Initial vendor packages

Signed-off-by: Valentin Popov <valentin@popov.link>

1 files changed, 869 insertions, 0 deletions

diff --git a/vendor/image/src/codecs/dxt.rs b/vendor/image/src/codecs/dxt.rs
new file mode 100644
index 0000000..8737fb3
--- /dev/null
+++ b/vendor/image/src/codecs/dxt.rs
@@ -0,0 +1,869 @@
+//!  Decoding of DXT (S3TC) compression
+//!
+//!  DXT is an image format that supports lossy compression
+//!
+//!  # Related Links
+//!  * <https://www.khronos.org/registry/OpenGL/extensions/EXT/EXT_texture_compression_s3tc.txt> - Description of the DXT compression OpenGL extensions.
+//!
+//!  Note: this module only implements bare DXT encoding/decoding, it does not parse formats that can contain DXT files like .dds
+
+use std::convert::TryFrom;
+use std::io::{self, Read, Seek, SeekFrom, Write};
+
+use crate::color::ColorType;
+use crate::error::{ImageError, ImageResult, ParameterError, ParameterErrorKind};
+use crate::image::{self, ImageDecoder, ImageDecoderRect, ImageReadBuffer, Progress};
+
+/// What version of DXT compression are we using?
+/// Note that DXT2 and DXT4 are left away as they're
+/// just DXT3 and DXT5 with premultiplied alpha
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum DxtVariant {
+    /// The DXT1 format. 48 bytes of RGB data in a 4x4 pixel square is
+    /// compressed into an 8 byte block of DXT1 data
+    DXT1,
+    /// The DXT3 format. 64 bytes of RGBA data in a 4x4 pixel square is
+    /// compressed into a 16 byte block of DXT3 data
+    DXT3,
+    /// The DXT5 format. 64 bytes of RGBA data in a 4x4 pixel square is
+    /// compressed into a 16 byte block of DXT5 data
+    DXT5,
+}
+
+impl DxtVariant {
+    /// Returns the amount of bytes of raw image data
+    /// that is encoded in a single DXTn block
+    fn decoded_bytes_per_block(self) -> usize {
+        match self {
+            DxtVariant::DXT1 => 48,
+            DxtVariant::DXT3 | DxtVariant::DXT5 => 64,
+        }
+    }
+
+    /// Returns the amount of bytes per block of encoded DXTn data
+    fn encoded_bytes_per_block(self) -> usize {
+        match self {
+            DxtVariant::DXT1 => 8,
+            DxtVariant::DXT3 | DxtVariant::DXT5 => 16,
+        }
+    }
+
+    /// Returns the color type that is stored in this DXT variant
+    pub fn color_type(self) -> ColorType {
+        match self {
+            DxtVariant::DXT1 => ColorType::Rgb8,
+            DxtVariant::DXT3 | DxtVariant::DXT5 => ColorType::Rgba8,
+        }
+    }
+}
+
+/// DXT decoder
+pub struct DxtDecoder<R: Read> {
+    inner: R,
+    width_blocks: u32,
+    height_blocks: u32,
+    variant: DxtVariant,
+    row: u32,
+}
+
+impl<R: Read> DxtDecoder<R> {
+    /// Create a new DXT decoder that decodes from the stream ```r```.
+    /// As DXT is often stored as raw buffers with the width/height
+    /// somewhere else the width and height of the image need
+    /// to be passed in ```width``` and ```height```, as well as the
+    /// DXT variant in ```variant```.
+    /// width and height are required to be powers of 2 and at least 4.
+    /// otherwise an error will be returned
+    pub fn new(
+        r: R,
+        width: u32,
+        height: u32,
+        variant: DxtVariant,
+    ) -> Result<DxtDecoder<R>, ImageError> {
+        if width % 4 != 0 || height % 4 != 0 {
+            // TODO: this is actually a bit of a weird case. We could return `DecodingError` but
+            // it's not really the format that is wrong However, the encoder should surely return
+            // `EncodingError` so it would be the logical choice for symmetry.
