to zero near the right side. If this is much less
than 255, then we can easily scale the image intensities using "imgexp"
Run imgexp like this:
imgexp mypic.rgb bright.rgb 0 205
ipaste bright.rgb
This would rescale the pixel values so that what used to be displayed
as 205 now will be full brightness.
4. Decide whether to apply color correction
Try creating a soft proof of your image as is, without any color
correction. To do this run:
sproof mypic.rgb sproof.rgb
ipaste sproof.rgb
If this looks all right, and you don't mind the color shifts, then
skip step 5, and zoom mypic.rgb in step 6.
5. Try color correcting your image.
Use the program "corimg" to transform the colors so that when these colors
are printed, the final print will closely match the colors of your
original. This is only possible if your image only uses printable colors.
To see what the printable colors look like on the screen do:
ipaste images/printable.rgb
Any color in this image, and any color that is darker, can be printed
accurately with the 4 color process inks. If your original steps outside
the color gamut of the 4 color process, "corimg" desaturates these colors
by adding white. Since these processes are fairly slow, you may want to
use "izoom" to make a smaller copy of your image to practice on. Try
this sequence of steps:
corimg mypic.rgb cor.rgb
sproof cor.rgb corsp.rgb
ipaste corsp.rgb
6. Resize the corrected image to make it print at 300 pixels per inch.
When the CT2T tape transferred to film, it is plotted plotted with a
resolution of exactly 300.00 pixels per inch in the X direction, and
exactly 304.80 pixels per inch in the Y direction. This means that if
you put an image on the tape that has 1536 pixels on each scan line,
and 2014 scanlines, the printed size will be exactly 5.12000 inches by
6.60761 inches. So now you have to consider exactly how big you
want the image printed, and resize the image to that size.
If you came from step 4 above, resize mypic.rgb, otherwise resize
cor.rgb from step 5. Resize the image like this
izoom cor.rgb bigcor.rgb 23.34 23.34
7. Write out the CT2T tape.
If you have an IRIS workstation with a 1/2 inch tape drive then put a
tape in the drive, put it on line, and use a command like this to
write the tape directly:
toscitex -o /dev/rmt/xmt0d0nr.6250 bigcor.rgb
This will transfer the image to the tape. A white border of 0.2
inches (60 pixels on each side) is added to the image as it is written
to the tape. This border will make it easier for the printer to use
the film.
The program "toscitex" will write out a text file called "tapelog" in
the current directory. Attach a listing of this file to the tape when
you send it out.
If you don't have a 1/2 inch tape drive connected to your IRIS
workstation, you can use the program "toscitex" to write out a
CMYK dump file, which can be transferred to tape on another machine.
To do this use a command like
toscitex -o out.ct2t bigcor.rgb
Copy the file "out.ct2t" to the machine with the tape drive. Now
use the unix program "dd" to transfer the image to the tape at
1600 or 6250 bpi. Look at the log file and get the xsize, and the
ysize of the image. The number of scanlines is given by ysize. The
number of pixels on each scanline is given by xsize. There are 4 bytes
in each pixel, one for each C, M, Y, and B, so the number of bytes
in each scanline in 4 times the xsize. Transfer the file out.ct2t
to the tape using dd to write one tape block of data for each scanline
in the image. For an image that is 720 by 400 (after adding the 60
pixel border) the dd command would look like this.
dd if=out.ct2t of=/dev/rmt bs=2880
8. Now you must get this information from your printer:
1. The highest density of the halftone screen they can print.
The preferred one is a 175 line screen. 150 line is ok.
2. Does the printer want right or wrong reading negatives. Most
printers want right reading negs.
3. Does the printer want emulsion side up or emulsion side down?
Most times they will want emulsion side down.
9. Create the package for Kedie/Orent
Call Kedie/Orent and tell them that you would like to get a CT2T tape
plotted to film. Arrange to send them the tape. You may want to agree
with them on the price for this service. Here is a brief price list:
Up to 4" x 5" $ 90.00
Up to 5" x 7" $120.00
Up to 8" x 10" $145.00
Up to 11" x 14" $240.00
Up to 12" x 18" $300.00
Up to 14" x 17" $335.00
.
.
.
Up to 30" x 40" $1200.00
Put in a note saying you want the enclosed ct2t tape plotted to film.
Describe the requirements of the printer with language like:
"Please plot this tape with 175 line screen
We want right reading negatives.
We want emulsion side down."
Send the tape to Kedie/Orent. The turn around should be about 3
working days.
Kedie/Orent
744 San Aleso Ave
Sunnyvale, CA 94086
1-408-734-9005
10. Get the tape, film, and a Chromalin(TM) proof back. If it looks good
then give it to your printer.