Implement floating-point predictor (Predictor 3) encoding

Add support for encoding TIFF images with the floating-point predictor,
which improves compression ratios for floating-point image data.

The floating-point predictor works by:
1. Converting floats to big-endian bytes
2. Shuffling bytes so all first bytes are together, all second bytes, etc.
3. Applying byte-level horizontal differencing across the buffer

This complements the existing decoding support for Predictor 3.

Supported types: Gray32Float, Gray64Float, RGB32Float, RGB64Float,
RGBA32Float, RGBA64Float, CMYK32Float, CMYK64Float.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/encoder/colortype.rs

@@ -1,5 +1,66 @@
 use crate::tags::{PhotometricInterpretation, SampleFormat};
 
+/// Apply floating-point predictor encoding to a row of f32 samples.
+///
+/// The floating-point predictor works by:
+/// 1. Converting each f32 to big-endian bytes
+/// 2. Shuffling bytes so all first bytes are together, all second bytes, etc.
+/// 3. Applying byte-level horizontal difference (wrapping_sub) across the entire buffer
+///
+/// The decoder reverses this by:
+/// 1. Applying cumulative sum (wrapping_add) on the entire byte stream
+/// 2. De-shuffling bytes back into f32 values
+fn fp_predict_f32(row: &[f32], samples: usize, result: &mut Vec<u8>) {
+    let num_values = row.len();
+    let byte_count = num_values * 4;
+    let start_len = result.len();
+
+    // Reserve space for the output
+    result.reserve(byte_count);
+
+    // Step 1 & 2: Convert to big-endian and shuffle bytes
+    // All first bytes, then all second bytes, etc.
+    for byte_idx in 0..4 {
+        for &value in row {
+            let bytes = value.to_be_bytes();
+            result.push(bytes[byte_idx]);
+        }
+    }
+
+    // Step 3: Apply byte-level horizontal difference across the entire buffer
+    // We iterate backwards so that when we compute output[i] - output[i-samples],
+    // output[i-samples] still has its original value.
+    // The prediction spans across quarter boundaries, just like the decoder's cumulative sum.
+    let output = &mut result[start_len..];
+    for i in (samples..byte_count).rev() {
+        output[i] = output[i].wrapping_sub(output[i - samples]);
+    }
+}
+
+/// Apply floating-point predictor encoding to a row of f64 samples.
+fn fp_predict_f64(row: &[f64], samples: usize, result: &mut Vec<u8>) {
+    let num_values = row.len();
+    let byte_count = num_values * 8;
+    let start_len = result.len();
+
+    // Reserve space for the output
+    result.reserve(byte_count);
+
+    // Step 1 & 2: Convert to big-endian and shuffle bytes
+    for byte_idx in 0..8 {
+        for &value in row {
+            let bytes = value.to_be_bytes();
+            result.push(bytes[byte_idx]);
+        }
+    }
+
+    // Step 3: Apply byte-level horizontal difference across the entire buffer
+    let output = &mut result[start_len..];
+    for i in (samples..byte_count).rev() {
+        output[i] = output[i].wrapping_sub(output[i - samples]);
+    }
+}
+
 macro_rules! integer_horizontal_predict {
     () => {
         fn horizontal_predict(row: &[Self::Inner], result: &mut Vec<Self::Inner>) {
@@ -23,6 +84,10 @@ macro_rules! integer_horizontal_predict {
                     .map(|(prev, current)| current.wrapping_sub(*prev)),
             );
         }
+
+        fn floating_point_predict(_: &[Self::Inner], _: &mut Vec<u8>) {
+            unreachable!("floating-point predictor is only valid for floating-point sample types")
+        }
     };
 }
 
@@ -38,6 +103,10 @@ pub trait ColorType {
     const SAMPLE_FORMAT: &'static [SampleFormat];
 
     fn horizontal_predict(row: &[Self::Inner], result: &mut Vec<Self::Inner>);
+
+    /// Apply floating-point predictor encoding to a row of samples.
+    /// This is only implemented for floating-point types; integer types will panic.
+    fn floating_point_predict(row: &[Self::Inner], result: &mut Vec<u8>);
 }
 
 pub struct Gray8;
@@ -108,7 +177,11 @@ impl ColorType for Gray32Float {
     const SAMPLE_FORMAT: &'static [SampleFormat] = &[SampleFormat::IEEEFP];
 
     fn horizontal_predict(_: &[Self::Inner], _: &mut Vec<Self::Inner>) {
-        unreachable!()
+        unreachable!("horizontal predictor is not valid for floating-point sample types")
+    }
+
+    fn floating_point_predict(row: &[Self::Inner], result: &mut Vec<u8>) {
+        fp_predict_f32(row, Self::SAMPLE_FORMAT.len(), result)
     }
 }
 