+            return Err(ImageError::Parameter(ParameterError::from_kind(
+                ParameterErrorKind::DimensionMismatch,
+            )));
+        }
+        let width_blocks = width / 4;
+        let height_blocks = height / 4;
+        Ok(DxtDecoder {
+            inner: r,
+            width_blocks,
+            height_blocks,
+            variant,
+            row: 0,
+        })
+    }
+
+    fn read_scanline(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        assert_eq!(u64::try_from(buf.len()), Ok(self.scanline_bytes()));
+
+        let mut src =
+            vec![0u8; self.variant.encoded_bytes_per_block() * self.width_blocks as usize];
+        self.inner.read_exact(&mut src)?;
+        match self.variant {
+            DxtVariant::DXT1 => decode_dxt1_row(&src, buf),
+            DxtVariant::DXT3 => decode_dxt3_row(&src, buf),
+            DxtVariant::DXT5 => decode_dxt5_row(&src, buf),
+        }
+        self.row += 1;
+        Ok(buf.len())
+    }
+}
+
+// Note that, due to the way that DXT compression works, a scanline is considered to consist out of
+// 4 lines of pixels.
+impl<'a, R: 'a + Read> ImageDecoder<'a> for DxtDecoder<R> {
+    type Reader = DxtReader<R>;
+
+    fn dimensions(&self) -> (u32, u32) {
+        (self.width_blocks * 4, self.height_blocks * 4)
+    }
+
+    fn color_type(&self) -> ColorType {
+        self.variant.color_type()
+    }
+
+    fn scanline_bytes(&self) -> u64 {
+        self.variant.decoded_bytes_per_block() as u64 * u64::from(self.width_blocks)
+    }
+
+    fn into_reader(self) -> ImageResult<Self::Reader> {
+        Ok(DxtReader {
+            buffer: ImageReadBuffer::new(self.scanline_bytes(), self.total_bytes()),
+            decoder: self,
+        })
+    }
+
+    fn read_image(mut self, buf: &mut [u8]) -> ImageResult<()> {
+        assert_eq!(u64::try_from(buf.len()), Ok(self.total_bytes()));
+
+        for chunk in buf.chunks_mut(self.scanline_bytes().max(1) as usize) {
+            self.read_scanline(chunk)?;
+        }
+        Ok(())
+    }
+}
+
+impl<'a, R: 'a + Read + Seek> ImageDecoderRect<'a> for DxtDecoder<R> {
+    fn read_rect_with_progress<F: Fn(Progress)>(
+        &mut self,
+        x: u32,
+        y: u32,
+        width: u32,
+        height: u32,
+        buf: &mut [u8],
+        progress_callback: F,
+    ) -> ImageResult<()> {
+        let encoded_scanline_bytes =
+            self.variant.encoded_bytes_per_block() as u64 * u64::from(self.width_blocks);
+
+        let start = self.inner.stream_position()?;
+        image::load_rect(
+            x,
+            y,
+            width,
+            height,
+            buf,
+            progress_callback,
+            self,
+            |s, scanline| {
+                s.inner
+                    .seek(SeekFrom::Start(start + scanline * encoded_scanline_bytes))?;
+                Ok(())
+            },
+            |s, buf| s.read_scanline(buf).map(|_| ()),
+        )?;
+        self.inner.seek(SeekFrom::Start(start))?;
+        Ok(())
+    }
+}
+
+/// DXT reader
+pub struct DxtReader<R: Read> {
+    buffer: ImageReadBuffer,
+    decoder: DxtDecoder<R>,
+}
+
+impl<R: Read> Read for DxtReader<R> {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        let decoder = &mut self.decoder;
+        self.buffer.read(buf, |buf| decoder.read_scanline(buf))
+    }
+}
+
+/// DXT encoder
+pub struct DxtEncoder<W: Write> {
+    w: W,
+}
+
+impl<W: Write> DxtEncoder<W> {
+    /// Create a new encoder that writes its output to ```w```
+    pub fn new(w: W) -> DxtEncoder<W> {
+        DxtEncoder { w }
+    }
+
+    /// Encodes the image data ```data```
+    /// that has dimensions ```width``` and ```height```
+    /// in ```DxtVariant``` ```variant```
+    /// data is assumed to be in variant.color_type()
+    pub fn encode(
+        mut self,
+        data: &[u8],
+        width: u32,
+        height: u32,
+        variant: DxtVariant,
+    ) -> ImageResult<()> {
+        if width % 4 != 0 || height % 4 != 0 {
+            // TODO: this is not very idiomatic yet. Should return an EncodingError.
