PNG export: Indexed Color keeps the document resolution (#3047) (#3271)

The palette PNG encoder never wrote a pHYs chunk, so an Indexed Color
document saved as PNG reopened at 72 ppi while the RGB path kept its
resolution. encode_png_indexed now takes an optional dpi and the flat
exporter passes the document resolution under the same embed_metadata
rule as the ordinary PNG encoder. Save for Web PNG-8 is unchanged.

Co-authored-by: Claude Opus 5.5 <[email protected]>

crates/codecs/src/codecs/png.rs

@@ -521,8 +521,16 @@ fn adler32_combine(a1: u32, a2: u32, len2: usize) -> u32 {
 }
 
 /// Writes an 8-bit palette PNG (PNG-8): `indices` row-major, `palette` RGB entries (≤ 256),
-/// optional fully transparent entry (`tRNS`). Used for Indexed Color documents.
-pub fn encode_indexed(width: u32, height: u32, indices: &[u8], palette: &[[u8; 3]], transparent: Option<u8>) -> Result<Vec<u8>, CodecError> {
+/// optional fully transparent entry (`tRNS`) and optional resolution in dpi (`pHYs`, written when
+/// both axes are positive and finite). Used for Indexed Color documents.
+pub fn encode_indexed(
+    width: u32,
+    height: u32,
+    indices: &[u8],
+    palette: &[[u8; 3]],
+    transparent: Option<u8>,
+    dpi: Option<(f32, f32)>,
+) -> Result<Vec<u8>, CodecError> {
     if width == 0 || height == 0 || indices.len() != width as usize * height as usize {
         return Err(CodecError::InvalidImage("index data does not match the size".into()));
     }
@@ -538,6 +546,15 @@ pub fn encode_indexed(width: u32, height: u32, indices: &[u8], palette: &[[u8; 3
         trns[usize::from(t)] = 0;
         info.trns = Some(trns.into());
     }
+    if let Some((x, y)) = dpi
+        && x.is_finite()
+        && y.is_finite()
+        && x > 0.0
+        && y > 0.0
+    {
+        info.pixel_dims =
+            Some(png::PixelDimensions { xppu: (x / METERS_PER_INCH).round() as u32, yppu: (y / METERS_PER_INCH).round() as u32, unit: png::Unit::Meter });
+    }
     let mut out = Vec::new();
     {
         let encoder = png::Encoder::with_info(&mut out, info).map_err(|e| CodecError::encode(F, e))?;
@@ -556,14 +573,14 @@ mod indexed_tests {
     fn png8_roundtrips_through_the_decoder() {
         let pal = [[255, 0, 0], [0, 0, 255], [0, 0, 0]];
         let idx = [0u8, 1, 2, 1, 0, 2];
-        let bytes = encode_indexed(3, 2, &idx, &pal, Some(2)).unwrap();
+        let bytes = encode_indexed(3, 2, &idx, &pal, Some(2), None).unwrap();
         let img = decode(&bytes, &Limits::default()).unwrap();
         assert_eq!(img.dimensions(), (3, 2));
         let px = img.convert(ChannelLayout::Rgba, SampleType::U8);
         assert_eq!(&px.data()[..8], &[255, 0, 0, 255, 0, 0, 255, 255]);
         assert_eq!(px.data()[11], 0, "transparent entry");
-        assert!(encode_indexed(3, 2, &[9; 6], &pal, None).is_err());
-        assert!(encode_indexed(3, 3, &idx, &pal, None).is_err());
+        assert!(encode_indexed(3, 2, &[9; 6], &pal, None, None).is_err());
+        assert!(encode_indexed(3, 3, &idx, &pal, None, None).is_err());
     }
 }
 

