Add basic mimpmap downscaling support

This is small patch to fix quality issues when scaling images down.
It'll create a number of mipmap/half-scaled versions of a given source
as required and use the next largest version as the source for bilinear
or bicubic scaling.

This uses the same downsampling functions as Skia, but similarly doesn't
run SIMD versions of these. As they are integer-based, these should be
reasonably quick.

src/lib.rs

@@ -46,6 +46,7 @@ mod geom;
 mod line_clipper;
 mod mask;
 mod math;
+mod mipmap;
 mod path64;
 mod path_geometry;
 mod pipeline;
@@ -60,6 +61,7 @@ pub use blend_mode::BlendMode;
 pub use color::{Color, ColorSpace, ColorU8, PremultipliedColor, PremultipliedColorU8};
 pub use color::{ALPHA_OPAQUE, ALPHA_TRANSPARENT, ALPHA_U8_OPAQUE, ALPHA_U8_TRANSPARENT};
 pub use mask::{Mask, MaskType};
+pub use mipmap::Mipmaps;
 pub use painter::{FillRule, Paint};
 pub use pixmap::{Pixmap, PixmapMut, PixmapRef, BYTES_PER_PIXEL};
 pub use shaders::{FilterQuality, GradientStop, PixmapPaint, SpreadMode};

