diff --git a/src/filters.rs b/src/filters.rs
index 5e18fa47..54b9bb63 100644
--- a/src/filters.rs
+++ b/src/filters.rs
@@ -62,30 +62,31 @@ pub fn filter_line(filter: u8, bpp: usize, data: &[u8], last_line: &[u8], buf: &
     }
 }
 
-pub fn unfilter_line(filter: u8, bpp: usize, data: &[u8], last_line: &[u8]) -> Vec<u8> {
-    let mut unfiltered = Vec::with_capacity(data.len());
+pub fn unfilter_line(filter: u8, bpp: usize, data: &[u8], last_line: &[u8], buf: &mut Vec<u8>) {
+    assert_eq!(buf.len(), 0);
+    buf.reserve(data.len());
     match filter {
         0 => {
-            unfiltered.extend_from_slice(data);
+            buf.extend_from_slice(data);
         }
         1 => {
             for (i, byte) in data.iter().enumerate() {
                 match i.checked_sub(bpp) {
                     Some(x) => {
-                        let b = unfiltered[x];
-                        unfiltered.push(byte.wrapping_add(b));
+                        let b = buf[x];
+                        buf.push(byte.wrapping_add(b));
                     }
                     None => {
-                        unfiltered.push(*byte);
+                        buf.push(*byte);
                     }
                 };
             }
         }
         2 => {
             if last_line.is_empty() {
-                unfiltered.extend_from_slice(data);
+                buf.extend_from_slice(data);
             } else {
-                unfiltered.extend(
+                buf.extend(
                     data.iter()
                         .zip(last_line.iter())
                         .map(|(cur, last)| cur.wrapping_add(*last)),
@@ -97,23 +98,23 @@ pub fn unfilter_line(filter: u8, bpp: usize, data: &[u8], last_line: &[u8]) -> V
                 if last_line.is_empty() {
                     match i.checked_sub(bpp) {
                         Some(x) => {
-                            let b = unfiltered[x];
-                            unfiltered.push(byte.wrapping_add(b >> 1));
+                            let b = buf[x];
+                            buf.push(byte.wrapping_add(b >> 1));
                         }
                         None => {
-                            unfiltered.push(*byte);
+                            buf.push(*byte);
                         }
                     };
                 } else {
                     match i.checked_sub(bpp) {
                         Some(x) => {
-                            let b = unfiltered[x];
-                            unfiltered.push(byte.wrapping_add(
+                            let b = buf[x];
+                            buf.push(byte.wrapping_add(
                                 ((u16::from(b) + u16::from(last_line[i])) >> 1) as u8,
                             ));
                         }
                         None => {
-                            unfiltered.push(byte.wrapping_add(last_line[i] >> 1));
+                            buf.push(byte.wrapping_add(last_line[i] >> 1));
                         }
                     };
                 };
@@ -124,25 +125,25 @@ pub fn unfilter_line(filter: u8, bpp: usize, data: &[u8], last_line: &[u8]) -> V
                 if last_line.is_empty() {
                     match i.checked_sub(bpp) {
                         Some(x) => {
-                            let b = unfiltered[x];
-                            unfiltered.push(byte.wrapping_add(b));
+                            let b = buf[x];
+                            buf.push(byte.wrapping_add(b));
                         }
                         None => {
-                            unfiltered.push(*byte);
+                            buf.push(*byte);
                         }
                     };
                 } else {
                     match i.checked_sub(bpp) {
                         Some(x) => {
-                            let b = unfiltered[x];
-                            unfiltered.push(byte.wrapping_add(paeth_predictor(
+                            let b = buf[x];
+                            buf.push(byte.wrapping_add(paeth_predictor(
                                 b,
                                 last_line[i],
                                 last_line[x],
                             )));
                         }
                         None => {
-                            unfiltered.push(byte.wrapping_add(last_line[i]));
+                            buf.push(byte.wrapping_add(last_line[i]));
                         }
                     };
                 };
@@ -150,7 +151,6 @@ pub fn unfilter_line(filter: u8, bpp: usize, data: &[u8], last_line: &[u8]) -> V
         }
         _ => unreachable!(),
     }
-    unfiltered
 }
 
 fn paeth_predictor(a: u8, b: u8, c: u8) -> u8 {
diff --git a/src/png/mod.rs b/src/png/mod.rs
index 33799f92..0077551b 100644
--- a/src/png/mod.rs
+++ b/src/png/mod.rs
@@ -282,17 +282,19 @@ impl PngImage {
         let bpp = ((self.ihdr.bit_depth.as_u8() * self.channels_per_pixel() + 7) / 8) as usize;
         let mut last_line: Vec<u8> = Vec::new();
         let mut last_pass = 1;
+        let mut unfiltered_buf = Vec::new();
         for line in self.scan_lines() {
             if let Some(pass) = line.pass {
                 if pass != last_pass {
-                    last_line = Vec::new();
+                    last_line.clear();
                     last_pass = pass;
                 }
             }
-            let unfiltered_line = unfilter_line(line.filter, bpp, &line.data, &last_line);
+            unfilter_line(line.filter, bpp, &line.data, &last_line, &mut unfiltered_buf);
             unfiltered.push(0);
-            unfiltered.extend_from_slice(&unfiltered_line);
-            last_line = unfiltered_line;
+            unfiltered.extend_from_slice(&unfiltered_buf);
+            std::mem::swap(&mut last_line, &mut unfiltered_buf);
+            unfiltered_buf.clear();
         }
         unfiltered
     }