gui: add docs for save_animated_gif -key

Signed-off-by: Matt Liberty <[email protected]>

Author: maliberty

src/gui/README.md

@@ -165,13 +165,19 @@ save_histogram_image
 
 This command can be used to generate an animated gif.
 
+When used with -start this command returns an integer key that can be
+used to distinguish files if multiple are generated.  That key can be
+provided when using -add or -end.  If only a single file is being used
+the key can be ignored.
+
 ```tcl
 save_animated_gif
     -start|-add|-end
     [-resolution microns_per_pixel]
     [-area {x0 y0 x1 y1}]
     [-width width]
     [-delay delay]
+    [-key key]
     [filename]
 ```
 
@@ -187,6 +193,7 @@ save_animated_gif
 | `-resolution`| resolution in microns per pixel to use when saving the image, default will match what the GUI has selected.|
 | `-width`| width of the output image in pixels, default will be computed from the resolution. Cannot be used with ``-resolution``.|
 | `-delay`| delay between frames in the GIF.|
+| `-key`| used to distinguish multiple GIF files (returned by -add).|
 
 ### Select Objects
 

src/gui/src/gui.tcl

@@ -142,12 +142,13 @@ sta::define_cmd_args "save_animated_gif" {-start|-add|-end \
                                           [-width width] \
                                           [-resolution microns_per_pixel] \
                                           [-delay delay] \
+                                          [-key key] \
                                           [path]
 }
 
 proc save_animated_gif { args } {
   sta::parse_key_args "save_animated_gif" args \
-    keys {-area -width -resolution -delay} flags {-start -end -add}
+    keys {-area -width -resolution -delay -key} flags {-start -end -add}
 
   set resolution 0
   if { [info exists keys(-resolution)] } {