Making a step gradient test file

[Dan Culbertson <danculb@concentric.net>]

Someone asked me how to make a step gradient to test paper/ink combinations.
I figure this has broader topic than the thread it was in so I started this
as a new one.  There are actually a few different ways but this one is
relatively numerically precise (but more tedious than some).

1.  Create a new *CMYK* file that fits on the size paper you want to use for
tests and a multiple of 10 for the resolution.  I use 8 inches wide by 10
inches tall at a resolution of 100 pixels per inch (you could use metric
measures also then the resolution would be perhaps 10 pixels per
centimeter).

2.  Determine how many steps you want in the gradient (if you are using it
to set Photoshop's dot area curves you need 14 steps for a general purpose
test you might want about 20).

3.  Divide the length of your page by the number of steps to get the size of
each step (subtract off any margins required including any area you might
want to use for labeling).

4.  Divide the width of the page by the number of columns you need (a single
full set of inks need a minimum of four, if you want to add a note block
next to each patch you need eight).

5.  Set the marquis tool to the "Fixed Size" style and set it at the number
of pixels that will give the size patch you want.  For example if you want a
patch that is one half inch by one half inch you would use 50 pixels by 50
pixels in the 100 pixel per inch resolution of the file.

6.  Make a grid with the marquis tool by starting at the top of you first
column and making a selection then stroke it (Edit/Stroke) at the number of
pixels you want for a dividing line. Using the Shift/Down Arrow (which
advances ten pixels at a time) continue down the page until you have a
column then do as many columns as you need until you have a matrix waiting
to be filled in with color (note, you may want to try this using stroked
paths to make a matrix or do it in Illustrator but the marquis tool works
okay as well)

7.  Once you have the matrix start filling it with the paint bucket tool.
Set the first color using the CMYK color picker to 100% Cyan, 0% everything
else and fill the first matrix cell with that color.  Fill second color with
(say) 95% Cyan, 0% everything else, etc.  If you were making a test sheet
for Photoshop dot area curves the patches would be in the sequence
100%/90%/80%/70%/60%/50%/40%/30%/20%/10%/8%/6%/4%/2%.  For a general test
page you might just want to use multiples of 5% going from 100% to 5% in
sequence.  Do that *in CMYK color* for all the inks, C,M,Y, and K.

8.  Add text to label the boxes if you left room for that and need it (for
comparing spectrometer measurements on the printouts for example). And that
is it for a CMYK test page.

Any time you open this file if be sure you tell Photoshop not to convert it
or color mange it (just say "no" to Photoshop).

RGB test page - take the CMYK test page and convert it to multichannel mode
then convert it to RGB mode.  *Don't* convert directly from CMYK to RGB as
this will scramble the pure ink colors.

Resizing - if you didn't get the size just right and want to resize or want
to increase the resolution be sure to use the Nearest Neighbor in the
Image/Resize Image command.  The other selection will make the nice sharp
rectangular cell boarders softer.

Multiple tests on a single sheet of paper - Resize the whole matrix page
making it narrower and holding it to the left of the page on resizing.  Copy
the matrix to a new layer on the right side of the page.  You can get two to
four test layers depending on how narrow you squeeze the matrix.  Turn off
all the layers then turn a new one on each time you run the paper through
the printer (remember to turn the last layer off or it will overprint the
previous print).

Alternate (simpler but less precise) methods - make four gradients using the
gradient tool in columnar rectangular selections (one for each of the CMYK
inks) with the foreground color set at 100% and the Background color set to
white (0% CMYandK).  Then Image/Adjust/Posterize the file selecting the
number of steps you want in the gradients.  This will make a pretty good set
of step gradients for casual visual tests.  You can also start in RGB
instead of CMYK if you translate the CMYK color values into RGB terminology
for the picker (ie 100% Cyan is 255 Green, 255% Blue, 0 Red).  Don't try to
specify CMYK colors in the picker when you are in an RGB document.  The
colors will not print correctly.

Dan Culbertson


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