+            return Err(ImageError::Parameter(ParameterError::from_kind(
+                ParameterErrorKind::DimensionMismatch,
+            )));
+        }
+        let width_blocks = width / 4;
+        let height_blocks = height / 4;
+
+        let stride = variant.decoded_bytes_per_block();
+
+        assert!(data.len() >= width_blocks as usize * height_blocks as usize * stride);
+
+        for chunk in data.chunks(width_blocks as usize * stride) {
+            let data = match variant {
+                DxtVariant::DXT1 => encode_dxt1_row(chunk),
+                DxtVariant::DXT3 => encode_dxt3_row(chunk),
+                DxtVariant::DXT5 => encode_dxt5_row(chunk),
+            };
+            self.w.write_all(&data)?;
+        }
+        Ok(())
+    }
+}
+
+/**
+ * Actual encoding/decoding logic below.
+ */
+use std::mem::swap;
+
+type Rgb = [u8; 3];
+
+/// decodes a 5-bit R, 6-bit G, 5-bit B 16-bit packed color value into 8-bit RGB
+/// mapping is done so min/max range values are preserved. So for 5-bit
+/// values 0x00 -> 0x00 and 0x1F -> 0xFF
+fn enc565_decode(value: u16) -> Rgb {
+    let red = (value >> 11) & 0x1F;
+    let green = (value >> 5) & 0x3F;
+    let blue = (value) & 0x1F;
+    [
+        (red * 0xFF / 0x1F) as u8,
+        (green * 0xFF / 0x3F) as u8,
+        (blue * 0xFF / 0x1F) as u8,
+    ]
+}
+
+/// encodes an 8-bit RGB value into a 5-bit R, 6-bit G, 5-bit B 16-bit packed color value
+/// mapping preserves min/max values. It is guaranteed that i == encode(decode(i)) for all i
+fn enc565_encode(rgb: Rgb) -> u16 {
+    let red = (u16::from(rgb[0]) * 0x1F + 0x7E) / 0xFF;
+    let green = (u16::from(rgb[1]) * 0x3F + 0x7E) / 0xFF;
+    let blue = (u16::from(rgb[2]) * 0x1F + 0x7E) / 0xFF;
+    (red << 11) | (green << 5) | blue
+}
+
+/// utility function: squares a value
+fn square(a: i32) -> i32 {
+    a * a
+}
+
+/// returns the squared error between two RGB values
+fn diff(a: Rgb, b: Rgb) -> i32 {
+    square(i32::from(a[0]) - i32::from(b[0]))
+        + square(i32::from(a[1]) - i32::from(b[1]))
+        + square(i32::from(a[2]) - i32::from(b[2]))
+}
+
+/*
+ * Functions for decoding DXT compression
+ */
+
+/// Constructs the DXT5 alpha lookup table from the two alpha entries
+/// if alpha0 > alpha1, constructs a table of [a0, a1, 6 linearly interpolated values from a0 to a1]
+/// if alpha0 <= alpha1, constructs a table of [a0, a1, 4 linearly interpolated values from a0 to a1, 0, 0xFF]
+fn alpha_table_dxt5(alpha0: u8, alpha1: u8) -> [u8; 8] {
+    let mut table = [alpha0, alpha1, 0, 0, 0, 0, 0, 0xFF];
+    if alpha0 > alpha1 {
+        for i in 2..8u16 {
+            table[i as usize] =
+                (((8 - i) * u16::from(alpha0) + (i - 1) * u16::from(alpha1)) / 7) as u8;
+        }
+    } else {
+        for i in 2..6u16 {
+            table[i as usize] =
+                (((6 - i) * u16::from(alpha0) + (i - 1) * u16::from(alpha1)) / 5) as u8;
+        }
+    }
+    table
+}
+
+/// decodes an 8-byte dxt color block into the RGB channels of a 16xRGB or 16xRGBA block.
