Small file size but high quality

[yllai]

Do have any method for making a picture to be very clear (scanned in high resolution) but the file size not huge? or do have any method for us to compress the file size but the picture quality still good?

Any suggestion, where can I get these info?

Thanks.

[tedster]

The visual quality depends on the software you use to create the compression - and not the resolution which is always 72 ppi. Of course you do need to start with a good scan that has a lot of color variation, no color cast, etc.

Here are some techniques to help your software get a better look from the raw image.

1. When you begin from a large image, don't resize it all in one jump. Instead, make several smaller resize steps, each step less than 50% smaller than the previous one. If one step loses too much detail or color variation, back up and appply some sharpening or increase the color saturation BEFORE you make the image smaller. If you make the image look slightly over-sharpened before the re-size, it usually looks just right after you re-size it.

2. It can also help to select all the colors (very light and very dark) that your eye cannot tell apart -- and make them exactly the same before you use jpg of gif compression. this makes for a greater compression with no obvious loss of quality.

3. If you are generating a jpg, one very good trick is using L*a*b color space, assuming your imaging software allows it. Because the L channel holds only lightness information, you can sharpen there without creating color artifacts.

Also, because the a and b channels hold only color information and not light-to-dark information, you can run very large blurs on those channels to even out the color without ruining the fine details.

Still, tricks and techniques aside, the best results come from the best software compression algorithms. Back at Photoshop version 6, Fireworks did a better job, IMO. Then with version 7, Photoshop took the lead again.

[edited by: tedster at 7:59 am (utc) on April 14, 2004]

[HelenDev]

To get maximum quality and minimum file size, choose the correct image format:

If the image is photographic, choose jpg

If the image is a line drawing or logo with big areas of flat colour, choose gif

Helen.

[Farix]

If the image is a line drawing or logo with big areas of flat colour, choose gif

Actually, choose PNG. It gives better compression and is just as loseless as GIF in the 8-bit format.

GIF is only good nowadays for animation, very small and very simplistic images, and the 1 pixel transparent spacer--which is a very small, very simple image.

[tedster]

Helen, I agree with your format choices as a general rule. But you'd be surprised how often you can get better results by going against it. For instance, if photographs are heavily focused in certain colors, GIF or PNG can sometimes do the job best. And if there are delicate shadings os a peacock reange of color in a drawing, jpg 'may' work out better.

With today's powerful algorithms, you can also get sharp looking type with a jpg. You can get gradients in gif or png that do not show visible banding.

In short, the old school rulebook is out the window often enough, that I almost always check both compression methods. Maybe 15% of the time, the best results are obtianed in the format I would not have guessed.