Re: 600 dpi Screen Captures?

[Max Wyss <prodok -at- PRODOK -dot- CH>]

Shifra,

there is always a confusion with image size and resolution. Let's look at
screenshots. In order to keep it simpler, assume that the screen is
greyscale.

The screen consists of a grid of 640 x 480 individual image points (they
are called "pixel"). All the information you see on screen is cut up into
this grid. When you take a screen shot, you get an image size of 640 x 480
pixel. A pixel does have a certain grey level. In most cases, it has one
out of 256 levels.

Let's also assumet hat your screen physically measures 8 by 6 inches. The
640 x 480 pixel grid is laid over the 8 by 6 inches. The result is a
resolution of your screen of 80 pixel per inch. This means you do have - if
you want to have the screen at its original size - to represent it at these
80 pixel per inch.

Now, what is the difference between "pixel per inch" and "dots per inch"?
If the screen would be purely black and white, there were none. A printer
prints either black or white (yes, I know there are printers who do print
grey levels). This means that in order to represent 256 grey levels you
need a "cell" of 16 by 16 dots for a pixel. This means that - if you want
to have the screen at its original size, and truely represent all 256 grey
levels - you would need a printer resolution of 16 x 80 = 1280 dots per
inch.  However, this would already mean high-end representation, as the
human eye can barely distinguish 256 grey levels. So, you can easily reduce
the cell size to 8 by 8 or even 6 x 6 dots, allowing you to represent 64 or
48 grey levels.

The definition of the cell size and the transforming of the picture
information into dots is handled by the raster image processor of your
pinter (which can be part of the printer, or be part of the printer
driver). You feed it a 640 x 480 pixel image, and the raster image
processor does the appropriate conversion It does essentiall not matter
which resolution the printer has; you will get either more different grey
levels (and a coarser picture) or a finer picture (with less grey levels).

Now, what does that mean for your friend's problem? She starts with a 80
dpi image. When she places the image, it will have a certain size (I am way
more familiar with Framemaker, and avoid Word for technical documents, so
her mileage may vary). In fact, the layout software tries to represent the
picture at its original size. (some programs assume that the picture is
already at printer resolution, and want to represent it at the size of the
printer resolution; with a 640 dpi printer, our example would become 1 by
0.75 inch; which is what you have noticed when you did your test).

So, if the printer insists on 600 dpi, you take your image, load it into a
image processing software, such as Adobe Photoshop, scale it to your target
image size (in inch or mm), and then change the resolution to 600 dpi _by
keeping the physical dimension the same_. Don't be surprised when you get
pretty big files.

The image now has a higher resolution, but the information contents is
still the same. You did not add any information to the original 640 x 480
pixel. You now may have blobs of the same grey level measuring 6 x 6 dots.

May sound a bit confusing, but I hope, this can help.


Max Wyss
PRODOK Engineering AG
Technical documentation and translations, Electronic Publishing
CH-8906 Bonstetten, Switzerland

Fax:  +41 1 700 20 37
e-mail:  mailto:prodok -at- prodok -dot- ch    or    100012 -dot- 44 -at- compuserve -dot- com


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>Hi again.
>I am posting again for a friend, I am going to make her subscribe herself
>soon....
>
>She was told by her client that "the screen shots are in 300 dpi low
>resolution.. I *told* you I wanted 600 dpi - do them all again."
>I was not sure what to advise -
>---------------------------------------------------------------------------
>------------------------------
>The people I asked said that 600 dpi is only a printer setting and screens
>cannot be captures at more than the resolution that you see - 75 dpi.  The
>screen resolution on my comp is set to the highest..... I have really no
>idea what I am talking about.  Are resolution and dpi the same thing - I
>thought so but now I am not sure.  After downloading 4 different screen
>capture utilities - nne of which have ANY refernce to dpi, I tried Corel
>Capture.  I was able to save in 600 dpi - but that just made it smaller -
>and I had to enlarge it.  Oh and one more thing - probably the most
>important - the program is currently being turned from DOS to Windows and
>the Screens I am taking now are in DOS.  I tried shrinking and then
>capturing the screen in order to retain some of the details I lose by
>capturing in full size and resizing in Word.(what the client suggested as
>the solution).I tried many other things but I know they are not correct, so
>I will not bother to expand.  My client still insists no matter what I send
>(he insisted on PCX for a while) that the quality is not good enough and
>his printer has grand cpabilities and he can't print a 600 dpi screen
>unless it was taken at 600 dpi.... HELP!
>---------------------------------------------------------------------------
>-----------------------------
>Please respond to gldmeier -at- actcom -dot- co -dot- il
>Thanks alot - i am sure someone out there can help her - At least that's
>what I told her.
>Shifra