+/// source should have a length of 8, dest a length of 48 (RGB) or 64 (RGBA)
+fn decode_dxt_colors(source: &[u8], dest: &mut [u8], is_dxt1: bool) {
+    // sanity checks, also enable the compiler to elide all following bound checks
+    assert!(source.len() == 8 && (dest.len() == 48 || dest.len() == 64));
+    // calculate pitch to store RGB values in dest (3 for RGB, 4 for RGBA)
+    let pitch = dest.len() / 16;
+
+    // extract color data
+    let color0 = u16::from(source[0]) | (u16::from(source[1]) << 8);
+    let color1 = u16::from(source[2]) | (u16::from(source[3]) << 8);
+    let color_table = u32::from(source[4])
+        | (u32::from(source[5]) << 8)
+        | (u32::from(source[6]) << 16)
+        | (u32::from(source[7]) << 24);
+    // let color_table = source[4..8].iter().rev().fold(0, |t, &b| (t << 8) | b as u32);
+
+    // decode the colors to rgb format
+    let mut colors = [[0; 3]; 4];
+    colors[0] = enc565_decode(color0);
+    colors[1] = enc565_decode(color1);
+
+    // determine color interpolation method
+    if color0 > color1 || !is_dxt1 {
+        // linearly interpolate the other two color table entries
+        for i in 0..3 {
+            colors[2][i] = ((u16::from(colors[0][i]) * 2 + u16::from(colors[1][i]) + 1) / 3) as u8;
+            colors[3][i] = ((u16::from(colors[0][i]) + u16::from(colors[1][i]) * 2 + 1) / 3) as u8;
+        }
+    } else {
+        // linearly interpolate one other entry, keep the other at 0
+        for i in 0..3 {
+            colors[2][i] = ((u16::from(colors[0][i]) + u16::from(colors[1][i]) + 1) / 2) as u8;
+        }
+    }
+
+    // serialize the result. Every color is determined by looking up
+    // two bits in color_table which identify which color to actually pick from the 4 possible colors
+    for i in 0..16 {
+        dest[i * pitch..i * pitch + 3]
+            .copy_from_slice(&colors[(color_table >> (i * 2)) as usize & 3]);
+    }
+}
+
+/// Decodes a 16-byte bock of dxt5 data to a 16xRGBA block
+fn decode_dxt5_block(source: &[u8], dest: &mut [u8]) {
+    assert!(source.len() == 16 && dest.len() == 64);
+
+    // extract alpha index table (stored as little endian 64-bit value)
+    let alpha_table = source[2..8]
+        .iter()
+        .rev()
+        .fold(0, |t, &b| (t << 8) | u64::from(b));
+
+    // alhpa level decode
+    let alphas = alpha_table_dxt5(source[0], source[1]);
+
+    // serialize alpha
+    for i in 0..16 {
+        dest[i * 4 + 3] = alphas[(alpha_table >> (i * 3)) as usize & 7];
+    }
+
+    // handle colors
+    decode_dxt_colors(&source[8..16], dest, false);
+}
+
+/// Decodes a 16-byte bock of dxt3 data to a 16xRGBA block
+fn decode_dxt3_block(source: &[u8], dest: &mut [u8]) {
+    assert!(source.len() == 16 && dest.len() == 64);
+
+    // extract alpha index table (stored as little endian 64-bit value)
+    let alpha_table = source[0..8]
+        .iter()
+        .rev()
+        .fold(0, |t, &b| (t << 8) | u64::from(b));
+
+    // serialize alpha (stored as 4-bit values)
+    for i in 0..16 {
+        dest[i * 4 + 3] = ((alpha_table >> (i * 4)) as u8 & 0xF) * 0x11;
+    }
+
+    // handle colors
+    decode_dxt_colors(&source[8..16], dest, false);
+}
+
+/// Decodes a 8-byte bock of dxt5 data to a 16xRGB block
+fn decode_dxt1_block(source: &[u8], dest: &mut [u8]) {
+    assert!(source.len() == 8 && dest.len() == 48);
+    decode_dxt_colors(source, dest, true);
+}
+
+/// Decode a row of DXT1 data to four rows of RGB data.
+/// source.len() should be a multiple of 8, otherwise this panics.
+fn decode_dxt1_row(source: &[u8], dest: &mut [u8]) {
+    assert!(source.len() % 8 == 0);
+    let block_count = source.len() / 8;
+    assert!(dest.len() >= block_count * 48);
+
+    // contains the 16 decoded pixels per block
+    let mut decoded_block = [0u8; 48];
+
+    for (x, encoded_block) in source.chunks(8).enumerate() {
+        decode_dxt1_block(encoded_block, &mut decoded_block);
+
+        // copy the values from the decoded block to linewise RGB layout
+        for line in 0..4 {
+            let offset = (block_count * line + x) * 12;
+            dest[offset..offset + 12].copy_from_slice(&decoded_block[line * 12..(line + 1) * 12]);
+        }
+    }
+}
+
+/// Decode a row of DXT3 data to four rows of RGBA data.
