Re: [Tuxnes-devel] Trying to add SDL Renderer

[Jim U. <ji...@3e...>]

At 07:41am on 2002 March 10, W. Michael Petullo did write:
> > I'm trying to add an SDL renderer to tuxnes.  Eventually this will give
> > us full-screen support and additional platforms for free.
> > [...]
> 
> I have the basic renderer working, though I don't really understand
> tuxnes's color code (UpdateColor.* functions).  As I understand it,
> the NES is only able to display 24 colors at once.  Because of this,
> the emulator's palette must be updated to the appropriate colors for each
> frame and these colors have to be mapped to a display's 8-, 16-, 24-,
> or 32-bit colorspace.

The palette stuff is confusing.  The first time it took me a while to
figure it out.  The second time (when I went back and converted it to use
hardware palette entries) again took me a while, so I took notes that time:

---- Palette conversion:
	paletteDC[] holds the 64 16-bit DC versions of the 64 total NES colors.
	palette[] holds 25 16-bit RGB565 values, all of which are cornfusing.
		The 2 NES palettes have 16 entries each, broken down into 4 groups of 4.
		Color 0 is transparent (letting the background (or background color) shine through).
		Color 0 is mirrored to 4, 8, 12.  Also, color 0 is mirrored between the two palettes.
		In this array:
		0..11  are the current tile colors.
		  0,1,2  correspond to colors 1, 2, and 3.
		  3,4,5 and 6,7,8  are 5, 6, 7 and 9, 10, 11.
		  9,10,11 are 13, 14, and 15.
		12..23 are the current sprite colors.
	    24 is the current background color, set in UpdateColors.
		Methinks this was a (misguided?) attempt to save space.
	tilecolor is the 2-bit value from the attribute table.
	curpal[] holds 4 16-bit RGB565 values corresponding to the 4 possible colors
	  for the current tile.  The index into curpal is the 2-bit value from the name table.
	  curpal[0] is always set to the value in palette[24] upon entering drawimage.
	  curpal[1,2,3] are set to palette[0,1,2], [3,4,5], [6,7,8], or [9,10,11]
	     depending on the value of tilecolor (2 bits provides 4 selections).
------------------------

The following code does not use hardware palette entries (i.e. not
PseudoColor).  The code in InitDisplay that converts the 32-bit NES_palette
values into 16-bit RGB565 native values is:

 for (x=0; x < 64; x++)
 {
 	/* RGB565 */
 	uint8 r,g,b;
 	r = (NES_palette[x] >> 16 >> 3) & 0x1f;
 	g = (NES_palette[x] >>  8 >> 2) & 0x3f;
 	b = (NES_palette[x] >>       3) & 0x1f;
 	paletteDC[x] = (r << 11) | (g << 5) | (b);
 }

 The update colors function is very simple (except for the unnecessarily
 complicated palette array organization):

 void
 UpdateColorsDC(void)
 {
 	int x;
 	palette[24] = paletteDC[VRAM[0x3f00] & 63];
 	for (x = 0; x < 24; x++) { 
 		palette[x] = paletteDC[VRAM[0x3f01 + x + (x / 3)] & 63];
 }
 
This should get you started.  I have hardware palette code too if you need
it.

-- 
"closing my eyes, i got a glimpse of several entities moving in front of
a giant complex control panel.  the creatures were bipedal and of about
human size. it was impossible to say more other than they did not move
like the giant insect creatures i have seen clearly under the influence of
stropharia mushrooms."             -- zarkov, "a hit of dmt 10/9/84"
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