Better anti-aliasing

[Bert Freudenberg]

On 25.04.2008, at 15:07, Paul Fox wrote:

> bert wrote:
>>
>> There is no need for "fancy color-adaptive subpixel rendering". The
>> framebuffer with its 1200x900 resolution maps 1:1 to physical display
>> pixels. The DCON simply selects the red channel of the first pixel,
>> and the green of the second, and the blue of the third, and so on.
>> This does not affect pixel geometry. Thus normal full-pixel anti-
>> aliased software rendering will do The Right Thing.
>
> i think there's something odd with this description.  (and, as
> someone who has clearly been confused about the display for some
> time, i'd like to be sure i understand.)
>
> unlike a traditional display, every pixel has a single color.
> given this, it seems wrong to talk about the "red channel of the
> first pixel".  you either use some of that pixel, or you don't.
> in effect, the display is implementing sub-(full-color-)pixel
> rendering all by itself.  (which i think is what bert was saying,
> but the "channel" thing confused me.)

In the display memory (a.k.a. framebuffer), each pixel has a red and a  
green and a blue value. I called these components "channel", which is  
not entirely correct. Still, the DCON selects only the red component  
of a framebuffer pixel to be displayed for a red physical pixel, the  
value of the green and blue components do not matter. If the 5-tap  
antialising filter is enabled, then the 4 adjacent pixel's red  
components are averaged, and mixed with the center pixel's red  
component. Again, the green and blue components are ignored, hence I  
spoke of "selecting". Makes sense?

> with the backlight off, this doesn't matter -- all pixels are the
> same color anyway.  but with the backlight on in monochrome mode,
> you see a lot of color in single-pixel-width lines.  if the line
> is perfectly aligned with the diagonal of the pixels, it will be
> completely colored (i just tried it), but mostly you just get a
> lesser or stronger fringing effect.
>
> for backlit ebook mode, this might not matter so much, since ebooks
> might not have a lot of diagonal single-pixel-width lines, but the
> effect is definitely there.


But it is greatly diminished by the antialiasing hardware filter. See  
if reading

	http://wiki.laptop.org/go/Display

clears things up. With "selecting the red channel" I meant the DCON  
"swizzling" as described on that page.


Btw, could someone replace the wrong close-up:

	http://wiki.laptop.org/go/Image:LCD-olpc.png

This illustrates MLJ's "color by refraction" idea but does not  
represent the actually shipping color filters. The image at wikipedia  
is much nicer:

	http://en.wikipedia.org/wiki/Image:XO_screen_01_Pengo.jpg

- Bert -