+/// source.len() should be a multiple of 16, otherwise this panics.
+fn decode_dxt3_row(source: &[u8], dest: &mut [u8]) {
+    assert!(source.len() % 16 == 0);
+    let block_count = source.len() / 16;
+    assert!(dest.len() >= block_count * 64);
+
+    // contains the 16 decoded pixels per block
+    let mut decoded_block = [0u8; 64];
+
+    for (x, encoded_block) in source.chunks(16).enumerate() {
+        decode_dxt3_block(encoded_block, &mut decoded_block);
+
+        // copy the values from the decoded block to linewise RGB layout
+        for line in 0..4 {
+            let offset = (block_count * line + x) * 16;
+            dest[offset..offset + 16].copy_from_slice(&decoded_block[line * 16..(line + 1) * 16]);
+        }
+    }
+}
+
+/// Decode a row of DXT5 data to four rows of RGBA data.
+/// source.len() should be a multiple of 16, otherwise this panics.
+fn decode_dxt5_row(source: &[u8], dest: &mut [u8]) {
+    assert!(source.len() % 16 == 0);
+    let block_count = source.len() / 16;
+    assert!(dest.len() >= block_count * 64);
+
+    // contains the 16 decoded pixels per block
+    let mut decoded_block = [0u8; 64];
+
+    for (x, encoded_block) in source.chunks(16).enumerate() {
+        decode_dxt5_block(encoded_block, &mut decoded_block);
+
+        // copy the values from the decoded block to linewise RGB layout
+        for line in 0..4 {
+            let offset = (block_count * line + x) * 16;
+            dest[offset..offset + 16].copy_from_slice(&decoded_block[line * 16..(line + 1) * 16]);
+        }
+    }
+}
+
+/*
+ * Functions for encoding DXT compression
+ */
+
+/// Tries to perform the color encoding part of dxt compression
+/// the approach taken is simple, it picks unique combinations
+/// of the colors present in the block, and attempts to encode the
+/// block with each, picking the encoding that yields the least
+/// squared error out of all of them.
+///
+/// This could probably be faster but is already reasonably fast
+/// and a good reference impl to optimize others against.
+///
+/// Another way to perform this analysis would be to perform a
+/// singular value decomposition of the different colors, and
+/// then pick 2 points on this line as the base colors. But
+/// this is still rather unwieldy math and has issues
+/// with the 3-linear-colors-and-0 case, it's also worse
+/// at conserving the original colors.
+///
+/// source: should be RGBAx16 or RGBx16 bytes of data,
+/// dest 8 bytes of resulting encoded color data
+fn encode_dxt_colors(source: &[u8], dest: &mut [u8], is_dxt1: bool) {
+    // sanity checks and determine stride when parsing the source data
+    assert!((source.len() == 64 || source.len() == 48) && dest.len() == 8);
+    let stride = source.len() / 16;
+
+    // reference colors array
+    let mut colors = [[0u8; 3]; 4];
+
+    // Put the colors we're going to be processing in an array with pure RGB layout
+    // note: we reverse the pixel order here. The reason for this is found in the inner quantization loop.
+    let mut targets = [[0u8; 3]; 16];
+    for (s, d) in source.chunks(stride).rev().zip(&mut targets) {
+        *d = [s[0], s[1], s[2]];
+    }
+
+    // roundtrip all colors through the r5g6b5 encoding
+    for rgb in &mut targets {
+        *rgb = enc565_decode(enc565_encode(*rgb));
+    }
+
+    // and deduplicate the set of colors to choose from as the algorithm is O(N^2) in this
+    let mut colorspace_ = [[0u8; 3]; 16];
+    let mut colorspace_len = 0;
+    for color in &targets {
+        if !colorspace_[..colorspace_len].contains(color) {
+            colorspace_[colorspace_len] = *color;
+            colorspace_len += 1;
+        }
+    }
+    let mut colorspace = &colorspace_[..colorspace_len];
+
+    // in case of slight gradients it can happen that there's only one entry left in the color table.
