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Author Topic: image resolution mystery  (Read 1054 times)

mwatkins

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image resolution mystery
« on: June 03, 2017, 11:30:36 pm »
Hello,

I'm having problems with image quality.

I've scanned some hand drawn illustrations for a book at 600dpi (the highest resolution setting available) and the resulting image files look great when I open them in an image editor, zoom in, etc.

However, when I drop them into my Scribus document, some end up looking horribly grainy. Strangely, they look better when "Preview Settings" are set at "Normal Resolution" rather than "Full Resolution". Disappointingly, when I've had draft copies printed, they don't look very good at all. I've tried resizing, changing resolution, etc. but nothing seems to be helping.

I've attached a couple of screenshots of the document opened in Scribus where the image is visible at Normal and Full Resolution.

It's 382 x 187pt in the document and its image info is as follows
size: 5808 x 2880 pixels
print size 246 x 122mm
resolution: 600dpi

Can someone advise as to how to I can process this image so that it looks as good as possible on the printed page?

Thanks!
MW

utnik

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Re: image resolution mystery
« Reply #1 on: June 04, 2017, 11:18:10 am »

hi matthew

…when I've had draft copies printed, they don't look very good at all.

your screenshots show the images in the scribus workspace.
  • how do they look in the .pdf file?
  • did you print from .pdf or from .sla?
  • did you uncheck 'maximum image resolution' in the .pdf export window?
  • is your printer good enough to deal with 600 ppi? (an office printer with 600 dpi isn't! – the difference between dpi and ppi matters here, even when applications like scribus use the wrong term…)
utnik

GarryP

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Re: image resolution mystery
« Reply #2 on: June 04, 2017, 12:06:03 pm »
Just to add to what utnik said above, there seems to be something wrong with the measurements you gave (as I interpret them).

Using Scribus to do the arithmetic, the 382 x 187pt you gave calculates as 134.761 x 65.969 mm and not the 246 x 122mm you gave. I don't know what's happened there. Why is your stated print size different to the size in the document?

Also, putting a 5808 x 2880 pixel image into a 382 x 187pt image frame would put the DPI at:
* 382 points / 72 points per inch = 5.305(recurring) inches;
* 5808 pixels / 5.305(recurring) inches = 1094 (approx) DPI.
* 187 points / 72 points per inch = 2.5972(recurring) inches;
* 2880 pixels / 2.5972(recurring) = 1108 (approx) DPI.

These numbers don't line up with what you gave. That might be causing a problem but I don't know which figures to trust.

I would suggest that you use GIMP - or something similar - to change the DPI of the images to 72 DPI (menu, "Image -> Scale Image") and then put the images back into Scribus before you try to do anything further. You won't lose any detail but it might help you to see what's going wrong.

When you tell Scribus to change the preview setting to "Full Resolution" you are telling Scribus to show the image at the highest resolution it can but you need to remember that your monitor - if it's not a retina/4K one - will only display at something like 90/96 DPI. This means that Scribus has to interpolate the pixels from a higher resolution to the lower screen resolution and that will normally cause problems, especially with very large images in small frames. Try to put 600 pixels into a 90 pixel space and you're going to have issues.

mwatkins

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Re: image resolution mystery
« Reply #3 on: June 04, 2017, 01:45:52 pm »
All the image info I supplied...

"size: 5808 x 2880 pixels
print size 246 x 122mm
resolution: 600dpi"

...is "pre-Scribus" (the data I'm getting from GIMP wherein I'm preparing the image).

The "382 x 187pt" is just the size of the image frame that I'm dropping it into in Scribus

Should I always first resize my image files (in GIMP, say) to their intended Scribus print size?

utnik

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Re: image resolution mystery
« Reply #4 on: June 04, 2017, 01:58:43 pm »
hi matthew
…Should I always first resize my image files (in GIMP, say) to their intended Scribus print size?

yes, that's good practice for the best control of your results. (i don't always do it, but when i really need the best image quality, i scale the files.)

utnik

mwatkins

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Re: image resolution mystery
« Reply #5 on: June 04, 2017, 02:55:10 pm »
OK, so for the current example, I need to reduce the print size to about 54% of its current value.

So... should I be resizing the image (and thereby reducing the number of pixels) by 54%, or increasing resolution by 54%?

I'm guessing the latter as (as I understand it), changing resolution doesn't actually reduce the amount of information in the image file.
« Last Edit: June 04, 2017, 02:58:23 pm by mwatkins »

GarryP

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Re: image resolution mystery
« Reply #6 on: June 04, 2017, 03:14:41 pm »
The resolution in GIMP - which is what is saved in the image file - is nothing more than a "serving suggestion". Scribus uses it when the image is first placed in a frame but after that it can be ignored as the only things that matter from then on are the pixel-size of the image and the size of the frame.

A 382 x 187pt frame is 5.3 x 2.6in.
A 5808 x 2880 pixel image, of 600DPI at 100%, will cover an area of 9.68 x 4.8in.
Proportionally shrinking this image into the 382 x 187pt frame gives an image of approx. 1108DPI.
This is way too high and is probably why you're getting strange results, unless you're using something like the printer mentioned here: http://www.nature.com/news/colour-printing-reaches-its-ultimate-resolution-1.11159

Anything higher than 300DPI is pointless for anything other than very specialised print jobs. Resizing/scaling and cropping the image manually is the best way to get the best results.

Work out what size frame you have and then rescale the image to fit the frame.

In GIMP, get to menu "Image -> Scale Image", then set the resolution you want (before anything else), then set the size (you can change the UOM), then OK the dialog and save the new copy of the image (keeping the old version in case you need it later). You might need to crop the image slightly if the frame and image aren't the same proportions but you can just use menu "Image -> Canvas Size" to sort this out.

utnik

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Re: image resolution mystery
« Reply #7 on: June 04, 2017, 08:22:26 pm »

Anything higher than 300DPI is pointless for anything other than very specialised print jobs.

well, there are reasons to save in resolutions up to something like 600 ppi (i prefere the correct term…)
i.e. with real black and white graphics (not grayscale) you'll see an improvement of the quality level from 300 to 600 ppi – but not on the output of a standard office printer… (this might be one of the mentioned 'very specialised print jobs‘…)

utnik
« Last Edit: June 04, 2017, 08:24:29 pm by utnik »

mwatkins

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Re: image resolution mystery
« Reply #8 on: June 04, 2017, 08:51:06 pm »
Thanks Garry P, that was exactly what was needed. I've started resizing my images in GIMP (changing resolution to 300dpi first), and everything's looking great in the PDFs I'm exporting now.

all best wishes,
MW

GarryP

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Re: image resolution mystery
« Reply #9 on: June 05, 2017, 10:53:11 am »
utnik, yeah, sorry about mixing up DPI and PPI.

Once I start using one of them I tend to keep using it without noticing, even if it's the wrong one. Hardly any of my output gets printed so there's no practical difference for me.

Maybe "very" specialised was too strong a phrase but I don't think many Scribus users will be printing real black and white. "Fairly specialised" might be more accurate.

The gist of what I was trying to say is that the "average" Scribus user - whatever one of those is - won't have any need for a PPI higher than 300.

I think it's reasonable to say that, with some exceptions:
* if someone is printing something on a normal home/office printer then 300PPI will usually be good enough;
* if someone is getting something printed at a print shop - or similar establishment - then they should ask the print shop what is best.

mwatkins, glad I could help.