example/stb-imagedumper: add -bg flag

cmd/handsum/main.go

diff --git a/cmd/handsum/main.go b/cmd/handsum/main.go
index 64aa2e2..fc8e09e 100644
--- a/cmd/handsum/main.go
+++ b/cmd/handsum/main.go
@@ -39,11 +39,19 @@
 	encodeFlag    = flag.Bool("encode", false, "")
 	roundtripFlag = flag.Bool("roundtrip", false, "")
 
-	bFlag = flag.String("b", "", "")
-	cFlag = flag.String("c", "rgb", "")
-	qFlag = flag.Int("q", 3, "")
+	bFlag  = flag.String("b", "", "")
+	bgFlag = flag.String("bg", "", "")
+	cFlag  = flag.String("c", "rgb", "")
+	qFlag  = flag.Int("q", 3, "")
 )
 
+func getBFlag() string {
+	if *bFlag != "" {
+		return *bFlag
+	}
+	return *bgFlag
+}
+
 const usageStr = `handsum decodes and encodes the Handsum lossy image file format.
 
 Usage: choose one of
@@ -61,10 +69,10 @@
 Roundtrip is equivalent to encode (to an ephemeral file) and then decode.
 
 For encode or roundtrip, the default color and quality is -c=rgb -q=3 (best
-quality; 147 bytes per file) but you can choose a lower setting. The -b flag
-sets a background color (in case the input has transparency). For example:
+quality; 147 bytes per file) but you can choose a lower setting. The -b or -bg
+flag sets a background color (in case the input is transparent). For example:
 
-handsum -encode -b=darkblue -c=rgb -q=2 foo.png > foo.handsum
+handsum -encode -bg=darkblue -c=rgb -q=2 foo.png > foo.handsum
 
 ----
 
@@ -74,7 +82,7 @@
   -encode     Whether to encode the input.
   -roundtrip  Whether to encode-and-decode the input.
 
-  -b  Encoding background color: e.g. red, white, 9c27b0 or #ff8.
+  -b or -bg  Encoding background color: e.g. red, white, 9c27b0 or #ff8.
       The default is ff9e9e9e (for -c=1 or -c=3) or 00000000 (for -c=4).
   -c  Encoding color: gray (1), rgb (3, default) or rgba (4).
   -q  Encoding quality: 0, 1, 2 or 3 (default).
@@ -100,7 +108,7 @@
 	}
 	quality := handsum.Quality(max(0, min(3, *qFlag)))
 
-	if (color <= handsum.ColorRGB) && (*bFlag == "") {
+	if (color <= handsum.ColorRGB) && (getBFlag() == "") {
 		*bFlag = "ff9e9e9e"
 	}
 
@@ -169,7 +177,7 @@
 }
 
 func applyBackgroundColor(src image.Image) image.Image {
-	backgroundColor, _ := parsecolor.Parse(*bFlag)
+	backgroundColor, _ := parsecolor.Parse(getBFlag())
 	r16, g16, b16, a16 := backgroundColor.RGBA()
 	if (r16 == 0) && (g16 == 0) && (b16 == 0) && (a16 == 0) {
 		return src

example/stb-imagedumper/stb-imagedumper.c

diff --git a/example/stb-imagedumper/stb-imagedumper.c b/example/stb-imagedumper/stb-imagedumper.c
index 34f7d80..1b32501 100644
--- a/example/stb-imagedumper/stb-imagedumper.c
+++ b/example/stb-imagedumper/stb-imagedumper.c
@@ -645,7 +645,8 @@
     "    -a or -ascii-art (use ASCII art instead of ANSI color codes)\n"
     "    -B or -braille-art-dark-mode (use white-on-black Braille art)\n"
     "    -b or -braille-art-light-mode (use black-on-white Braille art)\n"
-    "    -c=orange or -background-color=ff9800 (set a background color)\n"
+    "    -c=orange or -bg=f90 or -background-color=ff9800 (set a background\n"
+    "        color, for transparent input)\n"
     "    -demo (dump the built-in demonstration images)\n"
     "    -n or -natural-size (use each image's natural size); this\n"
     "        overrides -resize=N\n"
@@ -696,7 +697,9 @@
       g_flags.natural_size = true;
       continue;
     }
-    if (!strncmp(arg, "c=", 2) || !strncmp(arg, "background-color=", 17)) {
+    if (!strncmp(arg, "c=", 2) ||   //
+        !strncmp(arg, "bg=", 3) ||  //
+        !strncmp(arg, "background-color=", 17)) {
       while (*arg++ != '=') {
       }
       int64_t c = parse_color(arg, strlen(arg));