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Author Topic: Help with printing, resolution and interpolation  (Read 5459 times)
katad0t1s
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« on: February 05, 2013, 12:24:24 PM »
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Hi to all Smiley New member here!

I have decided to start printing some of my photos, but I am in a bit of a blur. To make things clear, please consider the following example:

Example:

You wish to send your photo to a service for printing at 5x7 inches. 

You have a photo that has been developed / retouched. It is now a Master File, with dimensions 1250x1750 pixels. What remains to be done is a) resize the photo and b) apply output sharpening (for the sake of conversation lets assume a standard Unsharp Mask application, tailored for a specific output, or Lightroom's output sharpening).

Case a) Photoshop.

1. Go to Image resize
2. set the dimensions to 5x7 inches
3. leave Resampling unchecked (unless you wanted to enlarge the photo)
4. Click ok
5. apply Output Sharpening
6. save and send the file
No interpolation used for this method

Case b) Lightroom

1. Go to print Module
2. Set Print to: Jpeg File
3. Click Custom Dimensions and set to 5x7
4. Set resolution to...anything (240, 300, whatever)
5. Set output sharpening to Normal, Strong, etc
5. Print File and send it to lab
This method used interpolation to resize the photo

My questions are:


1. Are the workflows described above correct or not? What mistakes am I making in Photoshop, Lightroom or both?
2. I was under the impression that I could print almost anything from above 180-200 ppi without any need for interpolation. Should I or should I not interpolate my photos?
3. The method I used to resize in photoshop does not actually alter the image size. Output sharpening should be applied at final output size. Is this kind of "resizing" ok for output sharpening or should the image size actually change (re-sample) to apply output sharpening properly?
4. Assuming that my Photoshop workflow is correct, is there any way to avoid interpolation in Lightroom? Or should I interpolate in Photoshop as well (ie to keep the file at 300ppi)
5. If I do have to always interpolate my photos, what should be my target ppi? whatever the lab asks for? 

Thanks in advance (and sorry for the long first post) Smiley
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bjanes
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« Reply #1 on: February 05, 2013, 01:31:57 PM »
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The forum guru on these matters is Jeff Schewe. A quick search of LuLa turns up this link. Undoubtedly, Jeff will elaborate in his upcoming Digital Print book (the first is the Digital Negative ala Ansell Adams' earieler series), but you can apply those recommendations for now.

You seem to want to avoid re-sampling, but the image will be re-sampled by the printer driver to its native resolution anyway so the question is whether you want to use re-sampling in your own software or just leave it to the printer driver. The beauty of LR printing is that it does the work for you and no intermediate image is required--you can just print from your master image. However, output sharpening should be done at the final print resolution, and this would require an intermediate image in Photoshop at the resolution of the printer.

Regards,

Bill
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Schewe
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« Reply #2 on: February 05, 2013, 01:42:17 PM »
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2. I was under the impression that I could print almost anything from above 180-200 ppi without any need for interpolation. Should I or should I not interpolate my photos?

Wrong...while you might get a "reasonable" print from 180-200 PPI, it won't be optimal. The current thinking (well, mine anyways) is to resize and resample to the native resolution of the output device in PS/LR and then apply output sharpening. You need to find out the native resolution of the printer you'll be sending your image to to determine the best final resolution. You might want to read this: The Right Resolution.
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katad0t1s
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« Reply #3 on: February 05, 2013, 02:05:27 PM »
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@bjanes @Jeff Schewe

The reason I wanted to avoid interpolation was this:

"In the grand scheme of things you are far better if the native resolution
of the file is between 180 ppi and 480 ppi and just send the native pixels to the printer"

                                                    Jeff Schewe "From Camera to Print" Ch. 9 - Resolution

I guess I should have been following Jeff and the forums here, much, much more closely to get the latest updates!

I will read the links both of you gave me. As I see it, I will be interpolating all my images to match the output device from now on.

Thanks guys!


ps any samples / table content / teaser of Jeff's upcoming book ?

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Schewe
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« Reply #4 on: February 05, 2013, 02:41:23 PM »
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"In the grand scheme of things you are far better if the native resolution
of the file is between 180 ppi and 480 ppi and just send the native pixels to the printer"

                                                    Jeff Schewe "From Camera to Print" Ch. 9 - Resolution


That was then and this is now. A lot changed when Lightroom incorporated the PixelGenius Inkjet Output Sharpening into Lightroom and Eric Chan improved the interpolation in LR/ACR. Also, new printers have helped advance the art...

