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Re: Screen capture
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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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References: 
 >Re: Screen capture (From: Walter Ian Kaye <email@hidden>)
 >Re: Screen capture (From: Emmanuel <email@hidden>)

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