(Experimental) Add support for iTerm2 inline image protocol

Close #1102

  fzf --preview 'imgcat -W $FZF_PREVIEW_COLUMNS -H $FZF_PREVIEW_LINES {}'

Notes:
* There is no good way to determine the height of the rendered image,
  so we assume that the image takes the full height of the preview
  window. So the image cannot be displayed with the other text.
* fzf-preview.sh script was updated to use `imgcat` if it's available
  but `chafa` is not.
* iTerm2 also supports Sixel, so adding support for this protocol is not
  quite necessary but it renders animated GIFs much better (e.g. looping).

3 files changed, 39 insertions, 13 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index a02e12cd..0a041f8d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,7 +10,12 @@ CHANGELOG
       ```sh
       fzf --preview='fzf-preview.sh {}'
       ```
-- (Experimental) Sixel and Kitty image support now also available on Windows
+- (Experimental) iTerm2 inline image protocol support in preview window
+  ```sh
+  # Using https://iterm2.com/utilities/imgcat
+  fzf --preview 'imgcat -W $FZF_PREVIEW_COLUMNS -H $FZF_PREVIEW_LINES {}'
+  ```
+- (Experimental) Sixel, Kitty, and iTerm2 image support now also available on Windows
 - HTTP server can be configured to accept remote connections
   ```sh
   # FZF_API_KEY is required for a non-localhost listen address
diff --git a/bin/fzf-preview.sh b/bin/fzf-preview.sh
index d72cd2d4..8991e3de 100755
--- a/bin/fzf-preview.sh
+++ b/bin/fzf-preview.sh
@@ -4,8 +4,9 @@
 # image in the preview window of fzf.
 #
 # Dependencies:
-# - https://github.com/hpjansson/chafa
 # - https://github.com/sharkdp/bat
+# - https://github.com/hpjansson/chafa
+# - https://iterm2.com/utilities/imgcat
 
 if [[ $# -ne 1 ]]; then
   >&2 echo "usage: $0 FILENAME"
@@ -44,6 +45,7 @@ elif ! [[ $KITTY_WINDOW_ID ]] && (( FZF_PREVIEW_TOP + FZF_PREVIEW_LINES == $(stt
   dim=${FZF_PREVIEW_COLUMNS}x$((FZF_PREVIEW_LINES - 1))
 fi
 
+# 1. Use kitty icat on kitty terminal
 if [[ $KITTY_WINDOW_ID ]]; then
   # 1. 'memory' is the fastest option but if you want the image to be scrollable,
   #    you have to use 'stream'.
@@ -52,10 +54,21 @@ if [[ $KITTY_WINDOW_ID ]]; then
   #    This confuses fzf and makes it render scroll offset indicator.
   #    So we remove the last line and append the reset code to its previous line.
   kitty icat --clear --transfer-mode=memory --stdin=no --place="$dim@0x0" "$file" | sed '$d' | sed $'$s/$/\e[m/'
+
+# 2. Use chafa with Sixel output
 elif command -v chafa > /dev/null; then
   chafa -f sixel -s "$dim" "$file"
   # Add a new line character so that fzf can display multiple images in the preview window
   echo
+
+# 3. If chafa is not found but imgcat is available, use it on iTerm2
+elif command -v imgcat > /dev/null; then
+  # NOTE: We should use https://iterm2.com/utilities/it2check to check if the
+  # user is running iTerm2. But for the sake of simplicty, we just assume
+  # that's the case here.
+  imgcat -W "${dim%%x*}" -H "${dim##*x}" "$file"
+
+# 4. Cannot find any suitable method to preview the image
 else
   file "$file"
 fi
diff --git a/src/terminal.go b/src/terminal.go
index 747001ef..f48e5f25 100644
--- a/src/terminal.go
+++ b/src/terminal.go
@@ -66,7 +66,8 @@ func init() {
 	// * https://github.com/tmux/tmux/wiki/FAQ#what-is-the-passthrough-escape-sequence-and-how-do-i-use-it
 	// * https://sw.kovidgoyal.net/kitty/graphics-protocol
 	// * https://en.wikipedia.org/wiki/Sixel
-	passThroughRegex = regexp.MustCompile(`\x1bPtmux;\x1b\x1b.*?[^\x1b]\x1b\\|\x1b(_G|P[0-9;]*q).*?\x1b\\\r?`)
+	// * https://iterm2.com/documentation-images.html
+	passThroughRegex = regexp.MustCompile(`\x1bPtmux;\x1b\x1b.*?[^\x1b]\x1b\\|\x1b(_G|P[0-9;]*q).*?\x1b\\\r?|\x1b]1337;.*?\a`)
 }
 
 type jumpMode int
@@ -125,7 +126,7 @@ type previewed struct {
 	numLines  int
 	offset    int
 	filled    bool
-	sixel     bool
+	image     bool
 	wipe      bool
 	wireframe bool
 }
@@ -2027,7 +2028,7 @@ func (t *Terminal) renderPreviewText(height int, lines []string, lineNo int, unc
 	var ansi *ansiState
 	spinnerRedraw := t.pwindow.Y() == 0
 	wiped := false
-	sixel := false
+	image := false
 	wireframe := false
 Loop:
 	for _, line := range lines {
@@ -2055,18 +2056,25 @@ Loop:
 				t.pwindow.Move(y, x)
 			}
 			for idx, passThrough := range passThroughs {
-				// Handling Sixel output
+				// Handling Sixel/iTerm image
 				requiredLines := 0
 				isSixel := strings.HasPrefix(passThrough, "\x1bP")
-				if isSixel {
+				isItermImage := strings.HasPrefix(passThrough, "\x1b]1337;")
+				isImage := isSixel || isItermImage
+				if isImage {
 					t.previewed.wipe = true
-					if t.termSize.PxHeight > 0 {
+					// NOTE: We don't have a good way to get the height of an iTerm image,
+					// so we assume that it requires the full height of the preview
+					// window.
+					requiredLines = height
+
+					if isSixel && t.termSize.PxHeight > 0 {
 						rows := strings.Count(passThrough, "-")
 						requiredLines = int(math.Ceil(float64(rows*6*t.termSize.Lines) / float64(t.termSize.PxHeight)))
 					}
 				}
 
-				// Render wireframe when Sixel image cannot be displayed entirely
+				// Render wireframe when the image cannot be displayed entirely
 				if requiredLines > 0 && y+requiredLines > height {
 					top := true
 					for ; y < height; y++ {
@@ -2081,17 +2089,17 @@ Loop:
 				}
 
 				// Clear previous wireframe or any other text
-				if (t.previewed.wireframe || isSixel && !t.previewed.sixel) && !wiped {
+				if (t.previewed.wireframe || isImage && !t.previewed.image) && !wiped {
 					wiped = true
 					for i := y + 1; i < height; i++ {
 						t.pwindow.MoveAndClear(i, 0)
 					}
 					// Required for tcell to clear the previous text
-					if !t.previewed.sixel {
+					if !t.previewed.image {
 						t.tui.Sync(false)
 					}
 				}
-				sixel = sixel || isSixel
+				image = image || isImage
 				if idx == 0 {
 					t.pwindow.MoveAndClear(y, x)
 				} else {
@@ -2154,7 +2162,7 @@ Loop:
 		}
 		lineNo++
 	}
-	t.previewed.sixel = sixel
+	t.previewed.image = image
 	t.previewed.wireframe = wireframe
 }