crates/engine/src/web_cmds.rs

@@ -490,7 +490,7 @@ pub fn optimize(px: &[[f32; 4]], bw: usize, rect: Rect, st: &WebSettings, icc: O
             let bytes = if st.format == WebFormat::Gif {
                 photocraft_codecs::web::encode_gif_indexed(ww, hh, &idx, &pal, tt, st.interlaced).map_err(e)?
             } else {
-                photocraft_codecs::encode_png_indexed(ww, hh, &idx, &pal, tt).map_err(e)?
+                photocraft_codecs::encode_png_indexed(ww, hh, &idx, &pal, tt, None).map_err(e)?
             };
             (bytes, Some(pal.len()))
         }

crates/io/src/flat.rs

@@ -664,7 +664,9 @@ fn export_mode_specific(doc: &Document, format: Format, opts: &ExportOptions) ->
                 }));
                 Ok(())
             });
-            let bytes = codecs::encode_png_indexed(doc.size.width, doc.size.height, &idx, &table.colors, table.transparent)?;
+            // Same rule as the ordinary PNG encoder: the resolution goes into `pHYs` with the metadata.
+            let dpi = opts.encode.embed_metadata.then_some((doc.resolution_dpi, doc.resolution_dpi));
+            let bytes = codecs::encode_png_indexed(doc.size.width, doc.size.height, &idx, &table.colors, table.transparent, dpi)?;
             let mut warnings = vec![format!("written as an 8-bit palette PNG ({} colours)", table.colors.len())];
             if opts.encode.embed_metadata && opts.xmp == XmpEmbed::All && !doc.metadata.text.is_empty() {
                 warnings.push("text metadata is not supported by the palette PNG exporter; it will be dropped".into());

crates/io/tests/flat.rs

@@ -313,8 +313,8 @@ fn transparent_flat_images_open_as_layer_0() {
         ("ga.png", enc(ChannelLayout::GrayA, CS::U8, Format::Png), true),
         ("a.tif", enc(ChannelLayout::Rgba, CS::U8, Format::Tiff), true),
         ("ga.tif", enc(ChannelLayout::GrayA, CS::U16, Format::Tiff), true),
-        ("pal.png", photocraft_codecs::encode_png_indexed(w, h, &[0, 1].repeat(n / 2), &[[255, 0, 0], [0, 0, 255]], Some(0)).unwrap(), true),
-        ("pal_opaque.png", photocraft_codecs::encode_png_indexed(w, h, &[0, 1].repeat(n / 2), &[[255, 0, 0], [0, 0, 255]], None).unwrap(), false),
+        ("pal.png", photocraft_codecs::encode_png_indexed(w, h, &[0, 1].repeat(n / 2), &[[255, 0, 0], [0, 0, 255]], Some(0), None).unwrap(), true),
+        ("pal_opaque.png", photocraft_codecs::encode_png_indexed(w, h, &[0, 1].repeat(n / 2), &[[255, 0, 0], [0, 0, 255]], None, None).unwrap(), false),
         ("rgb.png", enc(ChannelLayout::Rgb, CS::U8, Format::Png), false),
         ("g.png", enc(ChannelLayout::Gray, CS::U16, Format::Png), false),
         ("rgb.tif", enc(ChannelLayout::Rgb, CS::U8, Format::Tiff), false),

crates/io/tests/modes.rs

@@ -37,3 +37,15 @@ fn duotone_exports_the_inks_as_rgb() {
     let px = back.layers[0].surface().unwrap().rgba(0, 0);
     assert!(px[0] > px[2] + 0.05, "warm: {px:?}");
 }
+
+#[test]
+fn indexed_png_keeps_the_document_resolution() {
+    // #3047: the palette PNG lost its pHYs chunk and reopened at 72 ppi.
+    let mut d = doc(ColorMode::Indexed);
+    d.resolution_dpi = 300.0;
+    d.color_table = Some(ColorTable { colors: vec![[0, 0, 0], [255, 0, 0]], transparent: None });
+    let r = photocraft_io::export(&d, "x.png", &Default::default()).unwrap();
+    assert_eq!(r.bytes[25], 3, "still a palette PNG");
+    let back = photocraft_io::import("x.png", &r.bytes).unwrap().document;
+    assert!((back.resolution_dpi - 300.0).abs() < 0.01, "{}", back.resolution_dpi);
+}