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I'm writing a project in xlib and have a problem concerning the colors.I use unsigned long type variables for storing the color values.does anybody know how can i take the red green and blue values of each color?

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Dani AI

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— two different things can be stored in that unsigned long in Xlib, and the correct approach depends on which one you have: a pixel that is an index into a colormap (PseudoColor) or a packed RGB pixel (TrueColor / DirectColor). pointed you toward masking/shifting (that is the usual way for packed pixels), but in X you often have safer, Xlib-specific alternatives or need a more general extraction that works with any Visual.

If the value is a colormap index (palette): query the colormap. XQueryColor fills an XColor structure with 16-bit component values (0..65535). Scale those to 8-bit (0..255) afterwards — dividing by 257 maps the full 0..65535 range exactly to 0..255.

XColor xc;
xc.pixel = pixel;
XQueryColor(display, DefaultColormap(display, screen), &xc);
/* xc.red/green/blue are 0..65535; map to 0..255 */
unsigned char r8 = (unsigned char)(xc.red / 257);

If the value is a packed pixel (TrueColor/DirectColor): use the Visual’s masks rather than hard-coded hex constants. Find each channel’s mask, compute the low-bit shift and the mask’s maximum, then extract and scale the channel to 0..255. This works for any framebuffer layout instead of assuming 8 bits per channel or fixed positions.

Visual *vis = DefaultVisual(dpy, screen);
unsigned long rmask = vis->red_mask;
unsigned long tmp = rmask;
int rshift = 0;
while ((tmp & 1) == 0) { tmp >>= 1; rshift++; }
unsigned long rmax = rmask >> rshift;
unsigned long rval = (pixel & rmask) >> rshift;
unsigned char r8 = (unsigned char)((rval * 255) / rmax);

Troubleshooting notes: if you read raw bytes from an XImage, prefer XGetPixel to avoid byte-order pitfalls. On 64-bit hosts, XPixel values still live in the lower bits of unsigned long. Also, a common C gotcha to avoid: ^ is XOR, not exponentiation — use shifts or multiplication when you mean powers of two.

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Member Avatar for Greywolf333

I'm writing a project in xlib and have a problem concerning the colors.I use unsigned long type variables for storing the color values.does anybody know how can i take the red green and blue values of each color?

Are you saying that the 24 bits of color (8 for each color component) are stored together in one 32 bit integer? If that's the case you can obtain the values using the logical AND operation to zero the other bits.

Let's say you start with

/*
  alpha?      r       g        b
  00000000 10101010 10101010 10101010    your 32 bit integer might look like this
                 &                       logical AND operator
  00000000 00000000 00000000 11111111    a bit mask
                 =
  00000000 00000000 00000000 10101010    the result

so now your 32 bit integer only has the blue values.

To do this in codez...

*/
unsigned char B = (unsigned char) (your_integer & 0x000000ff) //000000ff is hex version of the bit mask

//but now what about the other two colors?  you can't just apply a bit mask like 0000ff00 because 00000000000000001010101000000000 is much larger than 255.  
//So you have to either divide the result by 256 to shift the bits right, or use >>8 to shift them to the right.

unsigned char G = (unsigned char) ((your_integer & 0x0000ff00) / 256)
unsigned char R = (unsigned char) ((your_integer & 0x00ff0000) / 256^2)
//or using the way I've used in the past... shifting before the mask.
unsigned char G = (unsigned char) ((your_integer >> 8) & 0x000000ff)
unsigned char R = (unsigned char) ((your_integer >> 16) & 0x000000ff)
Member Avatar for libathos

I'll try that , thx very much

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