From 15a9b4a3e79f50a758f3f0a4f7e9aaf5ae649a21 Mon Sep 17 00:00:00 2001
From: Andrew <andrew@digerati.co.nz>
Date: Sun, 20 Nov 2022 16:27:33 +1300
Subject: [PATCH] Fast deinterlace for 8-bit+

---
 src/interlace.rs | 105 +++++++++++++++++++++++++++++++++--------------
 1 file changed, 75 insertions(+), 30 deletions(-)

diff --git a/src/interlace.rs b/src/interlace.rs
index 33332724..4db6b6f8 100644
--- a/src/interlace.rs
+++ b/src/interlace.rs
@@ -95,6 +95,23 @@ pub fn interlace_image(png: &PngImage) -> PngImage {
 }
 
 pub fn deinterlace_image(png: &PngImage) -> PngImage {
+    PngImage {
+        data: match png.ihdr.bpp() {
+            8.. => deinterlace_bytes(png),
+            _ => deinterlace_bits(png),
+        },
+        ihdr: IhdrData {
+            interlaced: 0,
+            ..png.ihdr
+        },
+        aux_headers: png.aux_headers.clone(),
+        palette: png.palette.clone(),
+        transparency_pixel: png.transparency_pixel.clone(),
+    }
+}
+
+/// Deinterlace by bits, for images with less than 8bpp
+fn deinterlace_bits(png: &PngImage) -> Vec<u8> {
     let bits_per_pixel = png.ihdr.bpp();
     let bits_per_line = 8 + bits_per_pixel as usize * png.ihdr.width as usize;
     // Initialize each output line with a starting filter byte of 0
@@ -125,29 +142,9 @@ pub fn deinterlace_image(png: &PngImage) -> PngImage {
         // Calculate the next line and move to next pass if necessary
         current_y += pass_constants.y_step as usize;
         if current_y >= png.ihdr.height as usize {
-            if current_pass == 7 {
+            if !increment_pass(&mut current_pass, png.ihdr) {
                 break;
             }
-            current_pass += 1;
-            if current_pass == 2 && png.ihdr.width <= 4 {
-                current_pass += 1;
-            }
-            if current_pass == 3 && png.ihdr.height <= 4 {
-                current_pass += 1;
-            }
-            if current_pass == 4 && png.ihdr.width <= 2 {
-                current_pass += 1;
-            }
-            if current_pass == 5 && png.ihdr.height <= 2 {
-                current_pass += 1;
-            }
-            if current_pass == 6 && png.ihdr.width == 1 {
-                current_pass += 1;
-            }
-            if current_pass == 7 && png.ihdr.height == 1 {
-                break;
-            }
-
             pass_constants = interlaced_constants(current_pass);
             current_y = pass_constants.y_shift as usize;
         }
@@ -159,16 +156,64 @@ pub fn deinterlace_image(png: &PngImage) -> PngImage {
         }
         output.extend_from_slice(line.as_raw_slice());
     }
-    PngImage {
-        data: output,
-        ihdr: IhdrData {
-            interlaced: 0,
-            ..png.ihdr
-        },
-        aux_headers: png.aux_headers.clone(),
-        palette: png.palette.clone(),
-        transparency_pixel: png.transparency_pixel.clone(),
+    output
+}
+
+/// Deinterlace by bytes, for images with at least 8bpp
+fn deinterlace_bytes(png: &PngImage) -> Vec<u8> {
+    let bytes_per_pixel = png.ihdr.bpp() / 8;
+    let bytes_per_line = 1 + bytes_per_pixel as usize * png.ihdr.width as usize;
+    // Initialize each output line with a starting filter byte of 0
+    // as well as some blank data
+    let mut lines: Vec<Vec<u8>> = vec![vec![0; bytes_per_line]; png.ihdr.height as usize];
+    let mut current_pass = 1;
+    let mut pass_constants = interlaced_constants(current_pass);
+    let mut current_y: usize = pass_constants.y_shift as usize;
+    for line in png.scan_lines() {
+        for (i, byte) in line.data.iter().enumerate() {
+            let current_x: usize = pass_constants.x_shift as usize
+                + (i / bytes_per_pixel as usize) * pass_constants.x_step as usize;
+            // Copy this byte into the output line, offset by 1 because of filter byte
+            let index = 1 + (i % bytes_per_pixel as usize) + current_x * bytes_per_pixel as usize;
+            lines[current_y][index] = *byte;
+        }
+        // Calculate the next line and move to next pass if necessary
+        current_y += pass_constants.y_step as usize;
+        if current_y >= png.ihdr.height as usize {
+            if !increment_pass(&mut current_pass, png.ihdr) {
+                break;
+            }
+            pass_constants = interlaced_constants(current_pass);
+            current_y = pass_constants.y_shift as usize;
+        }
     }
+    lines.concat()
+}
+
+fn increment_pass(current_pass: &mut u8, ihdr: IhdrData) -> bool {
+    if *current_pass == 7 {
+        return false;
+    }
+    *current_pass += 1;
+    if *current_pass == 2 && ihdr.width <= 4 {
+        *current_pass += 1;
+    }
+    if *current_pass == 3 && ihdr.height <= 4 {
+        *current_pass += 1;
+    }
+    if *current_pass == 4 && ihdr.width <= 2 {
+        *current_pass += 1;
+    }
+    if *current_pass == 5 && ihdr.height <= 2 {
+        *current_pass += 1;
+    }
+    if *current_pass == 6 && ihdr.width == 1 {
+        *current_pass += 1;
+    }
+    if *current_pass == 7 && ihdr.height == 1 {
+        return false;
+    }
+    true
 }
 
 #[derive(Clone, Copy)]