More robust SVG auto-detection (#5878)

1 files changed, 16 insertions, 2 deletions

diff --git a/crates/typst-library/src/visualize/image/mod.rs b/crates/typst-library/src/visualize/image/mod.rs
index 97189e22..258eb96f 100644
--- a/crates/typst-library/src/visualize/image/mod.rs
+++ b/crates/typst-library/src/visualize/image/mod.rs
@@ -398,8 +398,7 @@ impl ImageFormat {
             return Some(Self::Raster(RasterFormat::Exchange(format)));
         }
 
-        // SVG or compressed SVG.
-        if data.starts_with(b"<svg") || data.starts_with(&[0x1f, 0x8b]) {
+        if is_svg(data) {
             return Some(Self::Vector(VectorFormat::Svg));
         }
 
@@ -407,6 +406,21 @@ impl ImageFormat {
     }
 }
 
+/// Checks whether the data looks like an SVG or a compressed SVG.
+fn is_svg(data: &[u8]) -> bool {
+    // Check for the gzip magic bytes. This check is perhaps a bit too
+    // permissive as other formats than SVGZ could use gzip.
+    if data.starts_with(&[0x1f, 0x8b]) {
+        return true;
+    }
+
+    // If the first 2048 bytes contain the SVG namespace declaration, we assume
+    // that it's an SVG. Note that, if the SVG does not contain a namespace
+    // declaration, usvg will reject it.
+    let head = &data[..data.len().min(2048)];
+    memchr::memmem::find(head, b"http://www.w3.org/2000/svg").is_some()
+}
+
 /// A vector graphics format.
 #[derive(Debug, Copy, Clone, Eq, PartialEq, Hash, Cast)]
 pub enum VectorFormat {