It could indeed be that today's monitors don't have a logarithmic gamma curve, but something more complex. Maybe a test pattern for various levels would tell. And indeed, if I look at a pure gradient, both the lower & the higher 10% stand out, so it could indeed be some S shape. To make it worse, tablet generally feature auto-brightness, & whether it's done by multiplication or shifting, it will still mess up the curve, unless it's done linearly which I doubt.
So perhaps that "2.2 gamma" standard should be re-evaluated, maybe gamma correction isn't as simple today & transfer functions are more complex. On Fri, Nov 8, 2013 at 7:12 PM, Dave Arnold <darn...@adobe.com> wrote: > I've attached a simple line pattern that I use to get a rough idea of > display gamma on various devices. > > For my laptop screen, at my normal viewing angle and distance, I measure > 2.8 at the top, 1.6 at the bottom and 2.2 in the middle. LCDs have always > varied with viewing angle, some technologies more than others. Luckily, the > exact gamma is not too critical. > > I agree that your 1.4 image looks more balanced in weight than your 2.2 > image. When I repeat this test with my tools, I find that 1.4 and 1.8 are > pretty close, while 2.2 (like yours) tends to make the white text too > heavy. (By the way, I don't know how to extract your images from the google > docs page, so I'm just relying on what Firefox does on my display.) > > I've noticed this effect with white text before, and don't completely > understand it. Some have argued that it is a visual perception issue. That > could be. Another idea is that it results from a display transfer function > that is not close enough to a pure power function. The physics of a CRT > naturally generate a transfer function that is close to a power curve. But > the physics of an LCD naturally generate a transfer function that looks > like an 'S' curve, flat at both top and bottom. I understand that display > manufacturers use electronics to modify this to get a better match to sRGB > (which is very close to a power curve). But the closeness of the match > varies and is never perfect. The greatest errors are close to the top end > (white). > > I agree in theory that gamma correction should make the weight of black > text and white text appear the same. But this is only one of many goals. > Smoother curves and diagonals is another goal, as is cancellation of color > in subpixel rendering. I believe that there is a single "gamma" function > that will do all of these at the same time, because it is primarily a > physics problem. > > Thanks. > > -Dave > > > On 11/7/2013 10:29 AM, another gol wrote: > > Still, IMHO gamma correction shouldn't be about the look, but the > -weight-. > I mean, the goal behind proper gamma correction is that black text on > white has the same weight as white text on black. I don't know how your > monitors are calibrated, but on the ones I have here, this (1.4) looks like > more or less equal weighting > > https://docs.google.com/file/d/0B6Cr7wjQ2EPudk1uV01NWjRjc2c/edit?usp=sharing > while this (2.2) doesn't at all > > https://docs.google.com/file/d/0B6Cr7wjQ2EPuZWpMYWl4T2ZobzQ/edit?usp=sharing > It could be that all of my monitors have a 1.4 gamma, but really, I > never saw it looking wrong on any monitor, & there must be a reason Windows > defaults to 1.4. I strongly suspect that the 2.2 standard gamma is either a > myth or an old truth that doesn't apply anymore. > > As for color fringing, reducing it is the role of the filter, and this > one only goes horizontally, I don't think that color fringing should be a > factor when picking gamma correction.If there's too much color fringing, > you can still enlarge the blur filter, it's always a tradeoff between > sharpness & color fringing. > > > > On Thu, Nov 7, 2013 at 7:02 PM, Dave Arnold <darn...@adobe.com> wrote: > >> You are correct that gamma 2.2 is a better approximation to sRGB. I agree >> with using that gamma for large area regions. But there are two reasons why >> I prefer gamma 1.8 for text. >> >> The first is something I call "effective gamma". We first encountered >> this back in the era of CRT displays. Unlike image data, text is mostly >> edges. The area covered by stems is on the order of a pixel wide. If you >> look at text as a video signal, it is very high frequency—often close to >> the limit of the display device. Rather than seeing a vertical stem as a >> perfect square wave, the video electronics tend to round it off and reduce >> its amplitude slightly. Both of these effects move the active signal closer >> to the middle of the gamma curve, where the curve is flatter. This has the >> effect of reducing the gamma at high frequencies. Everything in the signal >> path contributes to this. Even video cables could make a significant >> difference in the appearance of text. I know less about the electronics in >> an LCD but, empirically, the effect seems to still be present. >> >> The second reason is that not all text rendering systems are able to >> provide the stem darkening technology to compensate for the loss of >> contrast at small sizes. This is particularly challenging, given the way >> that TrueType hints work. For example, both Windows XP and Android chose >> gamma 1.4 as a default for text blending. It's not as accurate as a higher >> gamma, but it doesn't lose as much contrast as gamma 2.2. So, designing for >> gamma 1.8 rather than 2.2 will work better if the system gamma turns out to >> be 1.4. >> >> Both of these arguments pertain mostly to grayscale antialiasing. For >> subpixel rendering, I agree that you will see more color fringing at 1.4. >> Some people are more sensitive to this than others. I know several people >> who never could get used to ClearType on Windows XP. >> >> -Dave >> >> >> On 11/6/2013 11:47 AM, Antti Lankila wrote: >> >>> Dave Arnold <darn...@adobe.com> kirjoitti 6.11.2013 kello 20.43: >>> >>> Hi Antti, >>>> I've attached an image comparing darkened gamma 1.8 to darkened gamma >>>> 2.2. The differences between 1.8 and 2.2 are much more subtle than those >>>> between 1.0 and 1.8. >>>> >>> Yeah, there’s admittedly not much difference. I was just curious because >>> in theory gamma 2.2 is more correct when it comes to approximating the >>> behavior of the sRGB color space. If the rendering was based on subpixels, >>> there might be perceptibly more color fringing at small sizes. >>> >>> — >>> Antti >>> >> >> >> _______________________________________________ >> Freetype-devel mailing list >> Freetype-devel@nongnu.org >> https://lists.nongnu.org/mailman/listinfo/freetype-devel >> > > >
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