@@ -140,7 +213,11 @@ impl ColorType for Gray64Float {
     const SAMPLE_FORMAT: &'static [SampleFormat] = &[SampleFormat::IEEEFP];
 
     fn horizontal_predict(_: &[Self::Inner], _: &mut Vec<Self::Inner>) {
-        unreachable!()
+        unreachable!("horizontal predictor is not valid for floating-point sample types")
+    }
+
+    fn floating_point_predict(row: &[Self::Inner], result: &mut Vec<u8>) {
+        fp_predict_f64(row, Self::SAMPLE_FORMAT.len(), result)
     }
 }
 
@@ -180,8 +257,13 @@ impl ColorType for RGB32Float {
     const TIFF_VALUE: PhotometricInterpretation = PhotometricInterpretation::RGB;
     const BITS_PER_SAMPLE: &'static [u16] = &[32, 32, 32];
     const SAMPLE_FORMAT: &'static [SampleFormat] = &[SampleFormat::IEEEFP; 3];
+
     fn horizontal_predict(_: &[Self::Inner], _: &mut Vec<Self::Inner>) {
-        unreachable!()
+        unreachable!("horizontal predictor is not valid for floating-point sample types")
+    }
+
+    fn floating_point_predict(row: &[Self::Inner], result: &mut Vec<u8>) {
+        fp_predict_f32(row, Self::SAMPLE_FORMAT.len(), result)
     }
 }
 
@@ -201,8 +283,13 @@ impl ColorType for RGB64Float {
     const TIFF_VALUE: PhotometricInterpretation = PhotometricInterpretation::RGB;
     const BITS_PER_SAMPLE: &'static [u16] = &[64, 64, 64];
     const SAMPLE_FORMAT: &'static [SampleFormat] = &[SampleFormat::IEEEFP; 3];
+
     fn horizontal_predict(_: &[Self::Inner], _: &mut Vec<Self::Inner>) {
-        unreachable!()
+        unreachable!("horizontal predictor is not valid for floating-point sample types")
+    }
+
+    fn floating_point_predict(row: &[Self::Inner], result: &mut Vec<u8>) {
+        fp_predict_f64(row, Self::SAMPLE_FORMAT.len(), result)
     }
 }
 
@@ -242,8 +329,13 @@ impl ColorType for RGBA32Float {
     const TIFF_VALUE: PhotometricInterpretation = PhotometricInterpretation::RGB;
     const BITS_PER_SAMPLE: &'static [u16] = &[32, 32, 32, 32];
     const SAMPLE_FORMAT: &'static [SampleFormat] = &[SampleFormat::IEEEFP; 4];
+
     fn horizontal_predict(_: &[Self::Inner], _: &mut Vec<Self::Inner>) {
-        unreachable!()
+        unreachable!("horizontal predictor is not valid for floating-point sample types")
+    }
+
+    fn floating_point_predict(row: &[Self::Inner], result: &mut Vec<u8>) {
+        fp_predict_f32(row, Self::SAMPLE_FORMAT.len(), result)
     }
 }
 
@@ -263,8 +355,13 @@ impl ColorType for RGBA64Float {
     const TIFF_VALUE: PhotometricInterpretation = PhotometricInterpretation::RGB;
     const BITS_PER_SAMPLE: &'static [u16] = &[64, 64, 64, 64];
     const SAMPLE_FORMAT: &'static [SampleFormat] = &[SampleFormat::IEEEFP; 4];
+
     fn horizontal_predict(_: &[Self::Inner], _: &mut Vec<Self::Inner>) {
-        unreachable!()
+        unreachable!("horizontal predictor is not valid for floating-point sample types")
+    }
+
+    fn floating_point_predict(row: &[Self::Inner], result: &mut Vec<u8>) {
+        fp_predict_f64(row, Self::SAMPLE_FORMAT.len(), result)
     }
 }
 
@@ -306,7 +403,11 @@ impl ColorType for CMYK32Float {
     const SAMPLE_FORMAT: &'static [SampleFormat] = &[SampleFormat::IEEEFP; 4];
 
     fn horizontal_predict(_: &[Self::Inner], _: &mut Vec<Self::Inner>) {
-        unreachable!()
+        unreachable!("horizontal predictor is not valid for floating-point sample types")
+    }
+
+    fn floating_point_predict(row: &[Self::Inner], result: &mut Vec<u8>) {
+        fp_predict_f32(row, Self::SAMPLE_FORMAT.len(), result)
     }
 }
 
