Reducing size in KB - Need help!!

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jarbiebo
Posts: 1
Joined: Sat Dec 22, 2007 3:02 pm

Reducing size in KB - Need help!!

Post by jarbiebo »

I'm new at this. I need to reduce the overal size of a number of .jpgs. Right now, they are averaging about 470 KB.

I am trying to pull in about 260 jpgs into a slide show program and it states that the compucic is closing - I've been told because the jpgs are too large.

How do I use this XnView program to reduce the overal size (KB) to a smaller size. I don't want to lose quality though.

Thanks![/b]
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Drahken
Posts: 884
Joined: Sun Apr 10, 2005 4:29 pm

Post by Drahken »

Um... size pretty much equals quality (especially with JPGs). If you want to decrease the size to any significant degree, you need to sacrifice at least a little quality. The mere usage of the JPG format itself instantly reduces the quality of the image.
It should be noted however, that there is a difference between technical quality and visual quality. Everything, including using the JPG format, reduces the technical quality of the images. However, most such quality loss isn't visible to the human eye., so you can reduce the technical quality quite a bit before it starts to impact the visual quality.


You can shave some of the size without affecting quality by selecting the "optimize huffman table" option, and by removing any embedded thumbnails (those things are usualy huge, and not really useful).

You could reduce the dimensions of each image. (Depending on how large your images are going to be displayed, this may be your best option, in addition to the 2 things I listed above. If your images are 1000x1000, but are going to be displayed at only 400x400, you could easily reduce them to 400x400 without a noticable difference in the slideshow. Doing so would greatly decrease the filesize. (You should keep a copy of the originals though, in case you need them for something else. Reducing the dimensions of the images can't be undone.))

The best way to find a good balance between size & quality is to use the export function. Open one of the images, then hit ctrl+alt+s (or click file->export). In the top middle of the dialog box, click the 4th button down to zoom to 100%. Now play around with the settings until you get the smallest filesize you can without degrading the quality too much. Using a smoothing factor of 10, a DCT method of "float", checking progressive & optimize, and remove all metadata will get the best size without affecting image quality. (The smoothing does actually affect the quality, but not enough to be visible. Removing all metadata will remove the embedded thumbnail (thus greatly reducing size), but will also remove info such as when the photo was taken.)
The subsampling option affects color saturation (especially on bright/bold colors, and on sharp edges). Using the default setting will decrease filesize, but may look very bad. (Some photos don't look any different, some will look drastically different, you just hjave to look & see how yours looks.) Using the "best" setting will look the best, but will noticably increase filesize.
You can usually use a quality setting of about 75~85 without noticable quality loss. Again, you'll have to look at your own images & see what you find acceptable.

Once you figure out what settings will work best, you can then use those same settings in the batch convert option to optimize a bunch of images at once. You just need to use the exporter to figure out which settings will work best.


*Keep in mind that all these changes will be permanent, so it's a good idea to make a copy of all the images before you optimize them.