[FM Discuss] Optimizing images for printing

[Daniel James]

Hi Ana,

> It could be really very useful to set some rules for fm images.
> 
> As I understood we should you use 72 ppi for screen

This is a long-held convention, based on the idea that there are 72
points to the inch in desktop publishing software (points are an archaic
unit used by traditional printers, the human kind - see:
http://en.wikipedia.org/wiki/Point_%28typography%29). I recall this 72
ppi figure was roughly accurate with the CRT monitors of the 1990s.

However, if you measure the width of a current 24" diagonal HD screen
with a tape measure and divide it by the resolution, you will get
something like:

1920 pixels / 21 inches = 91 pixels per inch

If you do the same calculation for a laptop or netbook, which squeezes a
high resolution into a small physical size, you will probably get a
number between 100 ppi and 140 ppi. So the 72ppi figure is now
completely bogus.

Bear in mind that on the screen, unless you specify a width or height
attribute in the HTML IMG tag, you get exactly one pixel displayed for
each pixel in the image.

So in this case, ppi is only useful to determine how physically large
the image will appear to the end user, and this is hardware specific.
For example if you are writing a manual for people who don't have good
eyesight, you might want to make sure that they are not reading it on a
netbook.

The iPad 2 is 132 ppi with less than nine inches wide viewable, which
makes it a poor replacement for a printed book, especially in landscape
orientation with fake margins wasting space. (On my way back from the
Mozilla Festival last night, I noticed a senior citizen squinting at one).

> and 300 ppi at least
> for printing

That depends on the output resolution (toner dots per inch) of your
laser printer. Although they are commonly confused, ppi in the source
image and dpi on the printed page are not equivalent. For print on
demand, the laser printer might be 600 dpi output, but this doesn't mean
you need to print images at 600 ppi. That would make all flossmanuals
images very small on the printed page.

> so for booki kind of floss manuals images are commonly
> best on a 150 ppi?

For a screenshot, you get exactly the number of pixels on the screen. If
you resample the image to a higher resolution, you cannot get more
quality that way.

Because of the pixel width limitation in current Booki, I recommend
tightly cropping all screenshots to show just the feature you are
writing about, rather than a 'whole screen' screenshot reduced to 600
pixels wide. It takes longer, but the results are so much clearer that
it's worth it.

Cheers!

Daniel