[PATCH:xbacklight] Add a -mul option, which changes brightness by a multiplier.

[Jake Schmidt]

On my laptop (a Lenovo ThinkPad X1 Carbon, 4th generation), the physical
screen brightness seems to be a logarithmic function of the xbacklight
number. This means adding/subtracting a constant has a much larger effect
on a dim screen than a bright screen. Multiplying the brightness by, say,
1.5/0.66 gives more desirable behavior (approximately constant brightness
changes throughout the range).

Signed-off-by: Jake Schmidt <thejakeschmidt at gmail.com>
---
 xbacklight.c | 26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/xbacklight.c b/xbacklight.c
index 278043f..deba73f 100644
--- a/xbacklight.c
+++ b/xbacklight.c
@@ -36,7 +36,7 @@
 #include <string.h>
 #include <unistd.h>
 
-typedef enum { Get, Set, Inc, Dec } op_t;
+typedef enum { Get, Set, Inc, Dec, Mul } op_t;
 
 static char *program_name;
 
@@ -53,6 +53,7 @@ usage (int exitcode)
             "  -set <percentage> or = <percentage>\n"
             "  -inc <percentage> or + <percentage>\n"
             "  -dec <percentage> or - <percentage>\n"
+            "  -mul <multiplier> or * <multiplier>\n"
             "  -get\n"
             "  -time <fade time in milliseconds>\n"
             "  -steps <number of steps in fade>\n");
@@ -185,6 +186,19 @@ main (int argc, char **argv)
 	    value = atof (argv[i] + 1);
 	    continue;
 	}
+	if (!strcmp (argv[i], "-mul") || !strcmp (argv[i], "*"))
+	{
+	    if (++i >= argc) missing_arg (argv[i-1]);
+	    op = Mul;
+	    value = atof (argv[i]);
+	    continue;
+	}
+	if (argv[i][0] == '*' && isdigit (argv[i][1]))
+	{
+	    op = Mul;
+	    value = atof (argv[i] + 1);
+	    continue;
+	}
 	if (!strcmp (argv[i], "-get") || !strcmp (argv[i], "-g"))
 	{
 	    op = Get;
@@ -315,6 +329,16 @@ main (int argc, char **argv)
 			case Dec:
 			    new = cur - set;
 			    break;
+			case Mul:
+			    set = cur * value - cur;
+			    if (value > 1 && set < 1.0) {
+				set = 1.0;
+			    }
+			    if (value < 1 && set > -1.0) {
+				set = -1.0;
+			    }
+			    new = cur + set;
+			    break;
 			default:
 			    xcb_aux_sync (conn);
 			    return 1;
-- 
2.9.3