From 881b3455cb5620daca2cb620ecac9d471b917cea Mon Sep 17 00:00:00 2001
From: Laura <laura@wiese2.org>
Date: Mon, 11 Aug 2025 03:15:29 +0200
Subject: [PATCH] input-only codecs

---
 internal/codec/avif/avif.go     | 5 ++++-
 internal/codec/bmp/bmp.go       | 4 ++++
 internal/codec/codec.go         | 2 ++
 internal/codec/detect.go        | 8 ++++++++
 internal/codec/gif/gif.go       | 4 ++++
 internal/codec/ico/ico.go       | 4 ++++
 internal/codec/jpeg/jpeg.go     | 4 ++++
 internal/codec/jpegxl/jpegxl.go | 4 ++++
 internal/codec/png/png.go       | 4 ++++
 internal/codec/tiff/tiff.go     | 4 ++++
 internal/codec/webp/webp.go     | 4 ++++
 11 files changed, 46 insertions(+), 1 deletion(-)

diff --git a/internal/codec/avif/avif.go b/internal/codec/avif/avif.go
index 60bb6ea..1af4882 100644
--- a/internal/codec/avif/avif.go
+++ b/internal/codec/avif/avif.go
@@ -15,7 +15,6 @@ import (
 )
 
 var (
-	quality  int
 	qualityA int
 	speed    int
 	chroma   int
@@ -35,6 +34,10 @@ func (impl) Extensions() []string {
 	return []string{"avif"}
 }
 
+func (impl) CanEncode() bool {
+	return true
+}
+
 func (impl) Flags(flags []cli.Flag) []cli.Flag {
 	return append(flags,
 		&cli.IntFlag{
diff --git a/internal/codec/bmp/bmp.go b/internal/codec/bmp/bmp.go
index 8a731c7..e4bb68d 100644
--- a/internal/codec/bmp/bmp.go
+++ b/internal/codec/bmp/bmp.go
@@ -26,6 +26,10 @@ func (impl) Extensions() []string {
 	return []string{"bmp"}
 }
 
+func (impl) CanEncode() bool {
+	return true
+}
+
 func (impl) Flags(flags []cli.Flag) []cli.Flag {
 	return flags
 }
diff --git a/internal/codec/codec.go b/internal/codec/codec.go
index 1583c40..abc26c8 100644
--- a/internal/codec/codec.go
+++ b/internal/codec/codec.go
@@ -14,6 +14,8 @@ type Codec interface {
 	Flags([]cli.Flag) []cli.Flag
 	Extensions() []string
 
+	CanEncode() bool
+
 	Sniff(io.ReaderAt) (int, []byte, error)
 	Decode(io.Reader) (image.Image, error)
 	Encode(io.Writer, image.Image, opts.Common) error
diff --git a/internal/codec/detect.go b/internal/codec/detect.go
index 9b0402e..c377964 100644
--- a/internal/codec/detect.go
+++ b/internal/codec/detect.go
@@ -78,6 +78,10 @@ func Detect(output, override string) (Codec, error) {
 			return nil, fmt.Errorf("unsupported output codec: %q", override)
 		}
 
+		if !codec.CanEncode() {
+			return nil, fmt.Errorf("decode-only output codec: %q", override)
+		}
+
 		return codec, nil
 	}
 
@@ -93,6 +97,10 @@ func Detect(output, override string) (Codec, error) {
 	for _, codec := range codecs {
 		for _, alias := range codec.Extensions() {
 			if ext == strings.ToLower(alias) {
+				if !codec.CanEncode() {
+					return nil, fmt.Errorf("decode-only output codec: %q", override)
+				}
+
 				return codec, nil
 			}
 		}
diff --git a/internal/codec/gif/gif.go b/internal/codec/gif/gif.go
index ba81d79..cfed6ef 100644
--- a/internal/codec/gif/gif.go
+++ b/internal/codec/gif/gif.go
@@ -29,6 +29,10 @@ func (impl) Extensions() []string {
 	return []string{"gif"}
 }
 
