Chris and NOF list,
Must do some work today.....but a few quick thoughts.....
<snip>
> - In the publishing world, generally "standard" prints are scanned to
> 300 DPI/PPI, uncompressed.
Yes, indeed, but again this can be just as misleading ( Publishing
world have got the same sized feet as the Printers!). The key point
here is that 'prints are scanned "TO" 300 DPI/PPI'. In other words,
the outputted image file must have 300 Pixels for every inch of it's
size when made in the 'Real World'. It does not mean that you scan at
300DPI, which unfortunately is what many people think. If there is no
increase in size between the original and the output, then the scanning
res will also be 300PPI, but if the original was 5x4ins and the output
twice as big then you would need to apply a 'zoom factor' to expand the
image. This is a complicated way of just increasing the scanning res
to 600 PPI which will of course provide the additional pixels.
We have undertaken some tests on these files and
> have
> blown up a 5x4 scanned photo to print an A3 poster to an acceptable
> level of
> quality (we even took it up to A0 and it was useable, albeit at a lower
> quality.
This is impressive! If you scanned a 5x4 image at 300 PPI then by
definition you had an image sized 1500 x 1200. As you point out, the
print industry would only expect this to print to about 7x5in max. I
would agree that using modern stochastic techniques of printing (like
the error dithering in your Epson, Canon etc), you can get some much
better results.....and of course with continuous tone printers such as
the Fuji Pictography, they will be still better, but this is all a long
shout from standard commercial printing.
>
> - It also depends on the scanner you are using; I've seen people using
> ?70
> scanners at resolutions of 1200x1200 - because they wanted the, quote:
> "best
> quality" ..... er, think they've missed the point here!
>
Yes - they have! <G>
I always ask why a commercial bureau would want to pay 30,000GBP for a
scanner if they can get the same results from a 1,000GBP one.
> - Another consideration which I don't think has been covered in these
> postings is the use of resolution to prevent abuse of images. Most
> standard
> printers operate at 300x300 DPI as a minimum (many are higher). By
> having
> publicly accessible images at an acceptable "screen resolution" -
> typically
> 72-100 DPI/PPI, when printed, they come out too small generally for
> use/abuse.
Yup.....couple of points, Image size (not the res) is a brilliant way
to protect images from misuse.....simply being 72PPI does not make the
image lower quality(size). A 10ft x 8ft oil painting scanned at 72PPI
will make a 8640 x 6912 pixel image, which by anyone's standards will
be pretty high quality! But an image 500 x 300 pixels is going to
contain a limited amount of information whether it is 1 in long (at
500PPI) or 5in (at 100PPI) and of course the image is identical in both
cases.
Sorry, let me say that again, and please bear with me experts!
A 500 x 300 pixel image can be
1in x 0.6in at 500PPI or 5in x 3in at 100PPI
but in both cases the image is identical.
As I said giving 'PPI' is only useful if you also give the real-world
size of the image.
The second point is my old bugbear about why we should be fussy about
using the correct units 'DPI' 'PPI' and 'LPI'....each has a different
and specific meaning and use.
When you say 'standard printers operate at 300 DPI'.....I presume you
mean desktop printers as opposed to commercial printing which as you
say are much higher. But it is important to remember that DPI is the
(correct) unit to measure the smallest dot capable of being printed by
the printer not the screen res that measures the printers ability to
convey each pixel.
A 300 DPI printer can print Black or White at that resolution, but if
you want to print a grey, then the printer must introduce a print
screen or print resolution to mix some black dots with white areas to
convey a grey.
As I am sure you know, this resolution is called Lines per inch or LPI
and is a true indication of a printers ability to print half-tone
information. So if you were to print an 8 bit colour (256 values) you
would need a square containing 16 x 16 'dots' so if a 300 DPI printer
was to print 8bit colour (they rarely do!) you would only in fact be
able to print at about 20 LPI......a very low quality.....and why
images printed on old laser printers looked so very bad.
As you say, the commercial quality printers normally specify scans at
300PPI, this is because they print to a LPI of around 150 - 225(tops!)
and always specify an additional 'half-tone' or 'quality' factor of 1.5
- 2.0. This means that a 300ppi scan (at output size!) should provide
the correct amount of information for most uses.
But believe me a commercial printer running at 175 LPI is way better
than a desktop at 300 PPI.
