cmd_opacity: add relative opacity changes

This enhances the opacity command to support relative assignment as well
as the currently implemented absolute assignment. The syntax is copied
from the same format that gaps uses for relative and absolute setting.
An example usage in a sway config looks like:

// relative change (this feature)
bindsym button4 opacity plus .1
bindsym button5 opacity minus .1

// absolute change (this feature)
bindsym button4 opacity set 1
bindsym button5 opacity set .3

// old way, still supported
bindsym button4 opacity 1
bindsym button5 opacity .3

sway/commands/opacity.c

@@ -1,21 +1,13 @@
 #include <assert.h>
 #include <stdlib.h>
+#include <strings.h>
 #include "sway/commands.h"
 #include "sway/tree/view.h"
 #include "log.h"
 
-static bool parse_opacity(const char *opacity, float *val) {
-	char *err;
-	*val = strtof(opacity, &err);
-	if (*val < 0 || *val > 1 || *err) {
-		return false;
-	}
-	return true;
-}
-
 struct cmd_results *cmd_opacity(int argc, char **argv) {
 	struct cmd_results *error = NULL;
-	if ((error = checkarg(argc, "opacity", EXPECTED_EQUAL_TO, 1))) {
+	if ((error = checkarg(argc, "opacity", EXPECTED_AT_LEAST, 1))) {
 		return error;
 	}
 
@@ -25,15 +17,26 @@ struct cmd_results *cmd_opacity(int argc, char **argv) {
 		return cmd_results_new(CMD_FAILURE, "No current container");
 	}
 
-	float opacity = 0.0f;
+	char *err;
+	float val = strtof(argc == 1 ? argv[0] : argv[1], &err);
+	if (*err) {
+		return cmd_results_new(CMD_INVALID, "opacity float invalid");
+	}
 
-	if (!parse_opacity(argv[0], &opacity)) {
+	if (!strcasecmp(argv[0], "plus")) {
+		val = con->alpha + val;
+	} else if (!strcasecmp(argv[0], "minus")) {
+		val = con->alpha - val;
+	} else if (argc > 1 && strcasecmp(argv[0], "set")) {
 		return cmd_results_new(CMD_INVALID,
-				"Invalid value (expected 0..1): %s", argv[0]);
+				"Expected: set|plus|minus <0..1>: %s", argv[0]);
 	}
 
-	con->alpha = opacity;
+	if (val < 0 || val > 1) {
-	container_damage_whole(con);
+		return cmd_results_new(CMD_FAILURE, "opacity value out of bounds");
+	}
 
+	con->alpha = val;
+	container_damage_whole(con);
 	return cmd_results_new(CMD_SUCCESS, NULL);
 }

sway/sway.5.scd

@@ -664,9 +664,9 @@ The default colors are:
 	Any mark that starts with an underscore will not be drawn even if
 	*show_marks* is yes. The default is _yes_.
 
-*opacity* <value>
+*opacity* [set|plus|minus] <value>
-	Set the opacity of the window between 0 (completely transparent) and 1
+	Adjusts the opacity of the window between 0 (completely transparent) and
-	(completely opaque).
+	1 (completely opaque). If the operation is omitted, _set_ will be used.
 
 *tiling_drag*  enable|disable|toggle
 	Sets whether or not tiling containers can be dragged with the mouse. If