Re: Theory versus practice
Re: Theory versus practice
- Subject: Re: Theory versus practice
- From: Steve Kale <email@hidden>
- Date: Thu, 06 Oct 2005 09:50:48 +0100
- Thread-topic: Theory versus practice
Title: Re: Theory versus practice
I don’t think that is quite right. The driver colour adjustment controls remain but there is no ICC colour management going on. It is simply looking at each pixel value from the unadjusted file and printing the corresponding amount of black ink. (If no driver adjustment is made with the colour controls no pixel values are adjusted.) There is no profile of this response behaviour at work with any sort of rendering. Think of a Same as Source workflow on a greyscale RIP. We typically set the rip to determine which inks will comprise the greyscale in what order, we set their ink limits (max ink load to paper), print an unlinearised greyscale and then linearise it. The “linearisation” is typically done with respect to L*. So if we plotted the L* response of the resultant greyscale it would be a straight line from, say, (0,14), ie file pixel value zero printing a luminance L* of 14, to (255,96), L*=96 being paper white. (With the unrigorous Black Only it may not be linear because we don’t get the opportunity to linearise the Epson driver’s response behaviour.) So the printer simply gets the file values for each pixel – unadjusted – and looks up how much ink it needs to put down. By “unadjusted”, I mean the file pixel values have not been altered by a CMM to reflect new values that will produce the right tonality in a new space as they would be if you converted to the print space with your chosen rendering. They are left alone. There is no remapping of the file values to best fit (according to your rendering intent) the RIP’s print space.
You can see clearly that you have a severe difference in gamma in the print space vs the file. If your workspace was Lab (there exist workspace’s which are just L* for greyscale) you could plot this from (0,0) to (255,100) on the same chart as above. Even though the RIP has been linearised with respect to L*, the fact that the two end points are different means the slope of the line or gamma is different.
If instead you profile that linearised (or even unlinearised) greyscale with something like QTR Create ICC you can use colour management to manage the luminance compression. A pixel value of 0 still stays at zero and hence is printed as dark as possible (0 = full K ink load = L*14) and at the other end stays at 255 (255 = no ink = paper white = L*96), but the points in between are adjusted according to your selected rendering intent. The printer still receives a range from 0-255 (assuming 8 bit and assuming the image file ranges from pure black to pure white).
Hope this helps
Steve
From: eugene appert <email@hidden>
Your first paragraph reaffirmed my convictions of the inherent contradiction between “same as source” ie no conversion and “Pixel value 0 is printed at maximum K ink load”. It convinced me that I must have overlooked something. So I went back this evening to run the test again, this time paying closer attention, and I found my error.
Tonight I realised that the Epson 7500 printer diver does not allow deactivation of colour adjustments when printing with black ink. I hadn’t noticed that after deactivating colour adjustment it pops back into auto mode as soon as you move on to the next menu. So the results are simply the same as printing with perceptual rendering.
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