OK....so.....you say "but my epson.....is brilliant at 2400 DPI". Yup
they do, but they are still only a 150 (or lower) LPI printer. The
reason that they can look so good (and don't they!) is because they are
using FM (stochastic) printing screens which use a fixed size dot but
vary the frequency, rather than a fixed no of dots and varying the size
(as in normal print). This hides the visual effect of the screen and
smoothes out the image. They have worked really hard to improve the
visual appearance of the images....and they are getting damn good!
The exception to all this are the continuous tone printers such as
dye-subs like the Fuji Pictography. Here the printer is not limited to
cyan, magenta and yellow but can print any colour within gamut. This
means that when they say it's 300DPI....it is ....but it is also
300LPI....which is why they look so damn good.
If this area interests anyone.....you will find more information at:
http://www.tasi.ac.uk/advice/creating/dpi.html
>
> - Another aspect of image quality is colour depth
>
> - When looking at image quality, compressed JPEGs/JFIFs do not lend
> themselves to large/solid blocks of colour. The effects of compression
> will
> become very apparent in images containing material of this nature. Fine
> line
> drawings also suffer. This is because one of the ways in which the JPEG
> compression routine operates is by comparing each pixel to neighbouring
> pixels and establishing patterns. Instead of describing the attributes
> of
> each pixel (as in a bitmap, .bmp file), it will look at a block of
> similarly
> coloured pixels and store a summary of this (e.g. 5 pixels of dark red)
> - in
> computing terms it takes less storage space to do this than to describe
> each
> individual pixel's attributes. The greater the compression, the more
> information it "throws away" and the more it averages out blocks of
> colour,
> causing pixellated blocks or fuzziness in areas of fine details.
>
too right....one of the easiest ways to compress a file is always going
to be to remove the colour depth, our brain simply does not notice
taking colour away as much as taking detail away.
Personally I think the JFIF file format is well past it's sell-by date
anyway. It's not even the actual file format recommended by the Joint
Photographic Experts Group.....that was the SPIF format....which is a
much better format altogether.
In the wings there are formats based on vector and wavelet technology,
that simply way outperform the old JFIF file using the very dubious
JPEG compression.
Roll on JPEG2000........please!
sorry to take so long again, I will most probably have to leave it
there, as I really must get back to doing some work, or I will get my
knuckles rapped!
cheers all
and keep talking
eib
> Regards
>
> Chris Meaney (AIMC)
> Managing Director
>
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>
> -----Original Message-----
> From: This list is for people who are receiving New Opportunities Fund
> Digitisation funding. [mailto:[log in to unmask]]On Behalf Of Ed I
> Bremner
> Sent: 28 May 2002 16:48
> To: [log in to unmask]
> Subject: Re: Web image dimensions paper.
>
>
> Daniel and NOF-Digi list.
>
> thanks for putting together the report on the thorny issue of File
> sizes for Internet delivery. It makes interesting reading.
>
> My time these days is rather tight and I don't really have the
> opportunity to feed into what you guys are doing in NOF-Digi much, but
> believe me I read the list with great interest.
>
> These are a few thoughts on the content of the report and feedback that
> you had from your email, I hope it encourages a little more discussion
> from the list on these important topics. I am sorry it is so long, but
> they are all rather favorite topics of mine.
>
> If you find any of this interesting, please remember to take a look
> at the TASI site at http://www.tasi.ac.uk where you will find some much
> better material covering largely the same area.
>
> **Quoted image resolution**
>
> I am always a little worried when we start gauging the quality of
> digital images by their 'dpi' (ppi would be more accurate - but I won't
> start on that one). Using 'dpi' in this way is both misleading and
> inaccurate. Folklore suggests that a screen resolution is 72dpi but
> really this is neither here not there (in fact it's not even nearly
> true on modern monitors....just measured mine and its 106dpi). The
> only possible and reliable way of quantifying the size of an image is
> by its pixel dimensions (as you rightly point out in section 1).
>
> I know this can be hard to grasp so let me give an analogy....
>
> If you asked me how much I weighed and I answered 2.8 pounds per square
> inch, you would think that I was being less than helpful...but this is
> precisely what we do when we quote an image's resolution in pixels per
> inch. You have no way of working out how much I weigh unless you also
> ask me...."Ed...how big are your feet?" Assessing the quality of
> an image is exactly the same.....the ppi of an image only gives you an
> indication of the quality of the image if we also know what the
> physical size is of the original or the proposed output.
