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JPEG: (Joint Photographic Experts Group)
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256 Colours, Shades of Gray, or 16 Million Colours.
Recommended - 96 dpi.
24-bit color planes
remember to use 'JPEG Interchange' not 'JPEG Tiff'. (*.jpg)
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This format was derived and released into public domain. There are no laws or governing bodies which control or define it's use.
JPG can represent any number of RGB colors up to 16.7 million.
JPG Uses HSL (Hue, Saturation, Luminescence) in its compression method.
How it compresses...
RGB values are converted to mathematical formulas by your computer.
Data reduction begins using a process called sub-sampling.
- the brightness information is left alone while 1/2 of the other 2 scales are eliminated.
- they are eliminated by replacing two neighboring pixels with a single value representing their average
- this cuts the image size down by 2/3 in size.
- this also leaves the brightness intact
The image is partitioned into 8x8 blocks, using "DCT" (Direct Cosign Transform) and "Quantization"
- the changes in brightness and colors are identified and rounded off.
- (3's are rounded to 5's and 75's may be rounded to 100's)
Now, once all the rounding off has been done, there are a lot more standard and identical settings and values
- this allows it to compress via a standard and unencumbered "Lostless" compression method.
Points to note:
- JPG uses Lossy Compression which means it throws out bits of the image to make the image file smaller.
- Reducing the number of colors in a JPG won't help much in reducing the file size because JPEG sizes are determined by the amount of compression not by the number of colors.
- Progressive JPEG -allows the image to be seen as it is being downloaded.
Summary:
You do not have to reduce to 256 colors, but when it compresses, some image details are lost. The amount of detail lost can be determined by the user using an image editing program. The more detail lost, the smaller the file size will be.
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