add cli flag --alpha-layer

now that we have long-opts, we don't have to worry about exhausting the
alphabet list for short-opts. so adding a cli flag to set/unset the
checker background makes sense.

ref: https://codeberg.org/nsxiv/nsxiv/issues/404

config.def.h

@@ -45,11 +45,6 @@ static const double CONTRAST_MAX    = 4.0;
 /* command i_scroll pans image 1/PAN_FRACTION of screen width/height */
 static const int PAN_FRACTION = 5;
 
-/* if true, use a checkerboard background for alpha layer,
- * toggled with 'A' key binding
- */
-static const bool ALPHA_LAYER = false;
-
 /* percentage of memory to use for imlib2's cache size.
  *   3 means use 3% of total memory which is about 245MiB on 8GiB machine.
  *   0 or less means disable cache.
@@ -70,6 +65,11 @@ static const int CACHE_SIZE_FALLBACK = 32 * 1024 * 1024; /* fallback to 32MiB if
  */
 static const bool ANTI_ALIAS = true;
 
+/* if true, use a checkerboard background for alpha layer,
+ * toggled with 'A' key binding (overwritten via `--alpha-layer` option)
+ */
+static const bool ALPHA_LAYER = false;
+
 #endif
 #ifdef INCLUDE_THUMBS_CONFIG
 

etc/nsxiv.1

@@ -119,6 +119,11 @@ key-handler and the input of \-i will be separated by a NULL character.
 Enables anti-aliasing, when given
 .I no
 as an argument, disables it instead.
+.TP
+.BI "\-\-alpha\-layer" [=no]
+Enables checkerboard background for alpha layer, when given
+.I no
+as an argument, disables it instead.
 .SH KEYBOARD COMMANDS
 .SS General
 The following keyboard commands are available in both image and thumbnail modes:

image.c

@@ -91,7 +91,7 @@ void img_init(img_t *img, win_t *win)
 	img->checkpan = false;
 	img->dirty = false;
 	img->anti_alias = options->anti_alias;
-	img->alpha = ALPHA_LAYER;
+	img->alpha = options->alpha_layer;
 	img->multi.cap = img->multi.cnt = 0;
 	img->multi.animate = options->animate;
 	img->multi.framedelay = options->framerate > 0 ? 1000 / options->framerate : 0;

nsxiv.h

@@ -241,6 +241,7 @@ struct opt {
 	float zoom;
 	bool animate;
 	bool anti_alias;
+	bool alpha_layer;
 	int gamma;
 	unsigned int slideshow;
 	int framerate;

options.c

@@ -66,8 +66,13 @@ static void print_version(void)
 
 void parse_options(int argc, char **argv)
 {
-	enum { /* ensure these can't be represented in a single byte */
-		OPT_AA = UCHAR_MAX + 1
+	enum {
+		/* ensure these can't be represented in a single byte in order
+		 * to avoid conflicts with short opts
+		 */
+		OPT_START = UCHAR_MAX,
+		OPT_AA,
+		OPT_AL
 	};
 	static const struct optparse_long longopts[] = {
 		{ "framerate",      'A',     OPTPARSE_REQUIRED },
@@ -95,6 +100,7 @@ void parse_options(int argc, char **argv)
 		{ "zoom",           'z',     OPTPARSE_REQUIRED },
 		{ "null",           '0',     OPTPARSE_NONE },
 		{ "anti-alias",    OPT_AA,   OPTPARSE_OPTIONAL },
+		{ "alpha-layer",   OPT_AL,   OPTPARSE_OPTIONAL },
 		{ 0 }, /* end */
 	};
 
@@ -115,6 +121,7 @@ void parse_options(int argc, char **argv)
 	_options.scalemode = SCALE_DOWN;
 	_options.zoom = 1.0;
 	_options.anti_alias = ANTI_ALIAS;
+	_options.alpha_layer = ALPHA_LAYER;
 	_options.animate = false;
 	_options.gamma = 0;
 	_options.slideshow = 0;
@@ -247,6 +254,11 @@ void parse_options(int argc, char **argv)
 				error(EXIT_FAILURE, 0, "Invalid argument for option --anti-alias: %s", op.optarg);
 			_options.anti_alias = op.optarg == NULL;
 			break;
+		case OPT_AL:
+			if (op.optarg != NULL && !STREQ(op.optarg, "no"))
+				error(EXIT_FAILURE, 0, "Invalid argument for option --alpha-layer: %s", op.optarg);
+			_options.alpha_layer = op.optarg == NULL;
+			break;
 		}
 	}