JPEG Save Options


These options determine how PMView saves JPEG files.


Quality This control lets you trade off compressed file size against quality of the reconstructed image: the higher the quality setting, the larger the JPEG file, and the closer the output image will be to the original input. Normally you want to use the lowest quality setting (smallest file) that decompresses into something visually indistinguishable from the original image. For this purpose the quality setting should be between 50 and 95; the default of 75 is often about right. If you see defects at 75, then go up 5 or 10counts at a time until you are happy with the output image. The optimal setting will vary from one image to another.


Smooth This control lets you select a smoothing factor for filtering the input to eliminate fine-scale noise. This is often useful when converting GIF files to JPEG: a moderate smoothing factor of 10 to 50 gets rid of dithering patterns in the input file, resulting in a smaller JPEG file and a better-looking image. Too large a smoothing factor will visibly blur the image, however.


Optimize entropy encoding This option selects whether to perform optimization of entropy encoding parameters or not. Without this, default-encoding parameters are used. Using this option usually makes the JPEG file a little smaller, but PMView saves the file somewhat slower and needs much more memory. Image quality and speed of decompression are unaffected by this option.


Progressive JPEG This option creates a "progressive JPEG" file. In this type of JPEG file, the data is stored in multiple scans of increasing quality. If the file is being transmitted over a slow communications link, the decoder can use the first scan to display a low-quality image very quickly, and can then improve the display with each subsequent scan. The final image is exactly equivalent to a standard JPEG file of the same quality setting, and the total file size is about the same --- often a little smaller. CAUTION: progressive JPEG is not yet widely implemented, so many decoders will be unable to view a progressive JPEG file at all.


Related information:

The JPEG Format