LR-HDR-EXPORT: HLG HDR AVIF export (hdrTransfer) and a tested SDR/HDR display decision

AVIF HDR export takes hdrTransfer = pq | hlg (export dialog: HDR curve;
MCP export schema lists it). HLG writes CICP transfer 18 and no clli box.
The window logs whether it shows HDR; today always SDR (eframe configures
an 8-bit surface), with the platform / surface format / headroom / opt-out
rules unit-tested. Also satisfies clippy 1.99's chunks_exact_to_as_chunks
in spots.rs.

Part of #868.

Sources: ITU-R BT.2100 (HLG OETF, inverse OOTF with system gamma 1.2),
ITU-R BT.2408 (203 cd/m2 reference white at 75 % HLG); own design for
the display decision.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/engine/src/export.rs

@@ -623,6 +623,32 @@ pub enum MetadataPolicy {
     None,
 }
 
+/// Transfer curve of an HDR AVIF export (`hdrTransfer`): PQ (absolute, SDR white at 203 cd/m²) or
+/// HLG (relative to the display; BT.2100 with SDR white at signal 0.75).
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "lowercase")]
+pub enum HdrCurve {
+    #[default]
+    Pq,
+    Hlg,
+}
+
+impl HdrCurve {
+    pub fn parse(s: &str) -> Option<Self> {
+        match s.to_ascii_lowercase().as_str() {
+            "pq" => Some(Self::Pq),
+            "hlg" => Some(Self::Hlg),
+            _ => None,
+        }
+    }
+    fn codec(self) -> lightcraft_codecs::HdrTransfer {
+        match self {
+            Self::Pq => lightcraft_codecs::HdrTransfer::Pq,
+            Self::Hlg => lightcraft_codecs::HdrTransfer::Hlg,
+        }
+    }
+}
+
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
 #[serde(default, rename_all = "camelCase")]
 pub struct ExportOptions {
@@ -681,6 +707,8 @@ pub struct ExportOptions {
     pub add_to_photos: bool,
     /// The Photos album they go into, made when missing (empty = none).
     pub photos_album: String,
+    /// The curve of an HDR AVIF: PQ (default) or HLG.
+    pub hdr_transfer: HdrCurve,
 }
 
 impl Default for ExportOptions {
@@ -710,6 +738,7 @@ impl Default for ExportOptions {
             jxl_progressive: false,
             add_to_photos: false,
             photos_album: String::new(),
+            hdr_transfer: HdrCurve::Pq,
         }
     }
 }
@@ -751,6 +780,7 @@ pub const OPTION_PARAMS: &[&str] = &[
     "jxlProgressive",
     "addToPhotos",
     "photosAlbum",
+    "hdrTransfer",
 ];
 
 /// Why an export can't add its files to Apple Photos on this platform (`None`: it can, when the
@@ -923,6 +953,7 @@ impl ExportOptions {
                 }
                 "sharpen" => one_of(k, v, &["none", "screen", "matte", "glossy"])?,
                 "sharpenAmount" => one_of(k, v, &["low", "standard", "high"])?,
+                "hdrTransfer" => one_of(k, v, &["pq", "hlg"])?,
                 "conflict" => one_of(k, v, &["unique", "overwrite", "skip"])?,
                 "tiffCompression" => one_of(k, v, &["none", "lzw", "zip", "deflate"])?,
                 "jxlEffort" => one_of(k, v, &["fast", "normal"])?,
@@ -1061,6 +1092,7 @@ impl ExportOptions {
             jxl_progressive: p.get("jxlProgressive").and_then(Value::as_bool).unwrap_or(d.jxl_progressive),
             add_to_photos: p.get("addToPhotos").and_then(Value::as_bool).unwrap_or(d.add_to_photos),
             photos_album: s("photosAlbum").map_or(d.photos_album, |a| a.trim().to_string()),
+            hdr_transfer: s("hdrTransfer").and_then(HdrCurve::parse).unwrap_or(d.hdr_transfer),
         }
     }
 
@@ -1450,7 +1482,7 @@ pub fn encode_deep(img: &DeepImage, o: &ExportOptions, meta: Option<&Metadata>)
         (ExportFormat::Tiff, _) => encode::encode_tiff(&e, o.tiff_compression, &meta),
         (ExportFormat::Psd, _) => encode::encode_psd(&e, &meta),
         (ExportFormat::Avif, DeepSamples::F32(v)) if img.space == OutputSpace::Rec2020 => {
-            lightcraft_codecs::encode_avif_pq(w, h, v, o.quality, 8, &meta)
+            lightcraft_codecs::encode_avif_hdr(w, h, v, o.quality, 8, &meta, o.hdr_transfer.codec())
         }
         (ExportFormat::Avif, _) => encode::encode_avif(&e, o.quality, 8, &meta),
         (ExportFormat::Jxl, DeepSamples::F32(_)) if img.space == OutputSpace::Rec2020 => {

crates/engine/src/tests_export.rs

@@ -455,6 +455,21 @@ fn hdr_avif_export_is_pq_rec2020() {
     assert!(nclx(&sdr.bytes).is_none_or(|c| c[3] != 16));
 }
 
+#[test]
+fn hdr_avif_export_hlg() {
+    let mut s = Session::with_demo();
+    let id = sunset(&mut s);
+    let o = ExportOptions::from_json(&json!({"longEdge": 160, "format": "avif", "hdr": true, "hdrTransfer": "hlg", "quality": 70}));
+    assert_eq!(o.hdr_transfer, crate::export::HdrCurve::Hlg);
+    assert_eq!(o.to_json()["hdrTransfer"], "hlg");
+    s.execute("develop.hdr", &json!({"enabled": true, "maxEv": 2})).unwrap();
+    let hdr = export_photo(&mut s, id, &o, 1).unwrap();
+    // nclx: BT.2020 primaries (9), HLG (18), BT.2020 NCL (9), full range; no content light level box
+    let i = hdr.bytes.windows(4).position(|w| w == b"nclx").expect("colr nclx box");
+    assert_eq!(hdr.bytes[i + 4..i + 11].to_vec(), vec![0, 9, 0, 18, 0, 9, 0x80]);
+    assert!(!hdr.bytes.windows(4).any(|w| w == b"clli"));
+}
+
 /// "Remove location info" holds for the file-copying formats too: the sidecar and the DNG's
 /// packet carry no location, and an original that embeds GPS is refused rather than copied.
 #[test]