nanogui: Jpeg images and dithering
Subject:
Re: [nanogui] Jpeg images and dithering
From:
"Greg Haerr" ####@####.####
Date:
2 Nov 2005 04:55:27 +0000
Message-Id: <09b301c5df69$6de40f70$6401a8c0@winXP>
: I am using nano-x on a 220x176 color lcd with MWPF_TRUECOLOR565 frame
: buffer configuration. The problem is graphics are being dithered. In
: looking at the problem I have one really big question. None of the RGB
: values can be a full 8 (255 decimal) bits, red is 5, green is 6 and blue
: is 5 maximum bits respectivly. So, how does either nano-x or libjpeg,
: which ever one does it, get them to the 565 format correctly?
When nano-X is presented with an RGB color (that is, 8/8/8),
it uses the COLORVALTOPIXELVAL macros in mwtypes.h
to convert this to 5/6/5 format. As you mention above, all
8 bits can't fit in 565 format, so macros convert the color
by shifting left 3 or 2 bits and packing them into a 16-bit word.
When nano-X decodes an image, it is converted into an internal
MWIMAGEHDR format, which is decoder-specific. I think
we ask the jpeg decoder to convert an image to a 24-bit color
value. When images are displayed, they have to then be converted
to the hardware (in your case 565) format.
So the dithering issue is likely when outputing the 888 MWIMAGE
to 565 format, although that doesn't preclude the jpeg decoder
from doing something silly. This can be checked by converting
the image externally, and using a different decoder in nano-X.
Of course, if you're running in palette mode, then things are
quite a bit different.
Regards,
Greg
:
: In my welcome screen I have large area that is GR_RGB(255, 168, 167), in
: nano-x'ese, with all RGB values larger than the 565 limit. On the screen
: it pretty much matches the pc screen color, but it is dithered with
: white, and maybe other colors.
:
: So, my real question is can somebody tell me how colors larger then the
: 565 pattern are "mapped" to the actual colors as seen on the screen. And
: where that transformation takes place, in nano-x or libjpeg.
:
: Thanks,
: tj
:
:
: ---------------------------------------------------------------------
: To unsubscribe, e-mail: ####@####.####
: For additional commands, e-mail: ####@####.####
:
:
