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use std::{fmt, fmt::Display, mem::transmute};
mod strategies;
pub use strategies::FilterStrategy;
/// PNG delta filters
#[repr(u8)]
#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Clone, Copy, Hash)]
pub enum RowFilter {
None,
Sub,
Up,
Average,
Paeth,
}
impl TryFrom<u8> for RowFilter {
type Error = ();
fn try_from(value: u8) -> Result<Self, Self::Error> {
if value > 4 {
return Err(());
}
unsafe { transmute(value as i8) }
}
}
impl Display for RowFilter {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
Display::fmt(
match self {
Self::None => "None",
Self::Sub => "Sub",
Self::Up => "Up",
Self::Average => "Average",
Self::Paeth => "Paeth",
},
f,
)
}
}
impl RowFilter {
pub(crate) const ALL: [Self; 5] = [Self::None, Self::Sub, Self::Up, Self::Average, Self::Paeth];
pub(crate) fn filter_line(
self,
bpp: usize,
data: &mut [u8],
prev_line: &[u8],
buf: &mut Vec<u8>,
alpha_bytes: usize,
) {
assert!(data.len() >= bpp);
assert_eq!(data.len(), prev_line.len());
if alpha_bytes != 0 {
self.optimize_alpha(bpp, data, prev_line, bpp - alpha_bytes);
}
buf.reserve(data.len() + 1);
buf.push(self as u8);
match self {
Self::None => {
buf.extend_from_slice(data);
}
Self::Sub => {
buf.extend_from_slice(&data[0..bpp]);
buf.extend(
data.iter()
.skip(bpp)
.zip(data.iter())
.map(|(cur, last)| cur.wrapping_sub(*last)),
);
}
Self::Up => {
buf.extend(
data.iter()
.zip(prev_line.iter())
.map(|(cur, last)| cur.wrapping_sub(*last)),
);
}
Self::Average => {
buf.extend(
data.iter()
.zip(prev_line.iter())
.take(bpp)
.map(|(cur, &up)| cur.wrapping_sub(up >> 1)),
);
buf.extend(data.iter().enumerate().skip(bpp).map(|(i, &cur)| {
let left = data[i - bpp];
let up = prev_line[i];
cur.wrapping_sub(((u16::from(left) + u16::from(up)) >> 1) as u8)
}));
}
Self::Paeth => {
buf.extend(
data.iter()
.zip(prev_line.iter())
.take(bpp)
.map(|(cur, &up)| cur.wrapping_sub(up)),
);
buf.extend(data.iter().enumerate().skip(bpp).map(|(i, &cur)| {
let left = data[i - bpp];
let up = prev_line[i];
let left_up = prev_line[i - bpp];
cur.wrapping_sub(paeth_predictor(left, up, left_up))
}));
}
}
}
// Optimize fully transparent pixels of a scanline such that they will be zeroed when filtered
fn optimize_alpha(self, bpp: usize, data: &mut [u8], prev_line: &[u8], color_bytes: usize) {
if self == Self::None {
// Assume transparent pixels already set to 0
return;
}
let mut pixels: Vec<_> = data.chunks_exact_mut(bpp).collect();
let prev_pixels: Vec<_> = prev_line.chunks_exact(bpp).collect();
for i in 0..pixels.len() {
if pixels[i].iter().skip(color_bytes).all(|b| *b == 0) {
// If the first pixel in the row is transparent, find the next non-transparent pixel and pretend
// it is the previous one. This can help improve effectiveness of the Sub and Paeth filters.
let prev = match i {
0 => pixels
.iter()
.position(|px| px.iter().skip(color_bytes).any(|b| *b != 0))
.unwrap_or(i),
_ => i - 1,
};
// These assertions help eliminate a few bounds checks in the slice accesses below
assert!(prev < pixels.len());
assert!(i < prev_pixels.len());
match self {
Self::None => unreachable!(),
Self::Sub => {
// The code below is roughly equivalent to pixels[i][0..color_bytes].copy_from_slice(&pixels[prev][0..color_bytes]),
// if such a thing was possible to do without violating Rust aliasing rules. See:
// https://users.rust-lang.org/t/problem-borrowing-two-elements-of-vec-mutably/21446/2
if prev < i {
let (pixels_head, pixels_tail) = pixels.split_at_mut(prev + 1);
pixels_tail[i - prev - 1][0..color_bytes]
.copy_from_slice(&pixels_head[prev][0..color_bytes]);
} else if prev > i {
let (pixels_head, pixels_tail) = pixels.split_at_mut(i + 1);
pixels_head[i][0..color_bytes]
.copy_from_slice(&pixels_tail[prev - i - 1][0..color_bytes]);
} else {
// If prev == i, we'd be copying the pixels onto themselves, which is useless
}
}
Self::Up => {
pixels[i][0..color_bytes].copy_from_slice(&prev_pixels[i][0..color_bytes]);
}
Self::Average => {
for j in 0..color_bytes {
pixels[i][j] = match i {
0 => prev_pixels[i][j] >> 1,
_ => {
((u16::from(pixels[i - 1][j]) + u16::from(prev_pixels[i][j]))
>> 1) as u8
}
};
}
}
Self::Paeth => {
for j in 0..color_bytes {
pixels[i][j] = match i {
0 => pixels[prev][j].min(prev_pixels[i][j]),
_ => paeth_predictor(
pixels[i - 1][j],
prev_pixels[i][j],
prev_pixels[i - 1][j],
),
};
}
}
}
}
}
}
pub(crate) fn unfilter_line(
self,
bpp: usize,
data: &[u8],
prev_line: &[u8],
buf: &mut Vec<u8>,
) {
assert!(data.len() >= bpp);
assert_eq!(data.len(), prev_line.len());
let offset = buf.len();
buf.reserve(data.len());
match self {
Self::None => {
buf.extend_from_slice(data);
}
Self::Sub => {
buf.extend_from_slice(&data[0..bpp]);
for i in bpp..data.len() {
let left = buf[offset + i - bpp];
buf.push(data[i].wrapping_add(left));
}
}
Self::Up => {
buf.extend(
data.iter()
.zip(prev_line)
.map(|(&cur, &last)| cur.wrapping_add(last)),
);
}
Self::Average => {
for (cur, &up) in data.iter().take(bpp).zip(prev_line.iter().take(bpp)) {
buf.push(cur.wrapping_add(up >> 1));
}
for i in bpp..data.len() {
let left = buf[offset + i - bpp];
let up = prev_line[i];
buf.push(data[i].wrapping_add(((u16::from(left) + u16::from(up)) >> 1) as u8));
}
}
Self::Paeth => {
for (cur, &up) in data.iter().take(bpp).zip(prev_line.iter().take(bpp)) {
buf.push(cur.wrapping_add(up));
}
for i in bpp..data.len() {
let left = buf[offset + i - bpp];
let up = prev_line[i];
let left_up = prev_line[i - bpp];
buf.push(data[i].wrapping_add(paeth_predictor(left, up, left_up)));
}
}
}
}
}
fn paeth_predictor(a: u8, b: u8, c: u8) -> u8 {
let p = i32::from(a) + i32::from(b) - i32::from(c);
let pa = (p - i32::from(a)).abs();
let pb = (p - i32::from(b)).abs();
let pc = (p - i32::from(c)).abs();
if pa <= pb && pa <= pc {
a
} else if pb <= pc {
b
} else {
c
}
}