fix: default SamplesPerPixel to 1 when tag is absent

fixtures/other/readme.md

@@ -1,3 +1,11 @@
+`grayscale_no_samplesperpixel.tif` is a minimal (114-byte) little-endian classic
+TIFF: a 2x2 uint8 grayscale image, uncompressed, single strip. It deliberately
+**omits the `SamplesPerPixel` tag (277)**, which the TIFF 6.0 specification
+defines with a default of 1. It is a regression fixture for parsing files that
+omit the tag (e.g. PerkinElmer/Phenix microscopy images), which previously
+panicked with "samples_per_pixel not found". It was generated by hand-writing
+the TIFF header, IFD and pixel bytes with Python's `struct` module.
+
 `geogtowgs_subset_USGS_13_s14w171.tif` was created from "s3://prd-tnm/StagedProducts/Elevation/13/TIFF/current/s14w171/USGS_13_s14w171.tif" using these commands:
 
 ```bash

src/ifd.rs

@@ -372,7 +372,10 @@ impl ImageFileDirectory {
             geo_key_directory = Some(GeoKeyDirectory::from_tags(tags)?);
         }
 
-        let samples_per_pixel = samples_per_pixel.expect("samples_per_pixel not found");
+        // SamplesPerPixel (tag 277) defaults to 1 when the tag is absent.
+        // TIFF 6.0 spec, "SamplesPerPixel ... Default = 1": https://download.osgeo.org/libtiff/doc/TIFF6.pdf
+        // Many single-channel grayscale TIFFs (e.g. PerkinElmer/Phenix microscopy) omit it.
+        let samples_per_pixel = samples_per_pixel.unwrap_or(1);
         let planar_configuration = if let Some(planar_configuration) = planar_configuration {
             planar_configuration
         } else if samples_per_pixel == 1 {

src/test/missing_samples_per_pixel.rs

@@ -0,0 +1,23 @@
+use crate::test::util::open_tiff;
+use crate::tags::PhotometricInterpretation;
+
+/// Regression test for TIFFs that omit the `SamplesPerPixel` tag (277).
+///
+/// The TIFF 6.0 specification defines a default of 1 for `SamplesPerPixel`, and
+/// some single-channel grayscale TIFFs (e.g. PerkinElmer/Phenix microscopy
+/// images) omit the tag entirely. Parsing such files used to panic with
+/// "samples_per_pixel not found"; it must instead fall back to the spec default.
+#[tokio::test]
+async fn test_missing_samples_per_pixel_defaults_to_one() {
+    let filename = "other/grayscale_no_samplesperpixel.tif";
+    let (_reader, tiff) = open_tiff(filename).await;
+    let ifd = &tiff.ifds()[0];
+
+    assert_eq!(ifd.samples_per_pixel(), 1);
+    assert_eq!(ifd.image_width(), 2);
+    assert_eq!(ifd.image_height(), 2);
+    assert_eq!(
+        ifd.photometric_interpretation(),
+        PhotometricInterpretation::BlackIsZero
+    );
+}

src/test/mod.rs

@@ -1,5 +1,6 @@
 mod geo;
 mod geotiff_test_data;
 mod image_tiff;
+mod missing_samples_per_pixel;
 mod ome_tiff;
 pub(crate) mod util;