Add support for decoding XMP metadata

This change adds support for accessing the XMP metadata stored in a GIF.
This extends the StreamingDecoder implementation that now reads this
Extension section correctly.

Co-authored-by: Rasmus Piorr <piorr@google.com>

Author: 1c3t3a

src/reader/decoder.rs

@@ -18,6 +18,9 @@ use weezl::{decode::Decoder as LzwDecoder, BitOrder, LzwError, LzwStatus};
 
 /// GIF palettes are RGB
 pub const PLTE_CHANNELS: usize = 3;
+/// Headers for supported extensions.
+const EXT_NAME_NETSCAPE: &[u8] = b"\x0bNETSCAPE2.0\x03";
+const EXT_NAME_XMP: &[u8] = b"\x0bXMP DataXMP";
 
 /// An error returned in the case of the image not being formatted properly.
 #[derive(Debug)]
@@ -377,6 +380,8 @@ pub struct StreamingDecoder {
     current: Option<Frame<'static>>,
     /// Needs to emit `HeaderEnd` once
     header_end_reached: bool,
+    /// XMP metadata bytes.
+    xmp_metadata: Option<Vec<u8>>,
 }
 
 /// One version number of the GIF standard.
@@ -464,6 +469,7 @@ impl StreamingDecoder {
             },
             current: None,
             header_end_reached: false,
+            xmp_metadata: None,
         }
     }
 
@@ -530,6 +536,12 @@ impl StreamingDecoder {
         self.height
     }
 
+    /// XMP metadata stored in the image.
+    #[must_use]
+    pub fn xmp_metadata(&self) -> Option<&[u8]> {
+        self.xmp_metadata.as_deref()
+    }
+
     /// The version number of the GIF standard used in this image.
     ///
     /// We suppose a minimum of `V87a` compatibility. This value will be reported until we have
@@ -754,17 +766,36 @@ impl StreamingDecoder {
                         }
                     }
                 } else {
+                    self.ext.data.push(b);
                     self.ext.is_block_end = false;
                     goto!(ExtensionDataBlock(b as usize), emit Decoded::SubBlockFinished(self.ext.id))
                 }
             }
             ApplicationExtension => {
                 debug_assert_eq!(0, b);
+                // We can consume the extension data here as the next states won't access it anymore.
+                let mut data = std::mem::take(&mut self.ext.data);
+
                 // the parser removes sub-block lenghts, so app name and data are concatenated
-                if self.ext.data.len() >= 15 && &self.ext.data[1..13] == b"NETSCAPE2.0\x01" {
-                    let repeat = &self.ext.data[13..15];
-                    let repeat = u16::from(repeat[0]) | u16::from(repeat[1]) << 8;
+                if let Some(&[first, second, ..]) = data.strip_prefix(EXT_NAME_NETSCAPE) {
+                    let repeat = u16::from(first) | u16::from(second) << 8;
                     goto!(BlockEnd, emit Decoded::Repetitions(if repeat == 0 { Repeat::Infinite } else { Repeat::Finite(repeat) }))
+                } else if data.starts_with(EXT_NAME_XMP) {
+                    data.drain(..EXT_NAME_XMP.len());
+                    // XMP adds a "ramp" of 257 bytes to the end of the metadata to let the "pascal-strings"
+                    // parser converge to the null byte. The ramp looks like "0x01, 0xff, .., 0x01, 0x00".
+                    // For convenience and to allow consumers to not be bothered with this implementation detail,
+                    // we cut the ramp.
+                    const RAMP_SIZE: usize = 257;
+                    if data.len() >= RAMP_SIZE
+                        && data.ends_with(&[0x03, 0x02, 0x01, 0x00])
+                        && data[data.len() - RAMP_SIZE..].starts_with(&[0x01, 0x0ff])
+                    {
+                        data.truncate(data.len() - RAMP_SIZE);
+                    }
+
+                    self.xmp_metadata = Some(data);
+                    goto!(BlockEnd)
                 } else {
                     goto!(BlockEnd)
                 }

src/reader/mod.rs

@@ -473,6 +473,12 @@ where
         self.decoder.decoder.height()
     }
 
+    /// XMP metadata.
+    #[inline]
+    pub fn xmp_metadata(&self) -> Option<&[u8]> {
+        self.decoder.decoder.xmp_metadata()
+    }
+
     /// Abort decoding and recover the `io::Read` instance
     pub fn into_inner(self) -> io::BufReader<R> {
         self.decoder.into_inner()

tests/decode.rs

@@ -259,3 +259,13 @@ fn issue_209_exension_block() {
         }
     })();
 }
+
+#[test]
+fn xmp_metadata() {
+    const EXPECTED_METADATA: &str = "<?xpacket begin='\u{feff}' id='W5M0MpCehiHzreSzNTczkc9d'?>\n<x:xmpmeta xmlns:x='adobe:ns:meta/' x:xmptk='Image::ExifTool 13.25'>\n<rdf:RDF xmlns:rdf='http://www.w3.org/1999/02/22-rdf-syntax-ns#'>\n\n <rdf:Description rdf:about=''\n  xmlns:dc='http://purl.org/dc/elements/1.1/'>\n  <dc:subject>\n   <rdf:Bag>\n    <rdf:li>sunset, mountains, nature</rdf:li>\n   </rdf:Bag>\n  </dc:subject>\n </rdf:Description>\n</rdf:RDF>\n</x:xmpmeta>\n                                                                                                    \n                                                                                                    \n                                                                                                    \n                                                                                                    \n                                                                                                    \n                                                                                                    \n                                                                                                    \n                                                                                                    \n                                                                                                    \n                                                                                                    \n                                                                                                    \n                                                                                                    \n                                                                                                    \n                                                                                                    \n                                                                                                    \n                                                                                                    \n                                                                                                    \n                                                                                                    \n                                                                                                    \n                                                                                                    \n                                                                                                    \n                                                                                                    \n                                                                                                    \n                                                                                                    \n<?xpacket end='w'?>";
+
+    let image: &[u8] = include_bytes!("samples/beacon_xmp.gif");
+    let decoder = DecodeOptions::new().read_info(image).unwrap();
+
+    assert_eq!(decoder.xmp_metadata(), Some(EXPECTED_METADATA.as_bytes()))
+}

tests/results.txt

@@ -1,5 +1,6 @@
 tests/samples/alpha_gif_a.gif: 3871893825
 tests/samples/anim-gr.gif: 291646878
+tests/samples/beacon_xmp.gif: 2462153529
 tests/samples/beacon.gif: 2462153529
 tests/samples/interlaced.gif: 2115495372
 tests/samples/moon_impact.gif: 2438689726