fix: ghostty detection and halfblock bug

detect/detect.go

@@ -23,11 +23,11 @@ func Best() Protocol {
 		return Kitty
 	}
 
-	// Ghostty sets TERM=xterm-ghostty and supports Kitty graphics.
+	// Ghostty sets TERM=ghostty or TERM=xterm-ghostty, and TERM_PROGRAM=ghostty.
 	term := os.Getenv("TERM")
 	termProg := strings.ToLower(os.Getenv("TERM_PROGRAM"))
 
-	if strings.HasPrefix(term, "xterm-ghostty") {
+	if term == "ghostty" || strings.HasPrefix(term, "xterm-ghostty") || termProg == "ghostty" {
 		return Kitty
 	}
 

detect/detect_test.go

@@ -16,7 +16,17 @@ func TestBest(t *testing.T) {
 			want: Kitty,
 		},
 		{
-			name: "kitty via ghostty TERM",
+			name: "kitty via ghostty TERM=ghostty",
+			env:  map[string]string{"TERM": "ghostty"},
+			want: Kitty,
+		},
+		{
+			name: "kitty via ghostty TERM_PROGRAM=ghostty",
+			env:  map[string]string{"TERM_PROGRAM": "ghostty"},
+			want: Kitty,
+		},
+		{
+			name: "kitty via ghostty TERM=xterm-ghostty",
 			env:  map[string]string{"TERM": "xterm-ghostty"},
 			want: Kitty,
 		},

termimage.go

@@ -62,13 +62,13 @@ func Display(w io.Writer, src string, opts Options) error {
 // DisplayContext is Display with caller-supplied context for cancellation of
 // remote fetches and sandboxed decoding.
 func DisplayContext(ctx context.Context, w io.Writer, src string, opts Options) error {
-	maxW, maxH := effectiveDimensions(opts)
-
 	proto := opts.Protocol
 	if proto == Auto {
 		proto = detect.Best()
 	}
 
+	maxW, maxH := effectiveDimensions(opts, proto)
+
 	resolved, err := source.Resolve(ctx, src)
 	if err != nil {
 		return err
@@ -108,12 +108,24 @@ func renderWith(w io.Writer, img *image.NRGBA, proto Protocol) error {
 	}
 }
 
-func effectiveDimensions(opts Options) (int, int) {
+// effectiveDimensions returns the pixel bounds for image scaling.
+// For HalfBlock, terminal character dimensions drive the limit (1 char = 1px
+// wide, 2px tall) because the renderer emits one character per pixel column.
+// For Kitty/Sixel, the terminal's actual pixel viewport is used.
+func effectiveDimensions(opts Options, proto Protocol) (int, int) {
 	w, h := opts.MaxWidth, opts.MaxHeight
 	if w > 0 && h > 0 {
 		return w, h
 	}
-	tw, th := detectTermPixels()
+
+	var tw, th int
+	if proto == HalfBlock {
+		cols, rows := detectTermChars()
+		tw, th = cols, rows*2
+	} else {
+		tw, th = detectTermPixels()
+	}
+
 	if w <= 0 {
 		w = tw
 	}
@@ -131,3 +143,12 @@ func detectTermPixels() (int, int) {
 	defer func() { _ = f.Close() }()
 	return termPixels(f)
 }
+
+func detectTermChars() (int, int) {
+	f, err := os.OpenFile("/dev/tty", os.O_RDWR, 0)
+	if err != nil {
+		return 220, 50
+	}
+	defer func() { _ = f.Close() }()
+	return termChars(f)
+}

termimage_test.go

@@ -47,23 +47,22 @@ func TestProtocolConstants(t *testing.T) {
 }
 
 func TestEffectiveDimensions_ExplicitWidthAndHeight(t *testing.T) {
-	w, h := effectiveDimensions(Options{MaxWidth: 800, MaxHeight: 600})
+	w, h := effectiveDimensions(Options{MaxWidth: 800, MaxHeight: 600}, HalfBlock)
 	if w != 800 || h != 600 {
 		t.Errorf("got %dx%d, want 800x600", w, h)
 	}
 }
 
 func TestEffectiveDimensions_ZerosFallBackToDetected(t *testing.T) {
-	// With both 0, both come from detection. detectTermPixels returns 1920x1080
-	// when /dev/tty isn't available — accept any positive values.
-	w, h := effectiveDimensions(Options{})
+	// With both 0, both come from detection — accept any positive values.
+	w, h := effectiveDimensions(Options{}, HalfBlock)
 	if w <= 0 || h <= 0 {
 		t.Errorf("expected positive dimensions, got %dx%d", w, h)
 	}
 }
 
 func TestEffectiveDimensions_PartialOverride(t *testing.T) {
-	w, h := effectiveDimensions(Options{MaxWidth: 500})
+	w, h := effectiveDimensions(Options{MaxWidth: 500}, HalfBlock)
 	if w != 500 {
 		t.Errorf("MaxWidth=500 not honoured: got %d", w)
 	}

termsize_linux.go

@@ -20,3 +20,12 @@ func termPixels(f *os.File) (int, int) {
 	// Fallback: estimate from cell count (8×16 px per cell is a safe default).
 	return int(ws.Col) * 8, int(ws.Row) * 16
 }
+
+// termChars returns the terminal dimensions as (cols, rows).
+func termChars(f *os.File) (int, int) {
+	ws, err := unix.IoctlGetWinsize(int(f.Fd()), unix.TIOCGWINSZ)
+	if err != nil || ws.Col == 0 || ws.Row == 0 {
+		return 220, 50
+	}
+	return int(ws.Col), int(ws.Row)
+}

termsize_stub.go

@@ -5,3 +5,4 @@ package termimage
 import "os"
 
 func termPixels(_ *os.File) (int, int) { return 1920, 1080 }
+func termChars(_ *os.File) (int, int)  { return 220, 50 }