src/mipmap.rs

@@ -0,0 +1,297 @@
+// Copyright 2006 The Android Open Source Project
+// Copyright 2020 Yevhenii Reizner
+// Copyright 2024 Jeremy James
+//
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+use alloc::vec::Vec;
+
+use crate::pixmap::{Pixmap, PixmapRef};
+use crate::PremultipliedColorU8;
+
+#[cfg(all(not(feature = "std"), feature = "no-std-float"))]
+use tiny_skia_path::NoStdFloat;
+
+/// Mipmaps are used to scaling down source images quickly to be used instead
+/// of a pixmap as source for bilinear or bicubic scaling
+///
+/// These are created from a `PixmapRef` as a base source which can be fetched
+/// using level `0`
+///
+#[derive(Debug)]
+pub struct Mipmaps<'a> {
+    levels: Vec<Pixmap>,
+    base_pixmap: PixmapRef<'a>,
+}
+
+impl<'a> Mipmaps<'a> {
+    /// Allocates a new set of mipmaps from a base pixmap
+    pub fn new(p: PixmapRef<'a>) -> Self {
+        Mipmaps {
+            levels: Vec::new(),
+            base_pixmap: p,
+        }
+    }
+
+    /// Fetch a mipmap to be used - or base pixmap if zero is given
+    pub fn get(&self, level: usize) -> PixmapRef {
+        return if level > 0 {
+            self.levels.get(level - 1).unwrap().as_ref()
+        } else {
+            self.base_pixmap
+        };
+    }
+
+    /// Ensure this many levels of mipmap are available, returning
+    /// an index to be used with get()
+    pub fn build(&mut self, required_levels: usize) -> usize {
+        let mut src_level = self.levels.len();
+        let mut src_pixmap = self.get(src_level);
+        let mut level_width = src_pixmap.width();
+        let mut level_height = src_pixmap.height();
+
+        while src_level < required_levels {
+            level_width = (level_width as f32 / 2.0).floor() as u32;
+            level_height = (level_height as f32 / 2.0).floor() as u32;
+
+            // Scale image down
+            let mut dst_pixmap = Pixmap::new(level_width, level_height).unwrap();
+            let dst_width = dst_pixmap.width() as usize;
+            let dst_height = dst_pixmap.height() as usize;
+            let dst_pixels = dst_pixmap.pixels_mut();
+
+            let src_pixels = src_pixmap.pixels();
+            let src_width = src_pixmap.width() as usize;
+            let src_height = src_pixmap.height() as usize;
+
+            //  To produce each mip level, we need to filter down by 1/2 (e.g. 100x100 -> 50,50)
+            //  If the starting dimension is odd, we floor the size of the lower level (e.g. 101 -> 50)
+            //  In those (odd) cases, we use a triangle filter, with 1-pixel overlap between samplings,
+            //  else for even cases, we just use a 2x box filter.
+            //
+            //  This produces 4 possible isotropic filters: 2x2 2x3 3x2 3x3 where WxH indicates the number of
+            //  src pixels we need to sample in each dimension to produce 1 dst pixel.
+            let mut downsample: fn(
+                &[PremultipliedColorU8],
+                usize,
+                usize,
+                &mut [PremultipliedColorU8],
+                usize,
+                usize,
+            ) = downsample_2_2;
+
+            if src_height & 1 == 1 {
+                if src_width & 1 == 1 {
+                    downsample = downsample_3_3;
+                } else {
+                    downsample = downsample_2_3;
+                }
+            } else {
+                if src_width & 1 == 1 {
+                    downsample = downsample_3_2;
+                }
+            }
+
+            let mut src_y = 0;
+            for dst_y in 0..dst_height {
+                downsample(src_pixels, src_y, src_width, dst_pixels, dst_y, dst_width);
+                src_y += 2;
+            }
+
+            self.levels.push(dst_pixmap);
+            src_pixmap = self.levels.get(src_level).unwrap().as_ref();
+            src_level += 1;
+        }
+
+        src_level
+    }
+}
+
+/// Determine how many Mipmap levels will be needed for a given source and
+/// a given (approximate) scaling being applied to the source
+///
+/// Return the number of levels, and a pre-scale that should be applied to
+/// a transform that will 'correct' it to the right size of source
+///
+/// Note that this is different from Skia since only required levels will
+/// be generated
+pub fn compute_required_levels(
+    base_pixmap: PixmapRef,
+    scale_x: f32,
+    scale_y: f32,
+) -> (usize, f32, f32) {
+    let mut required_levels: usize = 0;
+    let mut level_width = base_pixmap.width();
+    let mut level_height = base_pixmap.height();
+    let mut prescale_x: f32 = 1.0;
+    let mut prescale_y: f32 = 1.0;
+
+    // Keep generating levels whilst required scale is
+    // smaller than half of previous level size
+    while scale_x * prescale_x < 0.5
+        && level_width > 1
+        && scale_y * prescale_y < 0.5
+        && level_height > 1
+    {
+        required_levels += 1;
+        level_width = (level_width as f32 / 2.0).floor() as u32;
+        level_height = (level_height as f32 / 2.0).floor() as u32;
+        prescale_x = base_pixmap.width() as f32 / level_width as f32;
+        prescale_y = base_pixmap.height() as f32 / level_height as f32;
+    }
+
+    (required_levels, prescale_x, prescale_y)
+}
+
+// Downsamples to match Skia (non-SIMD)
+macro_rules! sum_channel {
+    ($channel:ident, $($p:ident),+ ) => {
+        0u16 $( + $p.$channel() as u16 )+
