[SDL] help with displaying grayscale buffers

[James Lehman]

>From a totally generic hardware POV, 16 bit color can be done in usually 3
different ways. You need to know how many bits each color really gets and
what their positions are in the 16 bit value. You will have something like
5:6:5; meaning 5 bits of red, 6 bits of green and 5 bits of blue. From the
description of what you see, it sounds like this is what is out of order.

Could be:

6:5:5
5:6:5
5:5:6

Another thing to point out here is that there is no proper translation to 24
bit RGB. Or if there is, I don't get it. If you have only 5 bits to
represent from black to fully lit, that's 32 levels. However, if you take
those 5 bits and tack 3 zeros on the end, you do not get 255; fully lit in 8
bits. What I did for this in EZFB was shift the 5 bits up to 8 and shift the
3 MSBs down to fill in the missing bits. I think this distributes the error
evenly.

James.   :o)


----- Original Message ----- 
From: "grapesmoker" <grapesmoker at gmail.com>
To: <sdl at lists.libsdl.org>
Sent: Monday, April 14, 2008 5:05 PM
Subject: [SDL] help with displaying grayscale buffers


>
> I'm trying to use SDL to display a 16bpp grayscale buffer that is acquired
> from a camera. I am using the following code:
>
> SDLSurface *screen, *image, *temp;
> SDL_Rect src, dest;
> unsigned short buffer[1572864]
>
> temp = SDL_CreateRGBSurfaceFrom(buffer, 1536, 1024, bytesPerPixel * 8,
1536
> * bytesPerPixel, 255, 255, 255, 0); // bytesPerPixel has value 2
> image = SDL_DisplayFormat(temp);
> dest.x = src.x = 0;
> dest.y = src.y = 0;
> dest.h = src.h = temp->h;
> dest.w = src.w = temp->w;
> SDL_BlitSurface(image, &src, screen, &dest);
> SDL_FreeSurface(image);
> SDL_Flip(screen);
> SDL_FreeSurface(temp);
>
> That's it. Now, the weird thing that I'm seeing is that the image
displays,
> sort of. What I get when I try to display the surface is something like a
> negative of the image, with weird gradients instead of a nice smooth
> grayscale. I have a routine that saves the buffer to a TIFF and when I
look
> at the TIFF file, everything looks exactly the way it's supposed to.
> However, I would also like to have a real time display of the buffer, and
I
> can't figure out what I'm doing wrong.
>
> I've included a sample image to illustrate the problem. You can ignore the
> text on the screen but as you can see, it's grainy rather than smooth and
> there is some sort of weird sunburst on the right. I know that these are
> artifacts of the display code since I have a TIFF file that looks normal
(in
> this case I'm looking at a dark piece of posterboard).
>
> I would appreciate any help or hints as to how I can fix this problem.
>
> Jerry
> http://www.nabble.com/file/p16599288/camview.png camview.png
> -- 
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