diff --git a/src/png/mod.rs b/src/png/mod.rs
index 455b96cc..cd2d605b 100644
--- a/src/png/mod.rs
+++ b/src/png/mod.rs
@@ -341,17 +341,9 @@ impl PngImage {
         let mut prev_line = Vec::new();
         let mut prev_pass: Option<u8> = None;
         let mut f_buf = Vec::new();
-        let mut best_possible = 0;
         for line in self.scan_lines(false) {
             if prev_pass != line.pass || line.data.len() != prev_line.len() {
                 prev_line = vec![0; line.data.len()];
-                // Calculate the best possible result for this pass
-                best_possible = match filter {
-                    RowFilter::Entropy => ilog2i(line.data.len() as u32 + 1) as i32,
-                    RowFilter::Bigrams => 1,
-                    RowFilter::BigEnt => ilog2i(line.data.len() as u32) as i32,
-                    _ => 0,
-                }
             }
             // Alpha optimisation may alter the line data, so we need a mutable copy of it
             let mut line_data = line.data.to_vec();
@@ -368,6 +360,15 @@ impl PngImage {
                 prev_line = line_data;
             } else {
                 // Heuristic filter selection strategies
+
+                if line_data.iter().all(|&x| x == 0) {
+                    // Assume None if the line is all zeros
+                    filtered.push(RowFilter::None as u8);
+                    filtered.extend_from_slice(&line_data);
+                    prev_line = line_data;
+                    continue;
+                }
+
                 let mut best_line = Vec::new();
                 let mut best_line_raw = Vec::new();
                 // Avoid vertical filtering on first line of each interlacing pass
@@ -380,21 +381,17 @@ impl PngImage {
                     RowFilter::MinSum => {
                         // MSAD algorithm mentioned in libpng reference docs
                         // http://www.libpng.org/pub/png/book/chapter09.html
-                        let mut best_size = i32::MAX;
+                        let mut best_size = usize::MAX;
                         for f in try_filters {
                             f.filter_line(bpp, &mut line_data, &prev_line, &mut f_buf, alpha_bytes);
                             let size = f_buf.iter().fold(0, |acc, &x| {
                                 let signed = x as i8;
-                                acc + signed.unsigned_abs() as i32
+                                acc + signed.unsigned_abs() as usize
                             });
                             if size < best_size {
                                 best_size = size;
                                 std::mem::swap(&mut best_line, &mut f_buf);
                                 best_line_raw.clone_from(&line_data);
-                                if size == best_possible {
-                                    // Best possible result
-                                    break;
-                                }
                             }
                         }
                     }
@@ -418,17 +415,13 @@ impl PngImage {
                                 best_size = size;
                                 std::mem::swap(&mut best_line, &mut f_buf);
                                 best_line_raw.clone_from(&line_data);
-                                if size == best_possible {
-                                    // Best possible result
-                                    break;
-                                }
                             }
                         }
                     }
                     RowFilter::Bigrams => {
                         // Count distinct bigrams, from pngwolf
                         // https://bjoern.hoehrmann.de/pngwolf/
-                        let mut best_size = i32::MAX;
+                        let mut best_size = usize::MAX;
                         for f in try_filters {
                             f.filter_line(bpp, &mut line_data, &prev_line, &mut f_buf, alpha_bytes);
                             let mut set = bitarr![0; 0x10000];
@@ -436,15 +429,11 @@ impl PngImage {
                                 let bigram = (pair[0] as usize) << 8 | pair[1] as usize;
                                 set.set(bigram, true);
                             }
-                            let size = set.count_ones() as i32;
+                            let size = set.count_ones();
                             if size < best_size {
                                 best_size = size;
                                 std::mem::swap(&mut best_line, &mut f_buf);
                                 best_line_raw.clone_from(&line_data);
-                                if size == best_possible {
-                                    // Best possible result
-                                    break;
-                                }
                             }
                         }
                     }
@@ -465,10 +454,6 @@ impl PngImage {
                                 best_size = size;
                                 std::mem::swap(&mut best_line, &mut f_buf);
                                 best_line_raw.clone_from(&line_data);
-                                if size == best_possible {
-                                    // Best possible result
-                                    break;
-                                }
                             }
                         }
                     }