+    // as the resulting banding can be quite bad if we would just left the block at the closest
+    // encodable color, we have a special path here that tries to emulate the wanted color
+    // using the linear interpolation between gradients
+    if colorspace.len() == 1 {
+        // the base color we got from colorspace reduction
+        let ref_rgb = colorspace[0];
+        // the unreduced color in this block that's the furthest away from the actual block
+        let mut rgb = targets
+            .iter()
+            .cloned()
+            .max_by_key(|rgb| diff(*rgb, ref_rgb))
+            .unwrap();
+        // amplify differences by 2.5, which should push them to the next quantized value
+        // if possible without overshoot
+        for i in 0..3 {
+            rgb[i] =
+                ((i16::from(rgb[i]) - i16::from(ref_rgb[i])) * 5 / 2 + i16::from(ref_rgb[i])) as u8;
+        }
+
+        // roundtrip it through quantization
+        let encoded = enc565_encode(rgb);
+        let rgb = enc565_decode(encoded);
+
+        // in case this didn't land us a different color the best way to represent this field is
+        // as a single color block
+        if rgb == ref_rgb {
+            dest[0] = encoded as u8;
+            dest[1] = (encoded >> 8) as u8;
+
+            for d in dest.iter_mut().take(8).skip(2) {
+                *d = 0;
+            }
+            return;
+        }
+
+        // we did find a separate value: add it to the options so after one round of quantization
+        // we're done
+        colorspace_[1] = rgb;
+        colorspace = &colorspace_[..2];
+    }
+
+    // block quantization loop: we basically just try every possible combination, returning
+    // the combination with the least squared error
+    // stores the best candidate colors
+    let mut chosen_colors = [[0; 3]; 4];
+    // did this index table use the [0,0,0] variant
+    let mut chosen_use_0 = false;
+    // error calculated for the last entry
+    let mut chosen_error = 0xFFFF_FFFFu32;
+
+    // loop through unique permutations of the colorspace, where c1 != c2
+    'search: for (i, &c1) in colorspace.iter().enumerate() {
+        colors[0] = c1;
+
+        for &c2 in &colorspace[0..i] {
+            colors[1] = c2;
+
+            if is_dxt1 {
+                // what's inside here is ran at most 120 times.
+                for use_0 in 0..2 {
+                    // and 240 times here.
+
+                    if use_0 != 0 {
+                        // interpolate one color, set the other to 0
+                        for i in 0..3 {
+                            colors[2][i] =
+                                ((u16::from(colors[0][i]) + u16::from(colors[1][i]) + 1) / 2) as u8;
+                        }
+                        colors[3] = [0, 0, 0];
+                    } else {
+                        // interpolate to get 2 more colors
+                        for i in 0..3 {
+                            colors[2][i] =
+                                ((u16::from(colors[0][i]) * 2 + u16::from(colors[1][i]) + 1) / 3)
+                                    as u8;
+                            colors[3][i] =
+                                ((u16::from(colors[0][i]) + u16::from(colors[1][i]) * 2 + 1) / 3)
+                                    as u8;
+                        }
+                    }
+
+                    // calculate the total error if we were to quantize the block with these color combinations
+                    // both these loops have statically known iteration counts and are well vectorizable
+                    // note that the inside of this can be run about 15360 times worst case, i.e. 960 times per
+                    // pixel.
+                    let total_error = targets
+                        .iter()
+                        .map(|t| colors.iter().map(|c| diff(*c, *t) as u32).min().unwrap())
+                        .sum();
+
+                    // update the match if we found a better one
+                    if total_error < chosen_error {
+                        chosen_colors = colors;
+                        chosen_use_0 = use_0 != 0;
+                        chosen_error = total_error;
+
+                        // if we've got a perfect or at most 1 LSB off match, we're done
+                        if total_error < 4 {
+                            break 'search;
+                        }
+                    }
+                }
+            } else {
+                // what's inside here is ran at most 120 times.
+
+                // interpolate to get 2 more colors
+                for i in 0..3 {
+                    colors[2][i] =
+                        ((u16::from(colors[0][i]) * 2 + u16::from(colors[1][i]) + 1) / 3) as u8;
+                    colors[3][i] =
+                        ((u16::from(colors[0][i]) + u16::from(colors[1][i]) * 2 + 1) / 3) as u8;
+                }
+
+                // calculate the total error if we were to quantize the block with these color combinations
+                // both these loops have statically known iteration counts and are well vectorizable
+                // note that the inside of this can be run about 15360 times worst case, i.e. 960 times per
+                // pixel.