+    };
+}
+
+fn downsample_2_2(
+    src_pixels: &[PremultipliedColorU8],
+    src_y: usize,
+    src_width: usize,
+    dst_pixels: &mut [PremultipliedColorU8],
+    dst_y: usize,
+    dst_width: usize,
+) {
+    let mut src_x = 0;
+    for dst_x in 0..dst_width {
+        let p1 = src_pixels[src_y * src_width + src_x];
+        let p2 = src_pixels[src_y * src_width + src_x + 1];
+        let p3 = src_pixels[(src_y + 1) * src_width + src_x];
+        let p4 = src_pixels[(src_y + 1) * src_width + src_x + 1];
+
+        let r = (sum_channel!(red, p1, p2, p3, p4) >> 2) as u8;
+        let g = (sum_channel!(green, p1, p2, p3, p4) >> 2) as u8;
+        let b = (sum_channel!(blue, p1, p2, p3, p4) >> 2) as u8;
+        let a = (sum_channel!(alpha, p1, p2, p3, p4) >> 2) as u8;
+        dst_pixels[dst_y * dst_width + dst_x] =
+            PremultipliedColorU8::from_rgba_unchecked(r, g, b, a);
+
+        src_x += 2;
+    }
+}
+
+fn downsample_2_3(
+    src_pixels: &[PremultipliedColorU8],
+    src_y: usize,
+    src_width: usize,
+    dst_pixels: &mut [PremultipliedColorU8],
+    dst_y: usize,
+    dst_width: usize,
+) {
+    // Given pixels:
+    // a0 b0 c0 d0 ...
+    // a1 b1 c1 d1 ...
+    // a2 b2 c2 d2 ...
+    // We want:
+    // (a0 + 2*a1 + a2 + b0 + 2*b1 + b2) / 8
+    // (c0 + 2*c1 + c2 + d0 + 2*d1 + d2) / 8
+    // ...
+
+    let mut src_x = 0;
+    for dst_x in 0..dst_width {
+        let p1 = src_pixels[src_y * src_width + src_x];
+        let p2 = src_pixels[src_y * src_width + src_x + 1];
+        let p3 = src_pixels[(src_y + 1) * src_width + src_x];
+        let p4 = src_pixels[(src_y + 1) * src_width + src_x + 1];
+        let p5 = src_pixels[(src_y + 2) * src_width + src_x];
+        let p6 = src_pixels[(src_y + 2) * src_width + src_x + 1];
+
+        let r = (sum_channel!(red, p1, p3, p3, p5, p2, p4, p4, p6) >> 3) as u8;
+        let g = (sum_channel!(green, p1, p3, p3, p5, p2, p4, p4, p6) >> 3) as u8;
+        let b = (sum_channel!(blue, p1, p3, p3, p5, p2, p4, p4, p6) >> 3) as u8;
+        let a = (sum_channel!(alpha, p1, p3, p3, p5, p2, p4, p4, p6) >> 3) as u8;
+        dst_pixels[dst_y * dst_width + dst_x] =
+            PremultipliedColorU8::from_rgba_unchecked(r, g, b, a);
+
+        src_x += 2;
+    }
+}
+
+fn downsample_3_2(
+    src_pixels: &[PremultipliedColorU8],
+    src_y: usize,
+    src_width: usize,
+    dst_pixels: &mut [PremultipliedColorU8],
+    dst_y: usize,
+    dst_width: usize,
+) {
+    // Given pixels:
+    // a0 b0 c0 d0 e0 ...
+    // a1 b1 c1 d1 e1 ...
+    // We want:
+    // (a0 + 2*b0 + c0 + a1 + 2*b1 + c1) / 8
+    // (c0 + 2*d0 + e0 + c1 + 2*d1 + e1) / 8
+    // ...
+
+    let mut src_x = 0;
+    for dst_x in 0..dst_width {
+        let p1 = src_pixels[src_y * src_width + src_x];
+        let p2 = src_pixels[src_y * src_width + src_x + 1];
+        let p3 = src_pixels[src_y * src_width + src_x + 2];
+        let p4 = src_pixels[(src_y + 1) * src_width + src_x];
+        let p5 = src_pixels[(src_y + 1) * src_width + src_x + 1];
+        let p6 = src_pixels[(src_y + 1) * src_width + src_x + 2];
+
+        let r = (sum_channel!(red, p1, p2, p2, p3, p4, p5, p5, p6) >> 3) as u8;
+        let g = (sum_channel!(green, p1, p2, p2, p3, p4, p5, p5, p6) >> 3) as u8;
+        let b = (sum_channel!(blue, p1, p2, p2, p3, p4, p5, p5, p6) >> 3) as u8;
+        let a = (sum_channel!(alpha, p1, p2, p2, p3, p4, p5, p5, p6) >> 3) as u8;
+        dst_pixels[dst_y * dst_width + dst_x] =
+            PremultipliedColorU8::from_rgba_unchecked(r, g, b, a);
+
+        src_x += 2;
+    }
+}
+
+fn downsample_3_3(
+    src_pixels: &[PremultipliedColorU8],
+    src_y: usize,
+    src_width: usize,
+    dst_pixels: &mut [PremultipliedColorU8],
+    dst_y: usize,
+    dst_width: usize,
+) {
+    // Given pixels:
+    // a0 b0 c0 d0 e0 ...
+    // a1 b1 c1 d1 e1 ...
+    // a2 b2 c2 d2 e2 ...
+    // We want:
+    // (a0 + 2*b0 + c0 + 2*a1 + 4*b1 + 2*c1 + a2 + 2*b2 + c2) / 16
+    // (c0 + 2*d0 + e0 + 2*c1 + 4*d1 + 2*e1 + c2 + 2*d2 + e2) / 16
+    // ...
+
+    let mut src_x = 0;
+    for dst_x in 0..dst_width {
+        let p1 = src_pixels[src_y * src_width + src_x];
+        let p2 = src_pixels[src_y * src_width + src_x + 1];
+        let p3 = src_pixels[src_y * src_width + src_x + 2];
+        let p4 = src_pixels[(src_y + 1) * src_width + src_x];
+        let p5 = src_pixels[(src_y + 1) * src_width + src_x + 1];
+        let p6 = src_pixels[(src_y + 1) * src_width + src_x + 2];
+        let p7 = src_pixels[(src_y + 2) * src_width + src_x];
+        let p8 = src_pixels[(src_y + 2) * src_width + src_x + 1];
+        let p9 = src_pixels[(src_y + 2) * src_width + src_x + 2];
+
+        let r = (sum_channel!(red, p1, p2, p2, p3, p4, p4, p5, p5, p5, p5, p6, p6, p7, p8, p8, p9)
+            >> 4) as u8;
+        let g =
+            (sum_channel!(green, p1, p2, p2, p3, p4, p4, p5, p5, p5, p5, p6, p6, p7, p8, p8, p9)
+                >> 4) as u8;
+        let b = (sum_channel!(blue, p1, p2, p2, p3, p4, p4, p5, p5, p5, p5, p6, p6, p7, p8, p8, p9)
+            >> 4) as u8;
+        let a =
+            (sum_channel!(alpha, p1, p2, p2, p3, p4, p4, p5, p5, p5, p5, p6, p6, p7, p8, p8, p9)
+                >> 4) as u8;
+        dst_pixels[dst_y * dst_width + dst_x] =
+            PremultipliedColorU8::from_rgba_unchecked(r, g, b, a);
+
+        src_x += 2;
+    }
+}