This is because the format uses lossy compression, which removes
information that the eye doesn??™t need. As the compression level increases, this information
loss becomes increasingly obvious, as shown in the following images. As you can see
from the image on the right, which is much more compressed than the one on the left,
nasty artifacts become increasingly dominant as the compression level increases. At
extreme levels of compression, an image will appear to be composed of linked blocks (see
the following two images, the originals of which are in the chapter 4 folder as tree.jpg
and tree-compressed.jpg).
WORKING WITH IMAGES
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Although it??™s tricky to define a cutoff point, it??™s safe to say that for photographic work
where it??™s important to retain quality and detail, 50 to 60% compression (40 to 50% quality)
is the highest you should go for. Higher compression is sometimes OK in specific circumstances,
such as for very small image thumbnails, but even then, it??™s best not to go over
70% compression.
If the download time for an image is unacceptably high, you could always try reducing the
dimensions rather than the quality??”a small, detailed image usually looks better than a
large, heavily compressed image. Also, bear in mind that common elements??”that is,
images that appear on every page of a website, perhaps as part of the interface??”will be
cached and therefore only need to be downloaded once. Because of this, you can get away
with less compression and higher file sizes.
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