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Graphics

2002082401 Email to Ronx
 

Why Compress Graphics

Advantages (and perils) of graphic compression


Original picture Original
= 18.2KB

Compressing the image has a marked impact on its file size - 18KB original can be compressed down to less than 2KB.This in turn means that the image file takes up less space on the server, and, more importantly, will download faster.

However, excessive compression will ruin the image quality.In this example there is very little difference between the original and 40% compression, but at 70% artefacts appear, spoiling the appearance, and 90% is unusable.

Choosing the image format is also important - the gif example shows that a gif is not suitable for a photograph.The stippled effect is due to the lack of colour information.GIFs are most suited to line art and images with large blocks of solid colour.

General Advice

Never work on the original image.Always use a copy.It is preferable to bury the original where it can only be found in an emergency.

Never edit and save a .jpg twice - each save will cause an image degradation.If possible, never work on a .jpg image at all; use the native format for the graphics program (e.g. for Paint Shop Pro, edit the PSP format and save as a PSP file.When it is perfect save as a .jpg
and as a .psp as well.

If the picture has to be edited again, go back to the .psp file

Change the physical dimensions of the image in the image editor before saving.This results in a smaller file size - using HTML to resize the imagewill not alter the file size, and may introduce distortions.

20% Compression 20% compression
= 4.51KB
40% Compression 40% compression
= 3.29KB
70% Compression 70% compression
= 2.45KB
90% Compression 90% compression
= 1.69KB
GIF format gif format (256 colours)
= 10.9KB