From: WHS <wouters@cistron.nl>
  To  : ggi-develop@eskimo.com
  Date: Fri, 14 Aug 1998 15:50:46 +0200

X colourmap code [X/Xlib target]

Steve Cheng wrote:

> Didn't somebody say that they have the code to this?
> This is to minimize color flashing in X.

>From my x11 frambuffer code:
0.	/* get the deafult colour map (i.e. the shared one) */
	colourmap = DefaultColormap(display, DefaultScreen(display));

1.	/* Create X window with AllocAll */
	if (Bpp==1) {
		private_cmap = XCreateColormap(display, main_win,
DefaultVisual(display, XDefaultScreen(display)), AllocAll);
	}

2.	/* Fill the colourmap with the original colours */
	for (i=0; i<NUM_COLOURS; i++) {
		Free_cell(i); /* sets the private entry to the default one! */
		palette_trans[i]=i; /* no translation */
	}

NUM_COLOURS=256, but could be 4096 for SGI modes (12 bit)

3.	if (Bpp==1)
		XSetWindowColormap(display, main_win, private_cmap);


That initializes, now some functions:
-------------------------------------

/* Free allocated cell and restore to colour in original colour map */
int X11_framebuf:: Free_cell(byte cell) {
	XColor colour_cell;

	if (Bpp==1) {
		colour_cell.pixel=cell;
		colour_cell.flags=DoRed | DoGreen | DoBlue;
		XQueryColor(display, colourmap, &colour_cell);
		pixel_val[cell]=cell;
		pixel_colours[cell]=(((colour_cell.red>>8)&255)<<16) |
(((colour_cell.green>>8)&255)<<8) | ((colour_cell.blue>>8)&255);
		XStoreColor(display, private_cmap, &colour_cell);
	}
	private_cmap_used[cell]=0;
	return 0;
}


/* Allocate colour cell in private colour map */
int X11_framebuf:: Alloc_cell(byte cell, byte r, byte g, byte b) {
	XColor colour_cell;

	if (Bpp==1) {
		colour_cell.pixel=cell;
		colour_cell.flags=DoRed | DoGreen | DoBlue;
		colour_cell.red=((short)r)<<8;
		colour_cell.green=((short)g)<<8;
		colour_cell.blue=((short)b)<<8;

		XStoreColor(display, private_cmap, &colour_cell);
		pixel_val[cell]=cell;
	}
	else
		pixel_val[cell]=((((int) r)>>red_mask_bits_compl) <<red_shift) |
			((((int) g)>>green_mask_bits_compl) <<green_shift) |
			((((int) b)>>blue_mask_bits_compl) <<blue_shift);

	pixel_colours[cell]=(((int)r)<<16 ) | (((int)g)<<8 ) | (int)b;


	//15+ bpp
	palette_changed_bits[ cell >>5] |= 1<< (cell &31);
	palette_changed=True;

	private_cmap_used[cell]=1;
	return pixel_val[cell];
}
---------------------

Now what you want to do is allocate from 255 down, as X allocates
colours from 0 up.
There are other improvements possible analysing the colourmap (to work
round a few isolated used entries in the upper range, e.g. as happens
when lots of programs have started, then most are stopped again).


Wouter