QTR-Quadtone RIP

Hi Olivier,

I think the Epson article is just trying to help people who think in LPI to
understand comparisons.  I don't believe that any of the inkjet drivers really
work on a "halftone cell" basis.  Its a convenient method to describe the
general idea of how multiple grays can be achieved with just dots but its not
how it actually works.

The Garner article is incorrect in most of its assumptions so all the calculations
are not very useful.  I can't imagine where the 4x24 nozzle arrangement idea
came from -- the 96 nozzles per ink are in a straight line 1/180 inch apart.  
The nozzle check shows this clearly (note that the whole thing is done in one pass).
There are no cells so trying to calculate 4x4s and 5x5s aren't relevant.  What
he's really shown is just how moire patterns can arise.

All Epson drivers that I've seen resample the data at either 360x360 or 720x720
before applying a stochastic (random) dithering algorithm which may or may not
also include error diffusion.  

The simplest explanation for dithering is just that the probability of a dot 
existing at a particular location is proportional to the value of gray you want there.  
The trick though is that its pseudo-random in such a way that the dots are 
more evenly spaced.

Roy


--- In QuadtoneRIP@yahoogroups.com, "Olivier" <odesmais@y...> wrote:
> I would like to add/inquire about another factor in the 
> smoothness/banding issue.
> 
> In a technical article from Epson, the company claims :
> "
> Halftone printing
> 
> In order to simulate varying size halftone dots in computer printers, 
> dithering is used, which creates clusters of dots in a "halftone cell." 
> We recommend using Error Diffusion when printing halftoning. The more 
> dots printed in the cell, the darker the gray. As the screen frequency 
> gets higher (more lines per inch), there is less room for dots in the 
> cell, reducing the number of gray levels that can be generated.
> Epson's (LPI) lines per inch conversions are as follows:
> 720 dpi equates to 240 lpi
> 360 dpi equates to 120 lpi
> "
> 
> Question 1 :
> Am I correct assuming the cell is thus 3x3 e.g. 9+1 shades of grey 
> available per cell.
> 
> I now refer to an article by Rags Garner (http://www.rags-int-
> inc.com/PhotoTechStuff/Epson2200/).
> 
> The purpose of this article is to find out what should be the image 
> resolution vs printer dpi settings to output the best possible pattern. 
> I did not bother in the past and simply set image res at 360 and 
> printer dpi at 1440 for lack of information and common acceptance. Rags 
> findings are that the best image res/dpi settings would be 
> 288ppi/1440dpi based on a 2200 with 4x24 (96) nozzles e.g. 5x5 cell 
> (???). Besides and image of 288ppi would turn to be 144lpi and would be 
> slightly below average human "resolution" thus leading to some detail 
> loss or pixelisation.
> 
> Question 2:
> This is the first time I ever come to this settings ? What do you 
> think ? It seems to contradict Epson 3x3 cell.
> 
> This would assume a 5x5 cell yieling 25+1 possible tones, I would tend 
> to multiple this (less 1 for pure white, not drop) by the 3 variable 
> drop sizes to make 75+1 shades available per cell. It's seems that 
> human eye can perceive about 100 grey tones which would turn the number 
> of ink dilution of 3 more than enough.  
> 
> Question 3 :
> I'm wondering about the K7, but if the above is not too ridiculous, K7 
> would not be so usefull (with only gray inks) and linearisation on a 
> 21stepwedeg sould suffice to prevent banding and provide adequate 
> smoothness?
> 
> I'm a very beginner, I was just initially looking for the proper 
> ppi/dpi settings to print correctly and was wondering about the best 
> linearisation process.
> 
> Last, while Epson and QTR do not document the screening methods (I hope 
> this is the proper term in English, in French it's "tramage") whether 
> it is AM or FM (or a mix of both should also have an impact propably in 
> the hilglight rendering. Variable dot size call for AM method (dot at 
> same distance, different size)so precise setting ppi/dpi would be of 
> even higher importance (I feel).
> 
> I'm pretty sure I confuse a lot of things : your advise in the settings 
> and the best quadtone combination would be welcome.