+                let total_error = targets
+                    .iter()
+                    .map(|t| colors.iter().map(|c| diff(*c, *t) as u32).min().unwrap())
+                    .sum();
+
+                // update the match if we found a better one
+                if total_error < chosen_error {
+                    chosen_colors = colors;
+                    chosen_error = total_error;
+
+                    // if we've got a perfect or at most 1 LSB off match, we're done
+                    if total_error < 4 {
+                        break 'search;
+                    }
+                }
+            }
+        }
+    }
+
+    // calculate the final indices
+    // note that targets is already in reverse pixel order, to make the index computation easy.
+    let mut chosen_indices = 0u32;
+    for t in &targets {
+        let (idx, _) = chosen_colors
+            .iter()
+            .enumerate()
+            .min_by_key(|&(_, c)| diff(*c, *t))
+            .unwrap();
+        chosen_indices = (chosen_indices << 2) | idx as u32;
+    }
+
+    // encode the colors
+    let mut color0 = enc565_encode(chosen_colors[0]);
+    let mut color1 = enc565_encode(chosen_colors[1]);
+
+    // determine encoding. Note that color0 == color1 is impossible at this point
+    if is_dxt1 {
+        if color0 > color1 {
+            if chosen_use_0 {
+                swap(&mut color0, &mut color1);
+                // Indexes are packed 2 bits wide, swap index 0/1 but preserve 2/3.
+                let filter = (chosen_indices & 0xAAAA_AAAA) >> 1;
+                chosen_indices ^= filter ^ 0x5555_5555;
+            }
+        } else if !chosen_use_0 {
+            swap(&mut color0, &mut color1);
+            // Indexes are packed 2 bits wide, swap index 0/1 and 2/3.
+            chosen_indices ^= 0x5555_5555;
+        }
+    }
+
+    // encode everything.
+    dest[0] = color0 as u8;
+    dest[1] = (color0 >> 8) as u8;
+    dest[2] = color1 as u8;
+    dest[3] = (color1 >> 8) as u8;
+    for i in 0..4 {
+        dest[i + 4] = (chosen_indices >> (i * 8)) as u8;
+    }
+}
+
+/// Encodes a buffer of 16 alpha bytes into a dxt5 alpha index table,
+/// where the alpha table they are indexed against is created by
+/// calling alpha_table_dxt5(alpha0, alpha1)
+/// returns the resulting error and alpha table
+fn encode_dxt5_alpha(alpha0: u8, alpha1: u8, alphas: &[u8; 16]) -> (i32, u64) {
+    // create a table for the given alpha ranges
+    let table = alpha_table_dxt5(alpha0, alpha1);
+    let mut indices = 0u64;
+    let mut total_error = 0i32;
+
+    // least error brute force search
+    for (i, &a) in alphas.iter().enumerate() {
+        let (index, error) = table
+            .iter()
+            .enumerate()
+            .map(|(i, &e)| (i, square(i32::from(e) - i32::from(a))))
+            .min_by_key(|&(_, e)| e)
+            .unwrap();
+        total_error += error;
+        indices |= (index as u64) << (i * 3);
+    }
+
+    (total_error, indices)
+}
+
+/// Encodes a RGBAx16 sequence of bytes to a 16 bytes DXT5 block
+fn encode_dxt5_block(source: &[u8], dest: &mut [u8]) {
+    assert!(source.len() == 64 && dest.len() == 16);
+
+    // perform dxt color encoding
+    encode_dxt_colors(source, &mut dest[8..16], false);
+
+    // copy out the alpha bytes
+    let mut alphas = [0; 16];
+    for i in 0..16 {
+        alphas[i] = source[i * 4 + 3];
+    }
+
+    // try both alpha compression methods, see which has the least error.
+    let alpha07 = alphas.iter().cloned().min().unwrap();
+    let alpha17 = alphas.iter().cloned().max().unwrap();
+    let (error7, indices7) = encode_dxt5_alpha(alpha07, alpha17, &alphas);
+
+    // if all alphas are 0 or 255 it doesn't particularly matter what we do here.
