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Photoshop

Digital Graphics

JPEG Format

Joint Photographic Expert Group (JPEG) is the best format for photographs because they contain 24-bit colour. That means it can support millions of colours (16.8 million, actually). The JPEG format does not support transparency. All current browsers support JPEG (also expressed as JPG) images.

Progressive

The Progressive option for JPEG images works the same way as interlacing does for GIFs. Instead of loading each row of the image pixels from top to bottom, a blurry version of the image displays initially and then it sharpens as the rest of the image downloads.

Lossy Compression

Lossy compression means that, depending on the amount of compression you apply to an image, some of the original detail will be lost. The advantage is that you can reproduce a high quality photograph over the Web with as much as a 100:1 difference in file size! GIF cannot compress photographs anywhere close to this.

Photoshop allows you to specify the degree of file compression so you can create a balance between image quality and file size. If picture quality is more important, choose a higher quality setting when you compress the file. It will not be as small as it could be, but it will be smaller than the original and look quite good. If you need smaller file sizes, increase the compression. The file will be dramatically smaller than the original, but it won't look as good.

The table below shows the results of four different JPEG compression settings applied to the image shown here. The images were magnified by 300% so the differences in quality would be easier to see. The file sizes shown represent the results of compression on the original file, not on these particular images.

Comparison of JPEG Compression Settings
80 quality 1.8K	60 quality 1.259K
30 quality 849 bytes	10 quality 653 bytes

The rule of thumb: use GIF for icons, banners, and cartoons; use JPEG for photographs. If you are not sure, save your images in both formats and compare file size and image quality, and go with what fits your needs best.

JPEG Format Summary

• The JPEG format uses a 24-bit fixed colour depth.
• JPEG uses a variable, lossy, compression system. The higher the compression, the smaller the file size but the penalty is a greater loss in quality.
• JPEG is best used for photographs or any image with many colours.
• GIF allows for animation and transparency. JPEG does not.

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Photoshop - TOC - Introduction - Books -
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Photoshop - TOC - Introduction - Books -
Digital Graphics - Links - Questions -
1 - 2 - 3 - 4 - 5 - 6 - 7 - [ 8 ] - 9 - A -