Re: [ft-devel] gamma correction and FreeType

[Dave Arnold]

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 <address@hidden> 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