Incorrect dpi information in the Print dialog

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davidh
Posts: 835
Joined: June 9th, 2009, 2:16 am

Incorrect dpi information in the Print dialog

Post by davidh »

When I want to print the layout with an image, I see a different dpi information in the Print dialog: incorrect 296,5 dpi, than there is in the Layout dialog and the layout Image info, both show correct 288 dpi. See the attachment.

There is no such difference in the Print dialog when I try to print the image itself outside the Layout transformation.
If I am not mistaken, 296,5 dpi is what is sent to the printer. I would prefer it the other way round: 288 dpi sent to the printer.
Attachments
DPI diff.png
DPI diff.png (44.87 KiB) Viewed 3534 times
jsachs
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Joined: January 22nd, 2009, 11:03 pm

Re: Incorrect dpi information in the Print dialog

Post by jsachs »

From the screen shots, it looks like the physical dimensions of the image exceed the page size so the image was scaled to fit the page which resulted in the slightly increased dpi.
Jonathan Sachs
Digital Light & Color
davidh
Posts: 835
Joined: June 9th, 2009, 2:16 am

Re: Incorrect dpi information in the Print dialog

Post by davidh »

The image is just a test print patch abput 6,5 x 2 cm.
Attachments
Layout dpi.PNG
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davidh
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Joined: June 9th, 2009, 2:16 am

Re: Incorrect dpi information in the Print dialog

Post by davidh »

I used Add Border and Composite to create the same test print patch. It is composed of 4 individual crops. The Print dialog shows the correct 288 dpi, however when I use the cropping rectangle horizontal lines to make the image smaller, the dpi in the Print dialog changes. It varies between 281 to 288. I would not expect any changes in dpi when cropping an image for print.

By the way. The Juxtapose transformation of PWP 7 was unbeatable for purposes like preparing test patches for printing. Just adding crops next to each other. Easy and fast. No unnecesary settings for this simple goal. Very helpful.
jsachs
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Joined: January 22nd, 2009, 11:03 pm

Re: Incorrect dpi information in the Print dialog

Post by jsachs »

The Print command is working correctly as far as I can tell. The image size as shown in Image Info was too big for the available space between the margins and the image was reduced in size. If the image dimensions are small enough to print unscaled, the correct dpi is displayed in the Print dialog box and it does not change when you crop. If the image is too big to fit in the available space, it is scaled and the dpi changes. If you add margins to the image, the total size must fit between the print margins -- the image margins cannot overlap the print margins.
Jonathan Sachs
Digital Light & Color
davidh
Posts: 835
Joined: June 9th, 2009, 2:16 am

Re: Incorrect dpi information in the Print dialog

Post by davidh »

OK, the problem was that the Layout dimensions were larger than the printing area: the paper dimensions minus the margins. I mistakenly entered the printing paper dimensions as the layout dimensions not thinking about margins.

One more question.
Since the dpi value in the Print dialog is sometimes integer other times decimal, changing by tenths or rather hundredths of dpi depending on the layout dimensions, what does a printer do with a decimal dpi? Does the printer round off a decimal value or replaces it with some default integer value?
jsachs
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Joined: January 22nd, 2009, 11:03 pm

Re: Incorrect dpi information in the Print dialog

Post by jsachs »

The dpi setting displayed in the Print dialog box is simply the image width in pixels divided by the image width in inches -- the dpi is not part of the data sent to the printer. The printer driver is responsible for resampling the image data it receives to fit the desired print size as specified in printer coordinates.

One important point is that the native printer resolution as reported by the Epson driver (you can see this value if you print a small image and set the scale to one-to-one) is 360dpi, no matter what resolution you select when you print. Higher resolutions such as 1440 or 2880 dpi are generated within the driver when it computes the dither pattern to send to the print heads. To get a resolution for a 360dpi RGB image, it takes a much finer printer resolution since the printer is using a limited palette and dithering.

For this reason, 360dpi is the ideal image resolution to send to Epson printers -- any higher adds no information. I assume something similar applies to other color printer models.
Jonathan Sachs
Digital Light & Color
AHuntley
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Re: Incorrect dpi information in the Print dialog

Post by AHuntley »

I'm certainly no technician when it comes to the desktop, but I was always under the impression that sending 360PPI to an Epson printer avoided any data interpolation. The image will simply print at whatever dimensions are the results of that PPI. I'm sure there must be something going on with dithering, etc, to spew 1440 or 2880 dots-per-inch, but the Epson driver is controlling that. Regardless, I always send 360PPI to Epson printers and 300PPI to my HP. I believe Canon printers are also 300PPI native.
Regards,
AlanH
jsachs
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Joined: January 22nd, 2009, 11:03 pm

Re: Incorrect dpi information in the Print dialog

Post by jsachs »

The native printer dpi can depend on the printer make and model -- For example, I have a black and white Canon laser printer that has a native resolution of 600dpi, so you can output a 1-bit binary image at 600dpi and control every dot. The best way to check the native dpi, without having to write a program, is to invoke the Print dialog on a small image and set Scale to one-to-one -- PWP will then display the printer resolution as the dpi setting.
Jonathan Sachs
Digital Light & Color
davidh
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Joined: June 9th, 2009, 2:16 am

Re: Incorrect dpi information in the Print dialog

Post by davidh »

I used the recommended method with a small print and scale set to one to one and got the native resolution 720 for my Epson Photo R2880. I guess half of it would be good enough for what I do.
Thanks for making it simple.
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