Mine eyes - a saving test

[Michele Rushworth]

Looks pretty clear: when choosing the highest quality (ie not much compression) the jpeg file format seems to be more than adequate for our needs in storing reference photos to paint from.

[Holly Snyder]

The thing is that you're displaying them at only 72 dpi on the forum, so any loss in image quality will probably not be viewable. If you look at those same images in your graphics program, zoomed in significantly (300+ percent) so that you can see the enlarged individual pixels, can you see a difference?

While jpg is lossy, it is an excellent algorithm, so that when an image is saved at the lowest compression (highest image quality), the difference vs. the original isn't that noticeable. But try the same test with the highest jpg compression (lowest image quality), and the difference will be much more noticeable.

Holly

[Mike McCarty]

Holly,

In rereading your post I don't think I did exactly what you suggested. The images above are just severely cropped. I can zoom in as far as I want in my program and when I do they still look pretty similar.

This one is cropped about as much as I dare (expecting the ghost of Ansel Adams to appear).

I don't have any particular stake in proving this one way or the other, it's just something I got curious about.

But try the same test with the highest jpg compression (lowest image quality), and the difference will be much more noticeable.

It seems from what you say that if it is your intention to screw up the image you certainly can. Or, it may actually be screwed up but I am unable to see it. However, if you pay attention the information can be maintained sufficiently. Certainly to the extent that mine eyes are able to utilize it. This gets into the realm of "a wink is as good as a nod to a blind man."

[Holly Snyder]

You peaked my curiousity Mike, so I decided to spend too much time and do another test.

The first image is a section of your original image enlarged to 1600% and saved as a jpg at maximum quality. In the second, I began from the first image, and drew a T, pixel by pixel, each time saving it as a jpg at maximum quality. There's hardly any image degradation, just a very slight color change of some of the pixels.

The third image began as the first, but saved as low quality -obvious deterioration. The fourth began from the third, and drew a T in the same location as in image two, pixel by pixel, each time saving it as a jpg at low quality. Again more deterioration.

Images one and two are much closer than I would have suspected. It may be hard to see, but if you look at the top row, fifth and sixth pixels from the right, you'll see a color change.

I guess the upshot is that jpgs, at max quality, are certainly fine to paint from!

Ok, back to work!

Holly