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Visual quality is subjective has to do with proper contrast, brightness, sharpness and other factors. THe pixel size is important measure, but it also depends on how big the image will be and if in print or just on the screen.
ok thanks i guess this question isn't really answerable in the computational or objective solution im looking for -- well, at least in 2013
There is the Scott Kelby seven point thingie, but that is a book. I am sure you'd find enough to work through it if you Google. That is just going to fix black and white points, saturation etc. and not touch on composition. Th Michael Freeman books 'A Photographer's Eye' and 'A Photographer's Mind' are probably as good as any for learning composition. Or you could join a camera club and enter the competitions. You'd get good constructive assesments of your pictures, and be taking great shots within a few months. I see it time after time at our club New people join with loads of enthusiasm but limited ability, and six months later they are as good as anyone else in the club.
As Curt mentioned, the appreciation of most things IS subjective - very tough to quantify, and also not easy to qualify, as an individual.
I have a strong academic, and life-experiences, training in visual art, from design to photography, to cinematography, to print making, and on to various illustrative media. It would be impossible for me to distill what moves me, and even more difficult to define the why. I know what I like, and also what I do not. My sense of aesthetics is built up over many, many decades of studying art, and advertising communication, in various media. Still, a great deal comes from the heart, and not the head. I can spend hours pouring over a lovely B/W image by, say A. Adams, where my wife (the one with the library of advanced degrees in business, healthcare and finance) will quickly go to a vibrantly colored image.
It is like wine. One likes, what they like. A decade, or so, ago, several Japanese scientists quantified the "qualities," of many great wines (the ones about which leather-bound books are written), and declared that they had numerically separated every aspect of those great wines, and furthermore, could reproduce them in the lab, rather than the vineyard and the winery. They failed. Not one respected taster chose their laboratory wine, over the real versions. Heck, in a double-blind tasting, their "numerically perfect wine" came in last, even against rather mundane wines, that were somewhat similar.
Attaching a numerical quantification is likely to be an exercise in futility, though possibly great fodder for a PhD thesis.
Good luck, and sorry that I could not give you a definitive answer, beyond my anecdotes.
Your comment on "composition" hit home with me. In a Reply above, I cited a few differences between my "eye," and that of my wife. I have observed her commenting favorably on many brightly colored images, with what I would term "horrible composition." She still derives pleasure from those images, where their composition, alone, would cause me to grit my teeth. She loves them, and I hate them.
Now I feel that all generals aspects should be taken into consideration, and addressed. When I was shooting commercially, I would spend a great deal of time with the Polaroid, and my 9x loupe. I would study every aspect of my shot. A few art directors would ask what the heck I was doing, since they had approved the shot. Well, I was looking for any flaw. Even if the viewer could not pick up a problem, they still might feel that *something* was wrong, even if they could not tell what it was - a subconscious glitch, even if it was undefined. I did not want ANY glitch, obvious, or subconscious in any of my images.
That was one of the great temptations with digital image manipulation (Scitex then), in that I could shoot for perfection, and then had one more go at achieving it. Even after all my decades in the image creation business, I have yet to achieve 100% perfection. There is always a pixel here, or there, that could be better. I still revisit old images, and do thing to them. After pulling an "all-nighter," a great client put her foot down - "Give me the image NOW! You will tweak it forever, and I have a ****** deadline!" When things got slow, I revisited that image, though it had long before been printed. It was my duty to the image.