Re: Screen capture
Re: Screen capture
- Subject: Re: Screen capture
- From: Bill Briggs <email@hidden>
- Date: Tue, 9 Dec 2003 17:10:55 -0400
At 8:01 PM +0100 08/12/03, Emmanuel wrote:
At 9:29 AM -0800 08/12/03, Walter Ian Kaye wrote:
Actually, you would be best off with Snapz Pro X anyway, as PDF is
rather a bad format for sending over the Web. If you want to use
GIF or JPEG instead (for all of the obvious reasons), Snapz'll be
better.
This statement suggests that a JPEG would always be significantly
smaller than the corresponding PDF - making the latter "a bad format
for sending over the Web". I am afraid this is not totally true.
Depends on the fidelity (compression level) of the JPEG which is
smaller, but you have very good chance with screen captures of having
a smaller image with GIF or LZW compressed TIFF.
I've just made a screen shot, PDF = 180 KB. Give it to Preview, save
as JPEG with best quality (so that I don't loose any information) ->
248 KB. (Then if I lower the JPEG's quality I get much smaller
files).
More surprising yet: I stuff both files, both of them keep the exact
same size (180 and 248).
The image in the PDF is likely compressed from the outset, and you
know the jpeg is, so stuffing the files buys you nothing as no more
compression can be done. In fact the overhead from the stuffing
compression routine should add a bit to an already compressed file.
The PDF is using a lossless compression with which to store the
screen capture, which you can see if you magnify it. I just opened
the file in a text editor to see if the image type was evident, but
the file header doesn't give any indication.
Generally JPEG is a poor choice of compression methods for things
like screen captures where you have long runs of uniform colour and
high contrast edges. JPEG is lossy and the bicubic interpolation that
transform performs makes the high contrast edges blurr. The effect
around those areas is like looking through a glass smeared with
vaseline. As someone said, it's lossy (it loses information about the
image). JPEG was designed to be used to compress photographs and
other continuous tone images. Even there it tends to mangle some
parts of the image. Photos of faces show compression artifacts around
the eyes and nostrils which are high contrast areas. JPEGs basically
suck for screen shots. There are better lossy methods available using
wavlet transforms that are much more faithful when the image is
reconstructed.
Screen shots should be taken as TIFFs and LZW compressed or as GIFs.
These use lossless methods. They are dictionary based substitution
methods that are highly effective at compressing images with long
runs of the same colour, or large fields of the same colour (which
largely describes screen captures). They can be very small, and they
look MUCH better than JPEG versions of the same screen because they
un-compress to look exactly as the original did.
- web
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