@@ -328,7 +429,11 @@ impl ColorType for CMYK64Float {
     const SAMPLE_FORMAT: &'static [SampleFormat] = &[SampleFormat::IEEEFP; 4];
 
     fn horizontal_predict(_: &[Self::Inner], _: &mut Vec<Self::Inner>) {
-        unreachable!()
+        unreachable!("horizontal predictor is not valid for floating-point sample types")
+    }
+
+    fn floating_point_predict(row: &[Self::Inner], result: &mut Vec<u8>) {
+        fp_predict_f64(row, Self::SAMPLE_FORMAT.len(), result)
     }
 }
 

src/encoder/mod.rs

@@ -33,9 +33,10 @@ use self::writer::*;
 /// Image data can be very unpredictable, and thus very hard to compress. Predictors are simple
 /// passes ran over the image data to prepare it for compression. This is mostly used for LZW
 /// compression, where using [Predictor::Horizontal] we see a 35% improvement in compression
-/// ratio over the unpredicted compression !
+/// ratio over the unpredicted compression!
 ///
-/// [Predictor::FloatingPoint] is currently not supported.
+/// [Predictor::Horizontal] is for integer sample types.
+/// [Predictor::FloatingPoint] is for floating-point sample types (f32, f64).
 pub type Predictor = crate::tags::Predictor;
 #[cfg(feature = "deflate")]
 pub type DeflateLevel = compression::DeflateLevel;
@@ -635,11 +636,14 @@ pub struct ImageEncoder<'a, W: 'a + Write + Seek, C: ColorType, K: TiffKind> {
 impl<'a, W: 'a + Write + Seek, T: ColorType, K: TiffKind> ImageEncoder<'a, W, T, K> {
     fn sanity_check(compression: Compression, predictor: Predictor) -> TiffResult<()> {
         match (predictor, compression, T::SAMPLE_FORMAT[0]) {
+            // Horizontal predictor is not valid for floating-point types
             (Predictor::Horizontal, _, SampleFormat::IEEEFP | SampleFormat::Void) => {
                 Err(TiffError::UsageError(UsageError::PredictorIncompatible))
             }
+            // Floating-point predictor is only valid for IEEEFP sample format
+            (Predictor::FloatingPoint, _, SampleFormat::IEEEFP) => Ok(()),
             (Predictor::FloatingPoint, _, _) => {
-                Err(TiffError::UsageError(UsageError::PredictorUnavailable))
+                Err(TiffError::UsageError(UsageError::PredictorIncompatible))
             }
             _ => Ok(()),
         }
@@ -773,7 +777,14 @@ impl<'a, W: 'a + Write + Seek, T: ColorType, K: TiffKind> ImageEncoder<'a, W, T,
                 }
                 self.encoder.write_data(row_result.as_slice())?
             }
-            _ => unimplemented!(),
+            Predictor::FloatingPoint => {
+                let byte_size = mem::size_of::<T::Inner>();
+                let mut row_result = Vec::with_capacity(value.len() * byte_size);
+                for row in value.chunks_exact(self.row_samples as usize) {
+                    T::floating_point_predict(row, &mut row_result);
+                }
+                self.encoder.write_data(row_result.as_slice())?
+            }
         };
 
         let byte_count = self.encoder.last_written() as usize;

src/tags.rs

@@ -357,7 +357,9 @@ pub enum Predictor(u16) {
     /// This means that instead of having in order `[r1, g1. b1, r2, g2 ...]` you will find
     /// `[r1, g1, b1, r2-r1, g2-g1, b2-b1, r3-r2, g3-g2, ...]`
     Horizontal = 2,
-    /// Not currently supported
+    /// Floating-point predictor for IEEE floating-point sample types.
+    ///
+    /// This predictor byte-shuffles the data and applies byte-level horizontal differencing.
     FloatingPoint = 3,
 }
 }

tests/encode_images.rs

@@ -1,7 +1,7 @@
 extern crate tiff;
 
 use tiff::decoder::{ifd, Decoder, DecodingResult};
-use tiff::encoder::{colortype, Ifd, Ifd8, SRational, TiffEncoder};
+use tiff::encoder::{colortype, Ifd, Ifd8, Predictor, SRational, TiffEncoder};
 use tiff::tags::Tag;
 use tiff::ColorType;
 
@@ -379,6 +379,141 @@ fn test_ycbcr_u8_roundtrip() {
     test_u8_roundtrip::<colortype::YCbCr8>("tiled-jpeg-ycbcr.tif", ColorType::YCbCr(8));
 }
 
