How can I estimate ink cost from RGB or CMYK image data?

Question

I’m building an app to estimate ink usage and print cost. My first thought was to convert each RGB pixel to CMYK, then multiply each channel by a per-dot or per-unit ink cost, for example:

RGB (X,Y,Z) -> CMYK (0.5, 0.25, 0.9, 0.2)

cost = (0.5 × cyan_cost) + (0.25 × magenta_cost) + (0.9 × yellow_cost) + (0.2 × black_cost)

Is this a valid way to estimate print cost, or am I missing important factors such as color profiles, paper type, or printer behavior?

user37321 · Answer

Your first problem.

There are many ways to convert an RGB file to CMYK. This depends on the color profile and that gives you different CMYK values for the same RGB one. Some give let's say a 300% TAC and other can send a 330%. A color profile is some kind of transformation matrix, and it is not linear.

But not only for the same type of paper. If you use an uncoated paper the total TAC could be around 220%. Different materials suck the ink in different ways.

There are for example some variations on how to generate the same tone. Basically, there is an achromatic or chromatic one. (This paragraph is probably an addendum, this different methods ara not used very often anymore)

Problem 2.

You have not only one area, you have percentages of color.

Some pixels will have 100% ink. Some will have 99%, 98%, 97...

And finally

Amount of ink is not the same as cost. You need to feed the price base of the ink and compare it to the actual ink used, that also depends on the size of the print. One RGB file can be printed at different sizes.

I must add, this app could be useful. You could add one module for grayscale images or spot colors.

How can I estimate ink cost from RGB or CMYK image data?

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