When putting together Projected Digital Image competitions, we need to match
Current projectors display 1024x768 pixels at 24 bits per pixel, requiring image file sizes of 2.3MB. (We can expect projectors capable of 1600x1200 pixels at 32 bits per pixel, which will require image file sizes of 7.5MB.)
Thus a 2.3MB bitmap (.bmp) or TIFF (.tif) file will contain all the information that a current projector can display. Unfortunately this size of file is too large for convenient transmission, so we want to compress it. The current choice is JPEG (.jpg). This reduces the file size to somewhere between 400KB and 800KB depending on the gradations of colour within the image. However, it can also introduce errors, and if you edit images in JPEG, repeatedly opening and saving them, then the image quality is degraded (see below ).
The PNG format does not give such high levels of compression (typically a 1024x768 image results in a file size of 1.2MB, or 50% compression) but does not introduce errors and does not degrade with multiple edits. Moreover it can store Photoshop CS's 16 bit-per-channel images whereas JPEG is limited to 8 bits-per-channel.
Note that TIFF files are not per-se compressed. They can contain JPEGs or zipped bitmaps, or can use Unisys's LZW compression algorithm. This, like PNG, is lossless, but produces slightly larger file sizes.
The following table shows the amount of compression achieved by saving an image using different file formats. The results were obtained by saving files of 1024x768 pixels using Photoshop CS.
"Multi" was an image (shown below) that contained a large number of different colours, while "White" was an image that contained only white pixels.
"Picture 1" was a typical landscape photograph while "Picture 2" was a photograph of a face.
The Type and Compression columns show the compression used, and the other columns show the resulting file sizes in KB.
TIFF files can be saved with no compression, LZW compression (a Unisys Patent method), JPG or ZIP. For comparison purposes, the sizes of a zipped non-compressed TIFF file and a zipped bitmap file are also shown.
JPEG files can be saved at various qualities. "Maximum" in the table represents Photoshop setting of 12/12 while "Meduim" represents 5/12.
| Type | Compression | Multi | White | Picture 1 | Picture 2 |
| Bitmap | 2305 | 2035 | 2035 | 2305 | |
| TIF | None | 2314 | 2314 | 2322 | 2319 |
| ZIP of | Bitmap | 626 | 3 | 1822 | 1726 |
| ZIP of | TIF | 628 | 5 | 1578 | 1732 |
| TIF | LZW | 365 | 20 | 1737 | 1188 |
| TIF | ZIP | 402 | 14 | 1586 | 1163 |
| PNG | 389 | 4 | 1425 | 984 | |
| TIF | JPG | 159 | 63 | 787 | 498 |
| JPG | Maximum | 158 | 63 | 787 | 496 |
| JPG | Medium | 26 | 24 | 153 | 65 |
You can see that the compression achieved depends to a large extent on the content of the image.
Uncompressed TIFF files are slightly larger than the bitmap they contain. Saving a file with "JPEG" options of TIFF is much the same as saving it as a JPEG. Using the "ZIP" option is a little better than zipping an uncompressed TIFF or Bitmap file.
Here are the same results expressed as a percentage of the size of the bitmap ("Photo Av" is the average for the two photographic images).
| Type | Compression | Average | Photo Av |
| TIF | None | 107% | 107% |
| ZIP of | Bitmap | 48% | 82% |
| ZIP of | TIF | 45% | 76% |
| TIF | LZW | 38% | 67% |
| TIF | ZIP | 36% | 63% |
| PNG | 32% | 56% | |
| TIF | JPG | 17% | 30% |
| JPG | Maximum | 17% | 30% |
| JPG | Medium | 3% | 5% |
The following images show the result of saving and loading a JPEG file many times. You can see that after the third generation the image starts to degrade visibly. Click on an image to see it at full screen (the original size was 1024x768 pixels).
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| Generation 1 | Generation 2 | Generation 3 | Generation 4 | Generation 5 |
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| Generation 6 | Generation 7 | Generation 8 | Generation 9 | Generation 10 |
PNG stands for Portable Network Graphics, and is a lossless compression method developed as a replacement for the GIF format. (For further information on PNG see this site.)
PNG really is lossless: here are first and 10th generation images...
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| Generation 1 | Generation 10 |
JPEG and PNG can both store images with 8 bits per channel. PNG, however, can also store images with 16 bits per channel, increasing the colour information available. (Note: I believe this is actually 12 bits of usable information per channel, giving 3x12=36 bits per pixel.) 16 bit images are about 1.5 times the size of their 8 bit equivalents.