Making a screenshot using Xlib and Cairo libs [fail]

Question

I\'m trying to make a screenshot using Xlib and Cairo, however I\'m not sure to do it the good way, \"stride\" is really confusing me.

Answer 1

Instead of doing all this complicated magic, let cairo do it for you:

#include <cairo.h>
#include <cairo-xlib.h>
#include <X11/Xlib.h>

int main(int argc, char** argv) {
    Display *disp;
    Window root;
    cairo_surface_t *surface;
    int scr;

    disp = XOpenDisplay(NULL);
    scr = DefaultScreen(disp);
    root = DefaultRootWindow(disp);

    surface = cairo_xlib_surface_create(disp, root, DefaultVisual(disp, scr),
            DisplayWidth(disp, scr), DisplayHeight(disp, scr));
    cairo_surface_write_to_png(
            surface,
            "test.png");
    cairo_surface_destroy(surface);

    return 0;
}

Answer 2

TFM:

CAIRO_FORMAT_RGB24
    each pixel is a 32-bit quantity, with the upper 8 bits unused

TFM:

stride = cairo_format_stride_for_width (format, width);
data = malloc (stride * height);

Hence, the correct index calculation is

data[y * stride + x * 4 + 0] = blue;
data[y * stride + x * 4 + 1] = green;
data[y * stride + x * 4 + 2] = red;    /* yes, in this order */

Also, masks are taken from the image and shifts are hard-coded, which makes absolutely no sense. Calculate the shifts from the masks.