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The TIF (Tagged Image Format) or BMP (bit map) formats are simple: Everything will be stored to the fullest amount. | |
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Every pixel is stored with the full information for brightness and 16 million colors | |
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This means that a picture needs lots of memory space; it is thus perfectly unsuitable for HTML and cannot be incorporated directly. | |
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It is, however, the format of choice to store your originals. You then optimize a copy of the original and store it in a compressed and HTML compatible format. | |
The JPEG Format
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The JPEG format compresses the image data by using a discrete cosine transformation algorithm (similar to Fourier transforms). | |
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The compression algorithm is rather good; it also keeps the maximum color information (i.e. a palette of 16.7 million colors). Most of the graphic programs offer the possibility of choosing the compression factor. | |
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With Paint Shop Pro you can choose the compression factor by activating the options in the "save as" ("Speichern als") menu. | |
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A high compression factor leads to small file with a very low quality and vice versa: A low compression factor will lead to a larger file with a very high quality. Just have a look at the examples in the link or at the test picture below. | |
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The JPEG format is a common format to present scanned photos or pictures with very fine color gradients in the Internet or as embedded files in Word documents. JPEG offers no opportunity to declare a transparent background. | |
The GIF format
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The GIF format was developed by CompuServe especially for on-line use. GIF compresses the size of an image by reducing the palette to 256 colors. This reduces the bit size of an image to 1/10 to 1/30 compared to a regular BMP file without noticeable losses. The GIF format offers several options for including the image into a web page: | |
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Transparent background: One of the at most 256 colors of the image can be declared to be "transparent"; it will then be substituted by the background color of the page. | |
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Interlaced presentation: The image will be presented successively (layer by layer) while each layer shows more details. | |
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Animations: Several GIF images can be connected to an animation; i.e. a short "comic strip". | |
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GIFs are best used for buttons, cliparts and other images with only a few colors or color shades, and particularly for drawings or texts presented as image, but not for high resolution photos. Again, look at examples in the link to appreciate this. | |
The PNG format
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The PNG format (portable network graphics) combines the advantages of the JPEG and the GIF formats: | |
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Compression without noticeable losses | |
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16.7 million colors | |
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Transparent background and interlacing | |
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However, it does not allow to chose the compression factor. So it may look good, but look at the size in comparison to the others in the examples. | |
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Here is an example for the different ways of strong an image. It contains a photography, hard lines, many colors, and a continuous color transition. In the original TIF format it needs 152 kB | ||||||||||||||||||||||||||
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Note that JPEG needs the least amount of storage space, but has problems with hard lines on a clear background. The photography, however, still looks pretty good even at high compression. | ||||||||||||||||||||||||||
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You can see the differences more pronounced if you compare the images by exporting them to Paint Shop Pro and enlarging them. | ||||||||||||||||||||||||||
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© H. Föll (Matwis Seminar)
