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Author Topic: Unexpected results from the pre-flight verifier, can anyone explain?  (Read 3280 times)

GarryP

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I was doing some experimentation for some other thing and came across a situation I didn't understand. I'll explain...

I created an image frame 128pt by 80pt. Then I inserted an image that was 72x72 DPI at 1280x800 pixels, scaled to the frame size.

This resulted in Scribus saying that the "Actual DPI" (in Properties/Image) was 720x720. This seems fine to me - x and y scales are 10% so 72 DPI x 10 = 720. (This was also the case for the "Effective DPI" in the "Manage Images" dialog. By the way, is there a difference between "actual" and "effective"?)

However, after changing the pre-flight verifier setup to be "Lowest Resolution" = 300 DPI, "Highest Resolution" = 600 DPI, and then using the P-FV I was told that there were "No problems found".

This seems odd. The original image is 72 DPI so if the P-FV looks at original image resolution it should have shown a problem at the lowest end, or if it looks at "Actual DPI" (720 DPI) it should have shown a problem at the highest end. But it showed neither.

Can anyone explain this to me in simple terms please?

Is it a bug or am I missing some vital bit of information? I realise that DPI is more of a complex subject than it looks but what is the "DPI" that the P-FV is looking at? And how can I know?

utnik

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Re: Unexpected results from the pre-flight verifier, can anyone explain?
« Reply #1 on: August 23, 2013, 02:10:06 am »
hi garry

i just checked on my system (scribus 1.4.3, mac os 10.6.8) – the preflight verifier reports the high resolution as expected.
did you change the minimal and maximal resolution under. 'preferencies' → 'preflight verifier' or under: 'document setup' → 'preflight verifier'?
with 'preferencies' there wont be any change in the actual document – this is for general settings. they affect only new files!

btw: what matters is only the effective dpi – the other is a free guideline, how to represent the image…
utnik

Nermander

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Re: Unexpected results from the pre-flight verifier, can anyone explain?
« Reply #2 on: August 23, 2013, 08:46:58 am »
DPI isn't complex at all, it is: Number of pixels divided by physical (printed) size.

300x300 pixels printed as 2x2 inches is 150 DPI, 300x300 pixels printed as 1x1 inch is 300 DPI.

Many image formats though contain a DPI field that can be seen as a "suggested printing size", but that field is just a number in the file. It's not a property of the actual image, which unfortunately a lot of people seem to think.

When an image comes from scanning the DPI field in the image might have some relation to the actual physical image, but with images from a digital camera any DPI value inside the image file is just not relevant.

GarryP

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Re: Unexpected results from the pre-flight verifier, can anyone explain?
« Reply #3 on: August 23, 2013, 11:53:52 am »
utnik: Ah, damn, that's it exactly. I'd changed the "Preferences" rather than the "Document Setup". A school boy error. (There doesn't seem to be an emoticon for "Doh!".)

I'm now wondering if there might be a simple way of showing the user that each dialog is different in some way. Maybe a label at the top of the dialog telling the user where the changes will be applied, or something. Maybe it's just me but I don't go into either dialog very often - and I guess lots of users will be the same - so I keep forgetting the difference between them. Or maybe it's just a very simple thing that I'm just not "getting"; that wouldn't be a new concept by a long chalk!

Nermander: I think what you've said explains a bit of my confusion. There are a lot of "pseudo-DPIs" that can get in the way of the real info.

Here's another example of where I keep making a mistake…

I can create an image in GIMP which is 128x80 pixels but also give it a DPI of 720x720. GIMP has no problem with this so, on the surface and without giving it any thought, it sounds reasonable - it's a small image with very fine detail. But… just giving it a tiny bit of thought shows that it is clearly silly. On the horizontal you've only got 128 pixels so how can you have 720 "dots" in 128 pixels? You just can't, it's physically impossible to have more "dots" than you've got pixels. All you get is a really tiny image (which Scribus displays perfectly but it just looks very strange until you realise what you've done wrong). I've done this so many times it's becoming embarrassing.

Maybe it's just me getting old/stupid/useless [delete as appropriate], or maybe it's because I don't do this stuff very often (I only do DTP-related things a couple of hours a week on average, although I do try and visit the forum when I can), but I keep forgetting some very basic stuff so I keep making the same silly mistakes and have to re-learn things over and over again. Some things just don't stay in my head long enough to be permanent.

Anyway, thanks to you both for your, as usual, excellent answers. I just hope I remember so you won't have to give the same answers again a few months down the line!

Cheers.

Nermander

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Re: Unexpected results from the pre-flight verifier, can anyone explain?
« Reply #4 on: August 23, 2013, 02:24:55 pm »
128 x 80 pixels and 720 DPI means the image is supposed to be 0.177 x 0.111  inches.

But those 720 DPI is just a number in the file. If you change the DPI to 72 the image will still contain the very same pixels as before, you are just saying that they should be printed/displayed larger than before.

So, the DPI is never a property of a digital image, it is only a property of a displayed or printed image. And in that case the DPI depends on the "zoom", "scaling" or whatever you can call it.

GarryP

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Re: Unexpected results from the pre-flight verifier, can anyone explain?
« Reply #5 on: August 23, 2013, 04:19:57 pm »
Yeah, I think I must have learnt something ages ago which turns out to be wrong and that "bad learning" is interfering with my grasping the concept fully nowadays.

Like learning something at school (e.g. that we have five senses) and then later finding out that it was wrong (e.g. we actually have between 11 and 20-something depending on how you categorise them) but the old ways keep pushing themselves into your mind making you forget the real information.

At some point it'll click, I hope.

Thanks for your help anyway.