+    let alpha05 = alphas
+        .iter()
+        .cloned()
+        .filter(|&i| i != 255)
+        .max()
+        .unwrap_or(255);
+    let alpha15 = alphas
+        .iter()
+        .cloned()
+        .filter(|&i| i != 0)
+        .min()
+        .unwrap_or(0);
+    let (error5, indices5) = encode_dxt5_alpha(alpha05, alpha15, &alphas);
+
+    // pick the best one, encode the min/max values
+    let mut alpha_table = if error5 < error7 {
+        dest[0] = alpha05;
+        dest[1] = alpha15;
+        indices5
+    } else {
+        dest[0] = alpha07;
+        dest[1] = alpha17;
+        indices7
+    };
+
+    // encode the alphas
+    for byte in dest[2..8].iter_mut() {
+        *byte = alpha_table as u8;
+        alpha_table >>= 8;
+    }
+}
+
+/// Encodes a RGBAx16 sequence of bytes into a 16 bytes DXT3 block
+fn encode_dxt3_block(source: &[u8], dest: &mut [u8]) {
+    assert!(source.len() == 64 && dest.len() == 16);
+
+    // perform dxt color encoding
+    encode_dxt_colors(source, &mut dest[8..16], false);
+
+    // DXT3 alpha compression is very simple, just round towards the nearest value
+
+    // index the alpha values into the 64bit alpha table
+    let mut alpha_table = 0u64;
+    for i in 0..16 {
+        let alpha = u64::from(source[i * 4 + 3]);
+        let alpha = (alpha + 0x8) / 0x11;
+        alpha_table |= alpha << (i * 4);
+    }
+
+    // encode the alpha values
+    for byte in &mut dest[0..8] {
+        *byte = alpha_table as u8;
+        alpha_table >>= 8;
+    }
+}
+
+/// Encodes a RGBx16 sequence of bytes into a 8 bytes DXT1 block
+fn encode_dxt1_block(source: &[u8], dest: &mut [u8]) {
+    assert!(source.len() == 48 && dest.len() == 8);
+
+    // perform dxt color encoding
+    encode_dxt_colors(source, dest, true);
+}
+
+/// Decode a row of DXT1 data to four rows of RGBA data.
+/// source.len() should be a multiple of 8, otherwise this panics.
+fn encode_dxt1_row(source: &[u8]) -> Vec<u8> {
+    assert!(source.len() % 48 == 0);
+    let block_count = source.len() / 48;
+
+    let mut dest = vec![0u8; block_count * 8];
+    // contains the 16 decoded pixels per block
+    let mut decoded_block = [0u8; 48];
+
+    for (x, encoded_block) in dest.chunks_mut(8).enumerate() {
+        // copy the values from the decoded block to linewise RGB layout
+        for line in 0..4 {
+            let offset = (block_count * line + x) * 12;
+            decoded_block[line * 12..(line + 1) * 12].copy_from_slice(&source[offset..offset + 12]);
+        }
+
+        encode_dxt1_block(&decoded_block, encoded_block);
+    }
+    dest
+}
+
+/// Decode a row of DXT3 data to four rows of RGBA data.
+/// source.len() should be a multiple of 16, otherwise this panics.
+fn encode_dxt3_row(source: &[u8]) -> Vec<u8> {
+    assert!(source.len() % 64 == 0);
+    let block_count = source.len() / 64;
+
+    let mut dest = vec![0u8; block_count * 16];
+    // contains the 16 decoded pixels per block
+    let mut decoded_block = [0u8; 64];
+
+    for (x, encoded_block) in dest.chunks_mut(16).enumerate() {
+        // copy the values from the decoded block to linewise RGB layout
+        for line in 0..4 {
+            let offset = (block_count * line + x) * 16;
+            decoded_block[line * 16..(line + 1) * 16].copy_from_slice(&source[offset..offset + 16]);
+        }
+
+        encode_dxt3_block(&decoded_block, encoded_block);
+    }
+    dest
+}
+
+/// Decode a row of DXT5 data to four rows of RGBA data.
+/// source.len() should be a multiple of 16, otherwise this panics.
+fn encode_dxt5_row(source: &[u8]) -> Vec<u8> {
+    assert!(source.len() % 64 == 0);
+    let block_count = source.len() / 64;
+
+    let mut dest = vec![0u8; block_count * 16];
+    // contains the 16 decoded pixels per block
+    let mut decoded_block = [0u8; 64];
+
+    for (x, encoded_block) in dest.chunks_mut(16).enumerate() {
+        // copy the values from the decoded block to linewise RGB layout
+        for line in 0..4 {
+            let offset = (block_count * line + x) * 16;
+            decoded_block[line * 16..(line + 1) * 16].copy_from_slice(&source[offset..offset + 16]);
+        }
+
+        encode_dxt5_block(&decoded_block, encoded_block);
+    }
+    dest
+}