+/// Test that encoding with the floating-point predictor works correctly
+/// and can be decoded back to the original data.
+#[test]
+fn test_gray_f32_floating_point_predictor() {
+    // Generate test data: a gradient of f32 values
+    let width = 32u32;
+    let height = 32u32;
+    let mut image_data: Vec<f32> = Vec::with_capacity((width * height) as usize);
+    for y in 0..height {
+        for x in 0..width {
+            // Create a gradient pattern
+            image_data.push((x as f32 + y as f32 * 0.1) / 100.0);
+        }
+    }
+
+    let mut file = Cursor::new(Vec::new());
+    {
+        let mut tiff = TiffEncoder::new(&mut file)
+            .unwrap()
+            .with_predictor(Predictor::FloatingPoint);
+        tiff.write_image::<colortype::Gray32Float>(width, height, &image_data)
+            .unwrap();
+    }
+
+    file.seek(SeekFrom::Start(0)).unwrap();
+    {
+        let mut decoder = Decoder::new(&mut file).unwrap();
+        assert_eq!(decoder.colortype().unwrap(), ColorType::Gray(32));
+
+        let mut buffer = DecodingResult::F32(vec![]);
+        decoder.read_image_to_buffer(&mut buffer).unwrap();
+        if let DecodingResult::F32(img_res) = buffer {
+            assert_eq!(image_data.len(), img_res.len());
+            for (orig, decoded) in image_data.iter().zip(img_res.iter()) {
+                assert_eq!(
+                    *orig, *decoded,
+                    "Mismatch at value: orig={}, decoded={}",
+                    orig, decoded
+                );
+            }
+        } else {
+            panic!("Wrong data type");
+        }
+    }
+}
+
+/// Test floating-point predictor with RGB f32 data
+#[test]
+fn test_rgb_f32_floating_point_predictor() {
+    let width = 16u32;
+    let height = 16u32;
+    let mut image_data: Vec<f32> = Vec::with_capacity((width * height * 3) as usize);
+    for y in 0..height {
+        for x in 0..width {
+            // RGB pattern
+            image_data.push(x as f32 / width as f32); // R
+            image_data.push(y as f32 / height as f32); // G
+            image_data.push((x + y) as f32 / (width + height) as f32); // B
+        }
+    }
+
+    let mut file = Cursor::new(Vec::new());
+    {
+        let mut tiff = TiffEncoder::new(&mut file)
+            .unwrap()
+            .with_predictor(Predictor::FloatingPoint);
+        tiff.write_image::<colortype::RGB32Float>(width, height, &image_data)
+            .unwrap();
+    }
+
+    file.seek(SeekFrom::Start(0)).unwrap();
+    {
+        let mut decoder = Decoder::new(&mut file).unwrap();
+        assert_eq!(decoder.colortype().unwrap(), ColorType::RGB(32));
+
+        let mut buffer = DecodingResult::F32(vec![]);
+        decoder.read_image_to_buffer(&mut buffer).unwrap();
+        if let DecodingResult::F32(img_res) = buffer {
+            assert_eq!(image_data.len(), img_res.len());
+            for (i, (orig, decoded)) in image_data.iter().zip(img_res.iter()).enumerate() {
+                assert_eq!(
+                    *orig, *decoded,
+                    "Mismatch at index {}: orig={}, decoded={}",
+                    i, orig, decoded
+                );
+            }
+        } else {
+            panic!("Wrong data type");
+        }
+    }
+}
+
+/// Test floating-point predictor with f64 data
+#[test]
+fn test_gray_f64_floating_point_predictor() {
+    let width = 16u32;
+    let height = 16u32;
+    let mut image_data: Vec<f64> = Vec::with_capacity((width * height) as usize);
+    for y in 0..height {
+        for x in 0..width {
+            image_data.push((x as f64 + y as f64 * 0.1) / 100.0);
+        }
+    }
+
+    let mut file = Cursor::new(Vec::new());
+    {
+        let mut tiff = TiffEncoder::new(&mut file)
+            .unwrap()
+            .with_predictor(Predictor::FloatingPoint);
+        tiff.write_image::<colortype::Gray64Float>(width, height, &image_data)
+            .unwrap();
+    }
+
+    file.seek(SeekFrom::Start(0)).unwrap();
+    {
+        let mut decoder = Decoder::new(&mut file).unwrap();
+        assert_eq!(decoder.colortype().unwrap(), ColorType::Gray(64));
+
+        let mut buffer = DecodingResult::F64(vec![]);
+        decoder.read_image_to_buffer(&mut buffer).unwrap();
+        if let DecodingResult::F64(img_res) = buffer {
+            assert_eq!(image_data.len(), img_res.len());
+            for (orig, decoded) in image_data.iter().zip(img_res.iter()) {
+                assert_eq!(
+                    *orig, *decoded,
+                    "Mismatch: orig={}, decoded={}",
+                    orig, decoded
+                );
+            }
+        } else {
+            panic!("Wrong data type");
+        }
+    }
+}
+
 trait AssertDecode {
     fn assert_tag_u32(&mut self, tag: u16) -> u32;
     fn assert_tag_u32_vec(&mut self, tag: u16) -> Vec<u32>;