+func (impl) CanEncode() bool {
+	return true
+}
+
 func (impl) Flags(flags []cli.Flag) []cli.Flag {
 	return append(flags, &cli.IntFlag{
 		Name:        "gif.colors",
diff --git a/internal/codec/ico/ico.go b/internal/codec/ico/ico.go
index 2a0f64e..49698f0 100644
--- a/internal/codec/ico/ico.go
+++ b/internal/codec/ico/ico.go
@@ -30,6 +30,10 @@ func (impl) Flags(flags []cli.Flag) []cli.Flag {
 	return flags
 }
 
+func (impl) CanEncode() bool {
+	return true
+}
+
 func (impl) Sniff(reader io.ReaderAt) (int, []byte, error) {
 	magicICO := []byte{0x00, 0x00, 0x01, 0x00}
 	magicCUR := []byte{0x00, 0x00, 0x02, 0x00}
diff --git a/internal/codec/jpeg/jpeg.go b/internal/codec/jpeg/jpeg.go
index 6c325af..4ad8cdd 100644
--- a/internal/codec/jpeg/jpeg.go
+++ b/internal/codec/jpeg/jpeg.go
@@ -30,6 +30,10 @@ func (impl) Flags(flags []cli.Flag) []cli.Flag {
 	return flags
 }
 
+func (impl) CanEncode() bool {
+	return true
+}
+
 func (impl) Sniff(reader io.ReaderAt) (int, []byte, error) {
 	magic := []byte{0xFF, 0xD8, 0xFF}
 
diff --git a/internal/codec/jpegxl/jpegxl.go b/internal/codec/jpegxl/jpegxl.go
index 911a311..221f678 100644
--- a/internal/codec/jpegxl/jpegxl.go
+++ b/internal/codec/jpegxl/jpegxl.go
@@ -32,6 +32,10 @@ func (impl) Extensions() []string {
 	return []string{"jxl"}
 }
 
+func (impl) CanEncode() bool {
+	return true
+}
+
 func (impl) Flags(flags []cli.Flag) []cli.Flag {
 	return append(flags,
 		&cli.IntFlag{
diff --git a/internal/codec/png/png.go b/internal/codec/png/png.go
index c2d69fa..24728bd 100644
--- a/internal/codec/png/png.go
+++ b/internal/codec/png/png.go
@@ -31,6 +31,10 @@ func (impl) Extensions() []string {
 	return []string{"png"}
 }
 
+func (impl) CanEncode() bool {
+	return true
+}
+
 func (impl) Flags(flags []cli.Flag) []cli.Flag {
 	return append(flags, &cli.IntFlag{
 		Name:        "png.compression",
diff --git a/internal/codec/tiff/tiff.go b/internal/codec/tiff/tiff.go
index 2e36dd6..6f885a2 100644
--- a/internal/codec/tiff/tiff.go
+++ b/internal/codec/tiff/tiff.go
@@ -33,6 +33,10 @@ func (impl) Extensions() []string {
 	return []string{"tiff", "tif"}
 }
 
+func (impl) CanEncode() bool {
+	return true
+}
+
 func (impl) Flags(flags []cli.Flag) []cli.Flag {
 	return append(flags,
 		&cli.IntFlag{
diff --git a/internal/codec/webp/webp.go b/internal/codec/webp/webp.go
index 7161e5b..7969828 100644
--- a/internal/codec/webp/webp.go
+++ b/internal/codec/webp/webp.go
@@ -33,6 +33,10 @@ func (impl) Extensions() []string {
 	return []string{"webp"}
 }
 
+func (impl) CanEncode() bool {
+	return true
+}
+
 func (impl) Flags(flags []cli.Flag) []cli.Flag {
 	return append(flags,
 		&cli.IntFlag{