Added Gaussian filter for convolution algorythm.

CHANGELOG.md

@@ -1,5 +1,15 @@
 ## [Unreleased] - ReleaseDate
 
+### Added
+
+- Added Gaussian filter for convolution algorythm.
+
+### Fixed
+
+- Fixed dividing image by alpha channel.
+
+### Changed
+
 A lot of breaking changes have been done in this release:
 
 - Structures `ImageView` and `ImageViewMut` have been removed. They always
@@ -32,10 +42,6 @@ A lot of breaking changes have been done in this release:
   type from the `image` crate. It allows you to use `DynamicImage` instances
   as arguments for `Resize::resize()` method.
 
-### Fixed
-
-- Fixed dividing image by alpha channel.
-
 ## [3.0.4] - 2024-02-15
 
 ### Fixed

src/convolution/filters.rs

@@ -107,6 +107,10 @@ pub enum FilterType {
     ///
     /// Minimal kernel size 4x4 px.
     Mitchell,
+    /// Gaussian filter with a standard deviation of 0.5.
+    ///
+    /// Minimal kernel size 6x6 px.
+    Gaussian,
     /// Lanczos3 filter calculate the output pixel value using a high-quality
     /// Lanczos filter (a truncated sinc) on all pixels that may contribute
     /// to the output value.
@@ -159,6 +163,7 @@ pub(crate) fn get_filter_func(filter_type: FilterType) -> (FilterFn, f64) {
         FilterType::Hamming => (hamming_filter, 1.0),
         FilterType::CatmullRom => (catmul_filter, 2.0),
         FilterType::Mitchell => (mitchell_filter, 2.0),
+        FilterType::Gaussian => (gaussian_filter, 3.0),
         FilterType::Lanczos3 => (lanczos_filter, 3.0),
         FilterType::Custom(custom) => (custom.func, custom.support),
     }
@@ -225,6 +230,22 @@ fn mitchell_filter(mut x: f64) -> f64 {
     }
 }
 
+/// The Gaussian Function.
+/// `r` is the standard deviation.
+fn gaussian(x: f64, r: f64) -> f64 {
+    ((2.0 * PI).sqrt() * r).recip() * (-x.powi(2) / (2.0 * r.powi(2))).exp()
+}
+
+/// Calculate the gaussian function with a
+/// standard deviation of 0.5.
+fn gaussian_filter(x: f64) -> f64 {
+    if (-3.0..3.0).contains(&x) {
+        gaussian(x, 0.5)
+    } else {
+        0.0
+    }
+}
+
 #[inline]
 fn sinc_filter(mut x: f64) -> f64 {
     if x == 0.0 {

src/mul_div.rs

@@ -69,7 +69,7 @@ impl MulDiv {
         #[cfg(feature = "only_u8x4")]
         match src_pixel_type {
             PixelType::U8x4 => self.multiply::<U8x4>(src_image, dst_image),
-            _ => Err(MulDivImagesError::SrcImageError(
+            _ => Err(MulDivImagesError::ImageError(
                 ImageError::UnsupportedPixelType,
             )),
         }

tests/resize_tests.rs

@@ -307,6 +307,7 @@ where
                 FilterType::Hamming => "hamming",
                 FilterType::Mitchell => "mitchell",
                 FilterType::CatmullRom => "catmullrom",
+                FilterType::Gaussian => "gaussian",
                 FilterType::Lanczos3 => "lanczos3",
                 _ => "unknown",
             },
@@ -915,6 +916,12 @@ mod u8x4 {
             cpu_extensions_vec.push(CpuExtensions::Simd128);
         }
         for cpu_extensions in cpu_extensions_vec {
+            P::downscale_test(
+                ResizeAlg::Convolution(FilterType::Gaussian),
+                cpu_extensions,
+                [2939881, 2946811, 2884299, 6122867],
+            );
+
             P::downscale_test(
                 ResizeAlg::Convolution(FilterType::Lanczos3),
                 cpu_extensions,