by GreenLake » Fri Apr 03, 2015 2:05 am
Sorry Tim, wasn't sure what your were trying to get across, so my reply may have been at cross purposes.
Anyway, an 800x600 bitmap, stored with an 8-bit color index would need something a little over 240,000 bytes, With 24bit color values, it's three times that.
So,I ran a little test with Alan's image, which is a bit smaller than that. Saved from Paint as a BMP it takes 667KB, as JPG it takes 53KB and as PNG it takes 321 KB. The JPG does loose information, when I switch between it and either of the other formats there's a little flicker where it doesn't represent the roughness of the cast aluminum part quite the same, but without that direct comparison you'd never know. It's not an entirely fair test, because the source was a jpg file to begin with, so it never had more information in it than the 153KB of the file size listed for the attachment.
With PS, if you save the file with "maximum" fidelity in jpg format, you get 153KB, essentially the original file. If saved with 50% quality, form "save for devices" in PhotoShop, the size is an astonishing 22KB (and degradation is not obvious). The reason, of course is that the image is largely an expanse of detail-poor background, and the surfaces of the parts are smooth.
So, as a result the relation between pixel dimension and the data required for a usable image is highly variable. With a good image resizer, it's quick and easy to massage your forum pictures a bit to make them both more friendly to be viewed in a post, and save some storage.
I tried the same sample with ResizeMyPhotos on Windows and the result was 18KB for the "medium", or 70% quality setting. So, that's even smaller than Photoshop uses for the 50% setting.
Anyway, enough fun playing with image compression.
~ green ~ lake ~ ~