Picking colors at mouse position with LuaJIT

color_picker.lua

-- Load FFI
local ffi = require("ffi")

-- Define FFI functions & structures by OS
local x11
if ffi.os == "Windows" then
	ffi.cdef([[
	typedef int BOOL;
	typedef long LONG;
	typedef struct{
		LONG x, y;
	}POINT, *LPPOINT;
	typedef void* HANDLE;
	typedef HANDLE HDC;
	typedef HANDLE HWND;
	typedef unsigned long DWORD;
	typedef DWORD COLORREF;

	BOOL GetCursorPos(LPPOINT);
	HDC GetDC(HWND);
	int ReleaseDC(HWND, HDC);
	COLORREF GetPixel(HDC, int, int);
	]])
else
	x11 = ffi.load("X11")
	ffi.cdef(
[[
typedef void Display;
typedef unsigned long XID;
typedef XID Window;
typedef XID Colormap;
typedef struct{
	void* ext_data;
	void* display;
	Window root;
	int width, height;
	int mwidth, mheight;
	int ndepths;
	void* depths;
	int root_depth;
	void* root_visual;
	void* default_gc;
	Colormap cmap;
	// Rest doesn't matter
}Screen;
typedef char* XPointer;
typedef struct{
	void* ext_data;
	void* private1;
	int fd;
	int private2;
	int proto_major_version;
	int proto_minor_version;
	char* vendor;
	XID private3;
	XID private4;
	XID private5;
	int private6;
	XID (*resource_alloc)(void*);
	int byte_order;
	int bitmap_unit;
	int bitmap_pad;
	int bitmap_bit_order;
	int nformats;
	void* pixmap_format;
	int private8;
	int release;
	void* private9, *private10;
	int qlen;
	unsigned long last_request_read;
	unsigned long request;
	XPointer private11;
	XPointer private12;
	XPointer private13;
	XPointer private14;
	unsigned max_request_size;
	void* db;
	int (*private15)(void*);
	char* display_name;
	int default_screen;
	// Rest doesn't matter
}*_XPrivDisplay;
typedef int Bool;
typedef struct{
	int x, y;
	int width, height;
	int border_width;
	int depth;
	void* visual;
	Window root;
	int class;
	int bit_gravity;
	int win_gravity;
	int backing_store;
	unsigned long backing_planes;
	unsigned long backing_pixel;
	Bool save_under;
	Colormap colormap;
	Bool map_installed;
	int map_state;
	long all_event_masks;
	long your_event_mask;
	long do_not_propagate_mask;
	Bool override_redirect;
	Screen *screen;
}XWindowAttributes;
typedef void XImage;
typedef XID Drawable;
static const unsigned long AllPlanes = (unsigned long)~0L;
static const int XYPixmap = 1;
typedef struct{
	unsigned long pixel;
	unsigned short red, green, blue;
	char flags;
	char pad;
}XColor;

Display* XOpenDisplay(char*);
int XCloseDisplay(Display*);
Screen* XScreenOfDisplay(Display*, int);
Bool XQueryPointer(Display*, Window, Window*, Window*, int*, int*, int*, int*, unsigned int*);
int XGetWindowAttributes(Display*, Window, XWindowAttributes*);
XImage* XGetImage(Display*, Drawable, int, int, unsigned int, unsigned int, unsigned long, int);
int XFree(void*);
unsigned long XGetPixel(XImage*, int, int);
Colormap XDefaultColormap(Display*, int);
int XQueryColor(Display*, Colormap, XColor*);
]]
	)
end

-- Get screen color(s) at cursor position and cursor position
local function pick_screen_color(range)
	-- Check arguments
	if range ~= nil and (type(range) ~= "number" or range < 0) then
		error("Invalid color pick range!", 2)
	end
	-- Fix range to valid integer
	range = range and math.floor(range) or 0
	-- Output buffers
	local bmp, mx, my = {n = 0}
	-- Continue by OS
	if ffi.os == "Windows" then
		-- Get cursor position
		local ppoint = ffi.new("POINT[1]")
		if ffi.C.GetCursorPos(ppoint) == 0 then
			error("Couldn't get cursor position!", 2)
		end
		mx, my = ppoint[0].x, ppoint[0].y
		-- Get desktop device context
		local dc = ffi.gc(ffi.C.GetDC(nil), function(dc) ffi.C.ReleaseDC(nil, dc) end)
		if not dc then
			error("Couldn't get desktop device context!", 2)
		end
		-- Get desktop color under & around cursor
		for y=-range, range do
			for x=-range, range do
				bmp.n = bmp.n + 1
				local color = ffi.C.GetPixel(dc, mx + x, my + y)
				if color < 16777216 then	-- No error on pixel get
					bmp[bmp.n] = {r = color % 256, g = math.floor(color / 256) % 256, b = math.floor(color / 65536)}
				end
			end
		end
	else
		-- Get display & root window
		local display = ffi.gc(x11.XOpenDisplay(nil), x11.XCloseDisplay)
		if not display then
			error("Couldn't open display!", 2)
		end
		local root = x11.XScreenOfDisplay(display, ffi.cast("_XPrivDisplay", display)[0].default_screen)[0].root
		-- Get cursor position
		local root_window, child_window, root_x, root_y, win_x, win_y, mask = ffi.new("Window[1]"), ffi.new("Window[1]"), ffi.new("int[1]"), ffi.new("int[1]"), ffi.new("int[1]"), ffi.new("int[1]"), ffi.new("unsigned int[1]")
		if x11.XQueryPointer(display, root, root_window, child_window, root_x, root_y, win_x, win_y, mask) == 0 then
			error("Couldn't get cursor position!", 2)
		end
		mx, my = win_x[0], win_y[0]
		-- Get desktop color under & around cursor
		local attr = ffi.new("XWindowAttributes[1]")
		if x11.XGetWindowAttributes(display, root, attr) == 0 then
			error("Couldn't get window attributes!", 2)
		end
		local w, h, color, color_map = attr[0].width, attr[0].height, ffi.new("XColor[1]"), x11.XScreenOfDisplay(display, ffi.cast("_XPrivDisplay", display)[0].default_screen)[0].cmap
		for y=-range, range do
			for x=-range, range do
				bmp.n = bmp.n + 1
				local ix, iy = mx + x, my + y
				if ix >= 0 and iy >= 0 and ix < w and iy < h then
					local image = ffi.gc(x11.XGetImage(display, root, ix, iy, 1, 1, ffi.C.AllPlanes, ffi.C.XYPixmap), x11.XFree)
					if image then
						color[0].pixel = x11.XGetPixel(image, 0, 0)
						x11.XQueryColor(display, color_map, color)
						bmp[bmp.n] = {r = math.floor(color[0].red / 256), g = math.floor(color[0].green / 256), b = math.floor(color[0].blue / 256)}
					end
				end
			end
		end
	end
	-- Return collected pixels & mouse position
	return bmp, mx, my
end

-- Test
local colors, mouse_x, mouse_y = pick_screen_color(1)
print(mouse_x, mouse_y)
for i=1, colors.n do
	local color = colors[i]
	if color then
		print(color.r, color.g, color.b)
	else
		print(color)
	end
end