Sounds like you need to watch the newest of the LuLa video tuts...From Camera to Print & Screen. In that video Mike and I talked extensively about image and print resolution. The previous Camera to Print has been superseded by new knowledge...
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katad0t1s
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« Reply #5 on: February 05, 2013, 04:16:07 PM »
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That was then and this is now. A lot changed when Lightroom incorporated the PixelGenius Inkjet Output Sharpening into Lightroom and Eric Chan improved the interpolation in LR/ACR. Also, new printers have helped advance the art...

Sounds like you need to watch the newest of the LuLa video tuts...From Camera to Print & Screen. In that video Mike and I talked extensively about image and print resolution. The previous Camera to Print has been superseded by new knowledge...

Sounds like you are right Wink

Thanks Jeff.
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BartvanderWolf
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« Reply #6 on: February 05, 2013, 05:38:41 PM »
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The reason I wanted to avoid interpolation was this:

"In the grand scheme of things you are far better if the native resolution
of the file is between 180 ppi and 480 ppi and just send the native pixels to the printer"

This has never been good advice! Printers work at a given native resolution.
It has always been better to upsample to that native resolution, and then do output sharpening at that resolution.

Why anybody would want to output sharpen first, and then resample, beats me. It doesn't make sense.

Cheers,
Bart
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katad0t1s
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« Reply #7 on: February 06, 2013, 10:51:02 AM »
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The benefits of printing to native resolution are obvious in the article link Jeff Schewe posted above.

However, for the sake of technical conversation, what would you say to those that say that ppi != dpi, so in effect pixels are not mapped 1 to 1 to dots when printed?

Is it true? Does it matter?
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BartvanderWolf
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« Reply #8 on: February 06, 2013, 07:20:02 PM »
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However, for the sake of technical conversation, what would you say to those that say that ppi != dpi, so in effect pixels are not mapped 1 to 1 to dots when printed?

That's correct, ppi != dpi. A pixel has one out of several million possible RGB colors. For an approximation of that color with only a limited number of ink colors, the pixel will be represented by a dithered number of higher physical resolution ink droplets, sometimes of variable size. So, many dots make one output pixel in order to create a mixed ink/background color.

Cheers,
Bart
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Schewe
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« Reply #9 on: February 06, 2013, 10:14:18 PM »
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However, for the sake of technical conversation, what would you say to those that say that ppi != dpi, so in effect pixels are not mapped 1 to 1 to dots when printed?

I think it's important to understand there is a difference between dots per inch and droplets per inch. While printers report a specific PPI to the print pipeline, current stochastic ink jet printers actually print out at a higher droplets per inch. DPI generally relates to the specific nozzle resolution...in the case of Epson pro printers, that's 360 nozzles per inch. But because of dithering and multiple passes of the head, the actual resolution in terms of droplets is greater. With the highest output settings on an Epson pro printer, the droplet resolution is 2880x1400. If you set the driver to Finest Detail, the reported dots per inch is actually 720PPI and that is dithered to be 2880x1400 droplets per inch.

Don't confuse dots per inch and droplets per inch. Those units are not the same. And to add further confusion, Epson states a 3.5 picoliter droplet size. Problem is, picoliter is a unit of volume not a dimension.

It's all pretty confusing...which is why there's so much mis-information out there.
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HSakols
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« Reply #10 on: February 07, 2013, 09:52:19 AM »
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Thanks Jeff for sharing your knowledge.  I've always been curious and blown away at how you went from a wet darkroom artists to software genius / artist.  What you do, buy a dummies guide to computer software engineering? 

So am I OK printing my 14x21 inch prints from a d800 at 720 dpi.  If I divide a 7360 x 4912 image from the d800 by 360 I get 20 x 13.6 (a bit short)?

 
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Jim Kasson
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« Reply #11 on: February 07, 2013, 05:00:09 PM »
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However, for the sake of technical conversation, what would you say to those that say that ppi != dpi, so in effect pixels are not mapped 1 to 1 to dots when printed?

Is it true? Does it matter?

Yes, it's true. Does it matter? It depends...

