Digital BW, The Print

Hi.

This post is to let people know that a new file has been uploaded to 
the Image Processing folder in the Files area of this site. The file 
(testimage_b&w_linearization.jpg) is intended to help users evaluate 
the linearization of monochrome printing setups.

Although I produced the uploaded file (it was a cold, wet Sunday 
afternoon here yesterday) I can't claim to have originated the idea 
behind it : to the best of my knowledge, that honour belongs to Keith 
Cooper of Northlight Images (http://www.northlight-images.co.uk/) who 
included a small version of one of the targets in his own black and 
white test image. Keith has a lot of interesting stuff on his site, 
and I thoroughly recommend a visit to anyone who hasn't been there 
before.

As soon as I printed it, it was immediately obvious how effectively 
Keith's target showed up the problems in my profile, but I found his 
version too small to be really useful and I wanted to be able to 
relate what I could see to actual tonal values. Sad to say, I 
couldn't find anything better to do yesterday than produce a test-
file of my own.

At the risk of stating the obvious, a quick heads-up for anyone who 
decides to make use of the test image . . . .

1) The targets were created by posterising a gradient. Anyone who's 
created their own linear step-wedge by this method will know that the 
posterizations are larger at the extremes than they are in the 
centre. (You can clearly see this effect in the linear 21-step wedge 
included at the bottom of my test file.) In a sense this is helpful 
because it gives additional real-estate to the highlight and shadow 
tones, but it is done at the expense of the midtones. 

Because of the unequal step sizes, and because a circular gradient 
inevitably has a lot of perimeter, the target with white on the 
outside will always (and should) appear lighter than the target with 
black on the outside. In an ideal world they would both appear mid-
grey if viewed at a distance with somebody else's glasses on, but if 
you manage to achieve that effect using this test-image, then you've 
got yourself a seriously warped profile. :)

I don't see this as a serious drawback, so long as users are aware of 
the issue. If in doubt, cross-check with the posterized circular 
targets to see how much space they allocate to 100% black and 0% 
white.

2) Although we're all used to looking at linear gradients, circular 
ones take a bit of mental adjustment. I found that I only really 
began to make sense of what I was seeing when I printed using a 
couple of different profiles and compared the results.

And once I started to understand what my circular gradients were 
telling me, I came to realise that they're not very forgiving. It's 
important to keep in mind that any test image is a tool and not a 
picture : the important thing is that we're happy with our output, 
not that we're producing a perfect linearization.

Finally . . . I genuinely hope that other people find the uploaded 
file useful. And my thanks go to everyone who has made this group 
such a pleasure to belong to.

Happy printing.

Graham Preston