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Reduce alloc in filtering/unfiltering by reusing buffers (#191)
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@SethDusek @shssoichiro
SethDusek authored and shssoichiro committed Sep 4, 2019
1 parent 239ca81 commit 68b763b
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Showing 2 changed files with 60 additions and 51 deletions.
65 changes: 32 additions & 33 deletions src/filters.rs
View file
Original file line number Diff line number Diff line change
@@ -1,12 +1,12 @@
pub fn filter_line(filter: u8, bpp: usize, data: &[u8], last_line: &[u8]) -> Vec<u8> {
let mut filtered = Vec::with_capacity(data.len());
pub fn filter_line(filter: u8, bpp: usize, data: &[u8], last_line: &[u8], buf: &mut Vec<u8>) {
buf.reserve(data.len());
match filter {
0 => {
filtered.extend_from_slice(data);
buf.extend_from_slice(data);
}
1 => {
filtered.extend_from_slice(&data[0..bpp]);
filtered.extend(
buf.extend_from_slice(&data[0..bpp]);
buf.extend(
data.iter()
.skip(bpp)
.zip(data.iter())
Expand All @@ -15,9 +15,9 @@ pub fn filter_line(filter: u8, bpp: usize, data: &[u8], last_line: &[u8]) -> Vec
}
2 => {
if last_line.is_empty() {
filtered.extend_from_slice(data);
buf.extend_from_slice(data);
} else {
filtered.extend(
buf.extend(
data.iter()
.zip(last_line.iter())
.map(|(cur, last)| cur.wrapping_sub(*last)),
Expand All @@ -27,12 +27,12 @@ pub fn filter_line(filter: u8, bpp: usize, data: &[u8], last_line: &[u8]) -> Vec
3 => {
for (i, byte) in data.iter().enumerate() {
if last_line.is_empty() {
filtered.push(match i.checked_sub(bpp) {
buf.push(match i.checked_sub(bpp) {
Some(x) => byte.wrapping_sub(data[x] >> 1),
None => *byte,
});
} else {
filtered.push(match i.checked_sub(bpp) {
buf.push(match i.checked_sub(bpp) {
Some(x) => byte.wrapping_sub(
((u16::from(data[x]) + u16::from(last_line[i])) >> 1) as u8,
),
Expand All @@ -44,12 +44,12 @@ pub fn filter_line(filter: u8, bpp: usize, data: &[u8], last_line: &[u8]) -> Vec
4 => {
for (i, byte) in data.iter().enumerate() {
if last_line.is_empty() {
filtered.push(match i.checked_sub(bpp) {
buf.push(match i.checked_sub(bpp) {
Some(x) => byte.wrapping_sub(data[x]),
None => *byte,
});
} else {
filtered.push(match i.checked_sub(bpp) {
buf.push(match i.checked_sub(bpp) {
Some(x) => {
byte.wrapping_sub(paeth_predictor(data[x], last_line[i], last_line[x]))
}
Expand All @@ -60,33 +60,33 @@ pub fn filter_line(filter: u8, bpp: usize, data: &[u8], last_line: &[u8]) -> Vec
}
_ => unreachable!(),
}
filtered
}

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)),
Expand All @@ -98,23 +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));
}
};
};
Expand All @@ -125,33 +125,32 @@ 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]));
}
};
};
}
}
_ => unreachable!(),
}
unfiltered
}

fn paeth_predictor(a: u8, b: u8, c: u8) -> u8 {
Expand Down
46 changes: 28 additions & 18 deletions src/png/mod.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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
}
Expand All @@ -309,7 +311,9 @@ impl PngImage {
let bpp = ((self.ihdr.bit_depth.as_u8() * self.channels_per_pixel() + 7) / 8) as usize;
let mut last_line: &[u8] = &[];
let mut last_pass: Option<u8> = None;
let mut f_buf = Vec::new();
for line in self.scan_lines() {
f_buf.clear();
match filter {
0 | 1 | 2 | 3 | 4 => {
let filter = if last_pass == line.pass || filter <= 1 {
Expand All @@ -318,28 +322,34 @@ impl PngImage {
0
};
filtered.push(filter);
filtered.extend_from_slice(&filter_line(filter, bpp, &line.data, last_line));
filter_line(filter, bpp, &line.data, last_line, &mut f_buf);
filtered.extend_from_slice(&f_buf);
}
5 => {
// Heuristically guess best filter per line
// Uses MSAD algorithm mentioned in libpng reference docs
// http://www.libpng.org/pub/png/book/chapter09.html
let mut trials: Vec<(u8, Vec<u8>)> = Vec::with_capacity(5);
let mut best_filter = 0;
let mut best_line = Vec::new();
let mut best_size = std::u64::MAX;

// Avoid vertical filtering on first line of each interlacing pass
for filter in if last_pass == line.pass { 0..5 } else { 0..2 } {
trials.push((filter, filter_line(filter, bpp, &line.data, last_line)));
filter_line(filter, bpp, &line.data, last_line, &mut f_buf);
let size = f_buf.iter().fold(0u64, |acc, &x| {
let signed = x as i8;
acc + i16::from(signed).abs() as u64
});
if size < best_size {
best_size = size;
best_filter = filter;
std::mem::swap(&mut best_line, &mut f_buf);
}
f_buf.clear() //discard buffer, and start again

}
let (best_filter, best_line) = trials
.iter()
.min_by_key(|(_, line)| {
line.iter().fold(0u64, |acc, &x| {
let signed = x as i8;
acc + i16::from(signed).abs() as u64
})
})
.unwrap();
filtered.push(*best_filter);
filtered.extend_from_slice(best_line);
filtered.push(best_filter);
filtered.extend_from_slice(&best_line);
}
_ => unreachable!(),
}
Expand Down

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