First off, I'm assuming from your use of the "!=" operator, that you have programming experience, and thus a mathematical/logical/technical bent. Also, you've opened the door to "technical conversation". The above are my excuses for talking about things perhaps not appropriate for "Beginner's Questions".

The process of going from "continuous tone" or "contone" (an image processing word that encompasses pixel encodings of more than one bit per color plane) pixels, to printer "dots" is called "halftoning", a word that dates back to the days when part of the process for making plates for printing presses involved photographing the contone work through a piece of processed film called a screen onto a piece of lithographic film.

Now that printers can produce different sized dots, the dichotomy between the pixels in the file and the ink on the paper is not as stark. But the convention remains: contone data in the image file = pixels, and the data that the printer driver send to the print head = dots. The pitch of the contone file is therefore measured in pixels/inch, and the pitch of the "image" that the driver sends to the print head is measured in dots/inch. And, as you so succinctly put it ppi != dpi, at least not as a rule.

There are many halftoning algorithms. They are usually divided into two categories, which I will call “ordered dither” and “stochastic”. The division is somewhat arbitrary, and some techniques fall outside of the mainstream or borrow from both poles.

In ordered dither, each contone pixel is used to generate a pattern of dots that, when combined on the paper and viewed from a distance by a person, can represent some significant portion of the various tones that contone pixel can take on. Thus, for ordered dither, the original contone image has to be resampled into pixels that the printer driver will turn into groups of dots. A 10x10 array of dots can represent 101 different tones (warning: fencepost situation). If a printer using that sized dot array printed dots on a 1440x1440 dpi grid, the printer driver would want an image of 144 pixels/inch. If it didn’t get the contone image at that resolution from the app doing the printing, it would need to resample the image before printing it. The least computationally intensive resampling algorithm is called “nearest neighbor”, and that algorithm is typically used by printer drivers because it is, in programming vernacular, “inexpensive”.

In stochastic halftoning, there is usually some algorithm that looks at the contone pixel and makes a decision about what dot or dots to put down based on the value of the pixel and the dots that it has already put down, or already decided to put down. One such algorithm is called “error diffusion”, and the printer driver keeps track of the difference between the putative tones of the dot array it’s already generated and the tones in the image. It’s interesting to note that with error diffusion, decisions made by the printer driver at the upper left hand corner of the image can affect the ink that it lays down at the bottom right corner. Error diffusion by itself then to produce some visually unpleasant snaky patterns; spectrally shaped noise (“blue noise” is the term of art) is added to the dot array to break up those patterns. That last operation is what gives stochastic halftoning its name.

When using stochastic halftoning, there is no strict need for the driver to resample the contone image. It’s possible to write code that deals well with different dpi and ppi pitches. However, there will be interpolation operations in the code that algorithmically look like those involved in resampling.

From now on, what I have to say only pertains to Epson printers and Epson drivers, because those are the only implementations on which I claim some expertise. The Epson printer drivers resample images sent to them to 360 ppi, or 720 ppi if the “finest detail” box is checked.  About two years ago, I demonstrated that through a series of photomicrographs of test images that you can see in this post and the preceding ones. The resampling that the Epson driver uses is nearest neighbor, which I demonstrate in another post that part of that series.

Now, finally, we get to the “does it matter part”. In the case of the Epson printer driver, it does. See this post in which a test image is resampled by nearest neighbor and by other algorithms; if you have a choice, you don't want your images resampled by the algorithm used by the Epson driver, and you should resample to driver resolution before you print with the Epson driver.


« Last Edit: February 07, 2013, 07:11:00 PM by Jim Kasson » Logged

katad0t1s
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« Reply #12 on: February 10, 2013, 03:04:11 PM »
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First off, I'm assuming from your use of the "!=" operator, that you have programming experience, and thus a mathematical/logical/technical bent. Also, you've opened the door to "technical conversation". The above are my excuses for talking about things perhaps not appropriate for "Beginner's Questions".



No excuses needed. I welcome your technical information and examples. It really helps to feed my hunger for understanding the inner works of printing (as long as I can find a lab that actually has and puts all this knowledge to use, where I am located.....).

As Jeff said above, it can indeed be a very confusing matter and reading "expert's" opinions on the web only makes things worse. I am glad though that I joined the community at LuLa, since you guys seem to have given the most direct answers, backed up by facts and thorough testing.

Thanks to everyone who replied to this post.
It has been an eye opening experience.
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