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A Pixel Is Not A Little Square (1995) [pdf]

alvyray.com

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Re: A Pixel Is Not A Little Square (1995) [pdf]

#3

Post by raphlinus »

I imagine this came up partly as a result of the recent alpha compositing discussion.

We desperately need some research, based in user studies and using modern display technology, to settle some basic questions:

* What reconstruction filter gives the best results? Is it the same for vector (text) and natural images? By "best" I do mean contrast (sharpness) and lack of visible artifacts.

* For rendering of very thin lines (relevant to text), what gamma curve gives the perception of equal line thickness across the range of subpixel phase? How does it vary with display dpi? (Hint: it's likely not linear luminance)

* What gamma curve yields the perception of equal width of black-on-white and white-on-black thin lines (also relevant for text)? (Hint: likely not linear luminance)

I've seen a number of discussions where people feel they are able to answer these questions from first principles, and corresponding arguments that doing these in a "correct" way gives results that are less visually appealing than the common assumptions of treating a pixel as a little square (so doing a box filter for reconstruction) and ignoring gamma so effectively using a perceptual color space for the purposes of alpha compositing.

I posit that these questions cannot be solved by argumentation. I think user studies might not be very difficult to do; you could probably get "good enough" results by doing online surveys, though this wouldn't pass standards of academic rigor.

Re: A Pixel Is Not A Little Square (1995) [pdf]

#5

Post by raphlinus »

shawnz wrote: post #4

A great video which touches on this idea, but mostly in the context of audio: https://youtu.be/cIQ9IXSUzuM (relevant part around 8 minutes)

Be careful generalizing the audio results to pixels. The central lesson of xiph's work is that people simply cannot hear frequencies above, let's say 20kHz. Therefore, as long as your sampling rate is above the Nyquist limit (and under the assumption the signal chain is linear), any reconstruction filter that passes frequencies through 20kHz is effectively "perfect."

There are two ways this is not true for pixels. First, even for "retina" displays the human visual system can make out spatial frequencies beyond the Nyquist limit of the display (this will vary by viewing distance, so is more of an issue for young people who can get close to their displays). Second, even assuming perfect gamma, the display must clip at black and white because of physical device limitations. Thus, especially for text rendering, only a reconstruction filter with nonnegative support is generally useful. Such a reconstruction filter would be an extremely poor choice for audio.

It is true that many of the underlying signal processing principles are the same, and I encourage people to learn and understand those :)

Re: A Pixel Is Not A Little Square (1995) [pdf]

#8

Post by BubRoss »

raphlinus wrote: post #3

I imagine this came up partly as a result of the recent alpha compositing discussion. We desperately need some research, based in user studies and using modern display technology, to settle some basic questions: * What reconstruction filter gives the best results? Is it the same for vector (text) and natural images? By "best" I do mean contrast (sharpness) and lack of visible artifacts. * For rendering of very thin l…

I can tell you right now that you will have a very difficult time beating a normalized gauss filter with a diameter of around 2.2 pixels in a general case.

Color and luminance is a separate and orthogonal issue from filtering. I also know that people get away with compositing without converting to linear space, but I'm skeptical that any benefits they see aren't just a matter of getting the color curve they want for free, as opposed to doing a similar correction after something has been composited correctly.

Re: A Pixel Is Not A Little Square (1995) [pdf]

#9

Post by BubRoss »

jcelerier wrote: post #7

> A Pixel Is Not a Little Square! Microsoft Tech Memo 6 is this the reason why they forced this horrible bilinear filter on the windows image viewer for so long (guess they still do) ? It made me crazy, it's so ugly

No, this was written by Alvy Ray Smith 25 years ago in response to incorrect filtering in computer graphics in general. This has nothing to do with whatever you are seeing.

Re: A Pixel Is Not A Little Square (1995) [pdf]

#10

Post by jmull »

Well, it's an overloaded term (like pretty much every term in software development).

But there's good information in this article. In many contexts you should be thinking in terms of point-samples, not squares or other areas.

I think it's not such a great idea to try to simplify the definition of pixel in this article because it distracts from the useful info.

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