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CompressionImage Convert can optionally apply a simple RLE compression technique to the output data. The effectiveness of this compression depends on many factors. If the input image is a computer-generated image with few colors, RLE compression can be very effective. If the image was produced with a RAY-tracing package or from an actual photograph, RLE compression is less successful. When RLE compression is enabled, Image Convert is required to save the output data in 8-bpp format, regardless of what format was selected in the Output Colors field. This means that for 1-bpp, 2-bpp, and 4-bpp input images, turning on RLE compression forces Image Convert to first expand the image to 8-bpp format, and then apply RLE compression. Depending on the exact image file, this can actually cause the final output file to be larger than if compression is not used. For this reason, selecting RLE compression is actually only a suggestion to Image Convert. If RLE compression is effective at reducing image size, the compression is performed. If compression does not reduce the output image size, RLE compression is omitted. This decision is made automatically by Image Convert during the conversion process. Therefore, the only reason to disable RLE compression is if the PegScreen derived screen driver does not support RLE encoded PegBitmap formats. The PegScreen drivers provided with On Time RTOS-32 do support RLE encoding. RLE compression is almost always beneficial if you are using 8-bpp bitmap encoding, especially for very large images. Compression ratios typically vary from 10:1 to 3:2, depending again on the source of the image being processed. The use of dithering on the output bitmap has a negative impact on RLE compression effectiveness. Therefore, you should disable the Dither option if data size is the most important consideration in your application. This forces Image Convert to do a best match color mapping of input to output colors.
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