>
> If I have an original which is 10inch long and I scan it twice, once at
> 300ppi and once at 100ppi, I will have two images; one 3000 pixels long
> and the other 1000 pixels long. The quality will be identical, only
> the size is different. The resolution is only of interest (or
> relevance) in that scanning process. It provides no evidence of quality
> (unless we also know size of the original). Once created the only
> relevant figure is the size in pixels, and in this case one image
> contains 3 times as much information as the other.
>
> NEITHER HAVE ANY SIZE in inches, cms or any other unit of the real
> world. You simply have two images......at 3000 and 1000 pixels long.
>
> You only have to worry about their resolution when you want to output
> them to some physical form in the 'real world'.
>
> Or to put it the other way around.....the image provides information
> that can either make a very high quality print (high dpi) at a small
> size, or to provide a lower quality print (low dpi) at a bigger size.
>
> Again quality can only be ascertained by knowing the size of the
> output. If you share a large amount of information (3000 pix) over a
> small space (5 in), the quality will be high (600ppi). If you share a
> smaller image (1000 pix) over a larger space (20 in) the quality will
> be low (50ppi) but in both cases the image is identical even though one
> is 600ppi and the other 50ppi.
>
> Gosh, I do hope this makes this all a little clearer, but if I had a
> penny for every time I had heard:
>
> "We have scanned everything to 'preservation' quality at 300dpi... but
> I don't understand why the scanned slides are not as good quality as
> the A3 prints..."
>
>
> I have tried explaining this many times before and I apoligise if
> despite the repetition I am still unable to explain it any better. Try
> reading the link below for another attempt on the same material.
>
> http://www.tasi.ac.uk/advice/creating/digitalimage.html
>
>
> **Image sizes**
>
> I think that section 1 covers all the points. The key thing when
> working out any question of image size for any imaging project...is to
> work from the other end....
>
> How much information do you need to capture?
>
> How big do you want to print the images and at what quality?
>
> How much detail do you need to capture within the original to extract
> the useful informational content?
>
> If you can answer these questions, then you already know the answers.
>
> If you decide that you will need to make A4 magazine quality images,
> you can deduce that you need approx 25Mb or 3600 x 2400 pixels to
> make them. The required resolution will be that figure which provides
> you with that size of image.....and if you have differing size
> originals, then it will vary dependent upon their size (as shown in the
> "Gathering the Jewels" image parameters guide.....so again sorry....no
> magic resolution figure.
>
> If you want to provide the image for delivery on a monitor, then I
> totally agree with your advice to provide something that fits into a
> 640 x 480 area, however do consider the issues of future
> sustainability....the standard monitor size (dare I say resolution<G>)
> is certainly going to grow and you will certainly need to deliver
> larger images at a later date.
>
> Of course the points you raise in Section 2 are of paramount
> importance. You will simply find that some images, especially those
> with small text, simply can not be shown in any useful way at these
> small image sizes.
>
> Actually this is interesting not just for print, some modern
> paintings...and many watercolours simply have very little visual
> information in them and can be understood and appreciated at
> screen resolutions, whereas many 19th century engravings simply are not
> readable at all, at sizes under 800 x 600 pixels.
>
> Again there are no rules, only rigorous research, followed by setting
> standards which are fit for the proposed purpose.
> There will be as many different 'correct' image sizes as there are
> different projects with different purposes.
>
>
> **Compression **
>
> A couple of points:
>
> With JPEG compression (remember JPEG is the compression - JFIF the file
> type) you will be able to vary your visual quality (and file size) from
> 'no visual difference' to 'small - but totally ruined' - it's your
> choice. A few things to remember though, Some image types will
> compress better than others, Watercolours will need a different amount
> of compression to engravings....it's just the nature of the way JPEG
> works. So test your image types with different amounts of
> compression and then make a choice and stick to using the appropriate
> amount of compression.
>
> A general rule of thumb....over many thousand images....of all
> types....is that if you compress all images to a point where they are
> 'just visually undiscernable from the originals' then on average this
> should be approx 10% of the file size, but it will vary from 5% to 20%
> (my guess - no evidence for this). You might well wish to compress
> them more than this, at the cost of a lower image quality but it gives
> you a starting point.
