jxl-oxide-cli: Improve image viewer consistency with BT.709 images

crates/jxl-color/src/convert.rs

@@ -124,6 +124,7 @@ pub struct ColorTransformBuilder {
     detect_peak: bool,
     srgb_icc: bool,
     from_pq: bool,
+    xyb_from_bt709: bool,
 }
 
 impl Default for ColorTransformBuilder {
@@ -139,6 +140,7 @@ impl ColorTransformBuilder {
             detect_peak: false,
             srgb_icc: false,
             from_pq: false,
+            xyb_from_bt709: false,
         }
     }
 
@@ -157,6 +159,11 @@ impl ColorTransformBuilder {
         self
     }
 
+    pub fn xyb_from_bt709(&mut self, xyb_from_bt709: bool) -> &mut Self {
+        self.xyb_from_bt709 = xyb_from_bt709;
+        self
+    }
+
     pub fn build(
         self,
         from: &ColorEncodingWithProfile,
@@ -211,6 +218,7 @@ impl ColorTransform {
             detect_peak,
             srgb_icc,
             from_pq,
+            xyb_from_bt709,
         } = builder;
         let connecting_tf = if srgb_icc {
             TransferFunction::Srgb
@@ -249,6 +257,7 @@ impl ColorTransform {
         }
 
         let mut ops = Vec::new();
+        let mut need_bt709_fix = false;
 
         let mut current_encoding = match from.encoding {
             ColourEncoding::IccProfile(_) => {
@@ -287,6 +296,15 @@ impl ColorTransform {
                 rendering_intent,
                 ..
             }) => {
+                let to_bt709 = matches!(
+                    to.encoding,
+                    ColourEncoding::Enum(EnumColourEncoding {
+                        tf: TransferFunction::Bt709,
+                        ..
+                    })
+                );
+                need_bt709_fix = xyb_from_bt709 && !to_bt709;
+
                 let inv_mat = oim.inv_mat;
                 #[rustfmt::skip]
                 let matrix = [
@@ -361,6 +379,28 @@ impl ColorTransform {
             }
         };
 
+        if need_bt709_fix {
+            // Correct using BT.1886 EOTF.
+            ops.push(ColorTransformOp::TransferFunction {
+                tf: TransferFunction::Bt709,
+                hdr_params: HdrParams {
+                    luminances: [0f32; 3],
+                    intensity_target,
+                    min_nits,
+                },
+                inverse: false,
+            });
+            ops.push(ColorTransformOp::TransferFunction {
+                tf: TransferFunction::Bt709,
+                hdr_params: HdrParams {
+                    luminances: [0f32; 3],
+                    intensity_target,
+                    min_nits,
+                },
+                inverse: true,
+            });
+        }
+
         let target_encoding = match &to.encoding {
             ColourEncoding::Enum(encoding) => encoding,
             ColourEncoding::IccProfile(_) => {
@@ -1091,8 +1131,9 @@ fn apply_inverse_transfer_function(
             }
         }
         TransferFunction::Bt709 => {
+            // Matches Chromium and Firefox behavior.
             for ch in channels {
-                tf::bt709_to_linear(ch);
+                tf::srgb_to_linear(ch);
             }
         }
         TransferFunction::Unknown => {}

crates/jxl-color/src/tf/bt709.rs

@@ -100,14 +100,3 @@ unsafe fn linear_to_bt709_x86_64_avx2(samples: &mut [f32]) -> &mut [f32] {
 
     remainder
 }
-
-pub fn bt709_to_linear(samples: &mut [f32]) {
-    for x in samples {
-        let a = *x;
-        *x = if a <= 0.081 {
-            a / 4.5
-        } else {
-            crate::fastmath::fast_powf_generic(a.mul_add(1.0 / 1.099, 0.099 / 1.099), 1.0 / 0.45)
-        };
-    }
-}

crates/jxl-oxide-cli/src/output.rs

@@ -58,11 +58,25 @@ pub(crate) fn write_png<W: Write>(
 
     let mut writer = encoder.write_header()?;
 
-    tracing::debug!("Embedding ICC profile");
-    let compressed_icc = miniz_oxide::deflate::compress_to_vec_zlib(&source_icc, 7);
-    let mut iccp_chunk_data = vec![b'0', 0, 0];
-    iccp_chunk_data.extend(compressed_icc);
-    writer.write_chunk(png::chunk::iCCP, &iccp_chunk_data)?;
+    let skip_icc = matches!(
+        metadata.colour_encoding,
+        jxl_oxide::color::ColourEncoding::Enum(jxl_oxide::color::EnumColourEncoding {
+            tf: jxl_oxide::color::TransferFunction::Bt709,
+            ..
+        })
+    );
+
+    if skip_icc {
+        // XXX: This is necessary because libjxl encodes BT.709 images with BT.709 inverse
+        // OETF, but some image viewers use BT.1886 EOTF when it encounters BT.709 CICP tag.
+        tracing::debug!("Skipping ICC profile, because the transfer function is BT.709");
+    } else {
+        tracing::debug!("Embedding ICC profile");
+        let compressed_icc = miniz_oxide::deflate::compress_to_vec_zlib(&source_icc, 7);
+        let mut iccp_chunk_data = vec![b'0', 0, 0];
+        iccp_chunk_data.extend(compressed_icc);
+        writer.write_chunk(png::chunk::iCCP, &iccp_chunk_data)?;
+    }
 
     if let Some(cicp) = cicp {
         tracing::debug!(cicp = format_args!("{:?}", cicp), "Writing cICP chunk");

crates/jxl-render/src/lib.rs

@@ -825,9 +825,18 @@ impl RenderContext {
             tracing::trace!(requested_color_encoding = ?self.requested_color_encoding);
             tracing::trace!(do_ycbcr = frame_header.do_ycbcr);
 
+            let is_bt709 = matches!(
+                header_color_encoding,
+                ColourEncoding::Enum(EnumColourEncoding {
+                    tf: jxl_color::TransferFunction::Bt709,
+                    ..
+                })
+            );
+
             let mut transform = jxl_color::ColorTransform::builder();
             transform.set_srgb_icc(!self.cms.supports_linear_tf());
             transform.from_pq(self.suggested_hdr_tf() == Some(jxl_color::TransferFunction::Pq));
+            transform.xyb_from_bt709(metadata.xyb_encoded && is_bt709);
             let transform = transform.build(
                 &frame_color_encoding,
                 &self.requested_color_encoding,