sixel: transparency improvements

* don't set transparency when raster attributes suffice - it seems
  terminals don't background-fill in that case either.
* fix transparency in encoder standalone mode
* update comments

2 files changed, 9 insertions, 9 deletions

diff --git a/adapter/img/sixel.nim b/adapter/img/sixel.nim
index 11625547..3eeacaf1 100644
--- a/adapter/img/sixel.nim
+++ b/adapter/img/sixel.nim
@@ -9,10 +9,7 @@
 #
 # * DCS q set-raster-attributes is omitted.
 # * 32-bit binary number in header indicates the end of the following
-#   palette. (Note: this includes this 32-bit number's and the following 8-bit
-#   number's length as well.)
-# * This is followed by an 8-bit number indicating whether the image includes
-#   transparent pixels.
+#   palette. (Note: this includes this 32-bit number's length as well.)
 # * A lookup table is appended to the file end, which includes (height + 5) / 6
 #   32-bit binary numbers indicating the start index of every 6th row.
 #
@@ -384,7 +381,10 @@ proc encode(img: openArray[RGBAColorBE]; width, height, offx, offy, cropw: int;
   if halfdump: # reserve size for prelude
     outs &= "\0\0\0\0"
   else:
-    outs &= DCS & 'q'
+    outs &= DCS
+    if transparent:
+      outs &= "0;1"
+    outs &= 'q'
     # set raster attributes
     outs &= "\"1;1;" & $width & ';' & $height
   let nodes = root.flatten(outs, palette)
diff --git a/src/local/term.nim b/src/local/term.nim
index 2d5b6c14..2550ca69 100644
--- a/src/local/term.nim
+++ b/src/local/term.nim
@@ -815,10 +815,10 @@ proc outputSixelImage(term: Terminal; x, y: int; image: CanvasImage;
   if preludeLen > data.len:
     return
   var outs = term.cursorGoto(x, y)
-  # set transparency if we want to draw a non-6-divisible number
-  # of rows *or* the image is transparent; omit it otherwise, for then
-  # some terminals (e.g. foot) handle the image more efficiently
-  let trans = realh mod 6 != 0 or image.transparent
+  # set transparency if the image has transparent sixels; omit it
+  # otherwise, for then some terminals (e.g. foot) handle the image more
+  # efficiently
+  let trans = image.transparent
   outs &= DCS & "0;" & $int(trans) & "q"
   # set raster attributes
   outs &= "\"1;1;" & $realw & ';' & $realh