>
> Someone pointed out that a 40% compression in one software is not the
> same as another......it's tons worse than that! First these figures
> are totally arbitrary, 40% does not mean you get a filesize 40% of the
> original....just that you are 40% of the way up the scale. Secondly
> watch out that some software gives figures for 'Quality' whilst others
> for 'compression'........they are opposite values and again 40 quality
> does not equal 60 compression.
>
> Another thing that I would point out is that JPEG compression is done
> by many proprietary algorithms.....(this is true for re-sizing also)
> and that some simply do a much better job than others giving higher
> quality files.....at smaller file sizes. On the whole I would say that
> the algorithms used by Adobe are some of the best (I suspect there are
> better) but others such as those used by ULEAD (the imaging engine used
> in some image management systems for internal JPEG compression) are
> certainly not as good. I have never compared Adobe to
> Macromedia.....mmmm....nice little project for someone <G>.
>
> The golden rule of all compression?
>
> "Never JPEG a JPEG"
>
> OK should be never JPEG a JFIF....but simply does not sound so good!
>
> If there is any part of your workflow where a JFIF is being
> JPEGed.....change it.....JPEG must only be used once and as the last
> operation before delivery.
>
>
> **Watermarking**
>
> Very interesting section this! I was aware that watermarking increased
> file-size, but not that it could add 100% size with text based
> images....good point!
>
> a couple of thoughts. I suspect that watermarking should really come
> at the very very end of the image workflow. I know I always say "Do
> everything......and then JPEG for delivery" but in this case I suspect
> that JPEGing a watermarked file will simply undo the work done by the
> watermark, leaving it possibly more visible....and also less durable.
>
> For those who wish to keep to the most rigorous control over colour
> quality...I suspect there could be real problems about watermarking
> ....and then JPEGing. I would need to do more research before being
> too dogmatic about it though....just have worries...although I admit to
> needing to have a good think on this one.....anybody else got any
> thoughts?
>
> I see that a few of the projects are going for a workflow where they
> make some form of delivery master in a lossless compressed or
> uncompressed form such as TIFF or PNG and use that to create delivery
> images....with watermarking....on-the-fly....
>
> Now that's the way to go!
>
>
> Sorry this has been so very very long.....
>
> If you think that you shouldn't really have the time to read
> it.....mmmmm.....I know what you mean, because I certainly didn't have
> the time to write it <G>.
>
> cheers
>
> eib
>
>
> ----------------------------------------------------------------------------
> Ed I Bremner, QA-Focus/TASI Senior Technical Research Officer
> Institute for Learning and Research Technology
> University of Bristol, 8-10 Berkeley Square, Bristol, BS8 1HH
> Tel: +44 (0)117 928 7170 Fax: +44 (0)117 928 7112
> http://www.tasi.ac.uk/ A JISC Service
> ----------------------------------------------------------------------------
>
>
> On Mon, 27 May 2002 13:55:44 +0100 Daniel Merriman
> <[log in to unmask]> wrote:
>
> > Hi,
> >
> > Well, I really didn't want to send the document to the whole list but
> I
> > have been left with no choice due to high demand. Here goes...
> >
> > Apologies to those who do not require a copy.
> >
> > For those who are interested, please see attached.
> >
> > regards
> >
> > Dan.
> >
> > Daniel Merriman - Project Manager
> >
> > From Weaver to Web Project
> > Calderdale MBC
> > Reference Library
> > Northgate House
> > Halifax
> > HX1 1UN
> >
> > 01422 392632
> >
> >
> > *** SCANNED FOR VIRUSES ***
>
> ----------------------------------------------------------------------------
> Ed I Bremner, QA-Focus/TASI Senior Technical Research Officer
> Institute for Learning and Research Technology
> University of Bristol, 8-10 Berkeley Square, Bristol, BS8 1HH
> Tel: +44 (0)117 928 7170 Fax: +44 (0)117 928 7112
> http://www.tasi.ac.uk/ A JISC Service
> ----------------------------------------------------------------------------
----------------------------------------------------------------------------
Ed I Bremner, QA-Focus/TASI Senior Technical Research Officer
Institute for Learning and Research Technology
University of Bristol, 8-10 Berkeley Square, Bristol, BS8 1HH
Tel: +44 (0)117 928 7170 Fax: +44 (0)117 928 7112
http://www.tasi.ac.uk/ A JISC Service
----------------------------------------------------------------------------
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