forks
/
arsd
mirror of https://github.com/adamdruppe/arsd.git
1
0
Fork 0
Code Issues Packages Projects Releases Wiki Activity

initial commit

This commit is contained in:
Adam D. Ruppe 2013-05-13 09:17:53 -04:00
parent 4480c0a4fa
commit 21ebf87b4e
1 changed files with 870 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

870
lazypng.d Normal file
View File

@ -0,0 +1,870 @@
module arsd.imagedraft;
import arsd.color;
import std.stdio;
import std.range;
import std.traits;
import std.exception;
import std.string;
import std.conv;
/*
struct Color {
ubyte r;
ubyte g;
ubyte b;
ubyte a;
string toString() {
return format("#%2x%2x%2x %2x", r, g, b, a);
}
}
*/
import simpledisplay;
struct RgbaScanline {
Color[] pixels;
}
auto convertToGreyscale(ImageLines)(ImageLines lines)
if(isInputRange!ImageLines && is(ElementType!ImageLines == RgbaScanline))
{
struct GreyscaleLines {
ImageLines lines;
bool isEmpty;
this(ImageLines lines) {
this.lines = lines;
if(!empty())
popFront(); // prime
}
int length() {
return lines.length;
}
bool empty() {
return isEmpty;
}
RgbaScanline current;
RgbaScanline front() {
return current;
}
void popFront() {
if(lines.empty()) {
isEmpty = true;
return;
}
auto old = lines.front();
current.pixels.length = old.pixels.length;
foreach(i, c; old.pixels) {
ubyte v = cast(ubyte) (
cast(int) c.r * 0.30 +
cast(int) c.g * 0.59 +
cast(int) c.b * 0.11);
current.pixels[i] = Color(v, v, v, c.a);
}
lines.popFront;
}
}
return GreyscaleLines(lines);
}
/// Lazily breaks the buffered input range into
/// png chunks, as defined in the PNG spec
///
/// Note: bufferedInputRange is defined in this file too.
LazyPngChunks!(Range) readPngChunks(Range)(Range r)
if(isBufferedInputRange!(Range) && is(ElementType!(Range) == ubyte[]))
{
// First, we need to check the header
// Then we'll lazily pull the chunks
while(r.front.length < 8) {
enforce(!r.empty(), "This isn't big enough to be a PNG file");
r.appendToFront();
}
enforce(r.front[0..8] == PNG_MAGIC_NUMBER,
"The file's magic number doesn't look like PNG");
r.consumeFromFront(8);
return LazyPngChunks!Range(r);
}
/// Same as above, but takes a regular input range instead of a buffered one.
/// Provided for easier compatibility with standard input ranges
/// (for example, std.stdio.File.byChunk)
auto readPngChunks(Range)(Range r)
if(!isBufferedInputRange!(Range) && isInputRange!(Range))
{
return readPngChunks(BufferedInputRange!Range(r));
}
/// Given an input range of bytes, return a lazy PNG file
auto pngFromBytes(Range)(Range r)
if(isInputRange!(Range) && is(ElementType!Range == ubyte[]))
{
auto chunks = readPngChunks(r);
auto file = LazyPngFile!(typeof(chunks))(chunks);
return file;
}
struct LazyPngChunks(T)
if(isBufferedInputRange!(T) && is(ElementType!T == ubyte[]))
{
T bytes;
Chunk current;
this(T range) {
bytes = range;
popFront(); // priming it
}
Chunk front() {
return current;
}
bool empty() {
return (bytes.front.length == 0 && bytes.empty);
}
void popFront() {
enforce(!empty());
while(bytes.front().length < 4) {
enforce(!bytes.empty,
format("Malformed PNG file - chunk size too short (%s < 4)",
bytes.front().length));
bytes.appendToFront();
}
Chunk n;
n.size |= bytes.front()[0] << 24;
n.size |= bytes.front()[1] << 16;
n.size |= bytes.front()[2] << 8;
n.size |= bytes.front()[3] << 0;
bytes.consumeFromFront(4);
while(bytes.front().length < n.size + 8) {
enforce(!bytes.empty,
format("Malformed PNG file - chunk too short (%s < %s)",
bytes.front.length, n.size));
bytes.appendToFront();
}
n.type[0 .. 4] = bytes.front()[0 .. 4];
bytes.consumeFromFront(4);
n.payload.length = n.size;
n.payload[0 .. n.size] = bytes.front()[0 .. n.size];
bytes.consumeFromFront(n.size);
n.checksum |= bytes.front()[0] << 24;
n.checksum |= bytes.front()[1] << 16;
n.checksum |= bytes.front()[2] << 8;
n.checksum |= bytes.front()[3] << 0;
bytes.consumeFromFront(4);
enforce(n.checksum == crcPng(n.stype, n.payload), "Chunk checksum didn't match");
current = n;
}
}
/// Lazily reads out basic info from a png (header, palette, image data)
/// It will only allocate memory to read a palette, and only copies on
/// the header and the palette. It ignores everything else.
///
/// FIXME: it doesn't handle interlaced files.
struct LazyPngFile(LazyPngChunksProvider)
if(isInputRange!(LazyPngChunksProvider) &&
is(ElementType!(LazyPngChunksProvider) == Chunk))
{
LazyPngChunksProvider chunks;
this(LazyPngChunksProvider chunks) {
enforce(!chunks.empty(), "There are no chunks in this png");
header = PngHeader.fromChunk(chunks.front());
chunks.popFront();
// And now, find the datastream so we're primed for lazy
// reading, saving the palette and transparency info, if
// present
chunkLoop:
while(!chunks.empty()) {
auto chunk = chunks.front();
switch(chunks.front.stype) {
case "PLTE":
// if it is in color, palettes are
// always stored as 8 bit per channel
// RGB triplets Alpha is stored elsewhere.
// FIXME: doesn't do greyscale palettes!
enforce(chunk.size % 3 == 0);
palette.length = chunk.size / 3;
auto offset = 0;
foreach(i; 0 .. palette.length) {
palette[i] = Color(
chunk.payload[offset+0],
chunk.payload[offset+1],
chunk.payload[offset+2],
255);
offset += 3;
}
break;
case "tRNS":
// 8 bit channel in same order as
// palette
enforce(chunk.size < palette.length);
foreach(i, a; chunk.payload)
palette[i].a = a;
break;
case "IDAT":
// leave the datastream for later
break chunkLoop;
default:
// ignore chunks we don't care about
}
chunks.popFront();
}
this.chunks = chunks;
enforce(!chunks.empty() && chunks.front().stype == "IDAT",
"Malformed PNG file - no image data is present");
}
/// Lazily reads and decompresses the image datastream, returning chunkSize bytes of
/// it per front. It does *not* change anything, so the filter byte is still there.
///
/// If chunkSize == 0, it automatically calculates chunk size to give you data by line.
auto rawDatastreamByChunk(int chunkSize = 0) {
assert(chunks.front().stype == "IDAT");
if(chunkSize == 0)
chunkSize = bytesPerLine();
struct DatastreamByChunk(T) {
std.zlib.UnCompress decompressor;
int chunkSize;
T chunks;
this(int cs, T chunks) {
decompressor = new std.zlib.UnCompress();
this.chunkSize = cs;
this.chunks = chunks;
popFront(); // priming
}
ubyte[] front() {
assert(current.length == chunkSize);
return current;
}
ubyte[] current;
ubyte[] buffer;
void popFront() {
if(buffer.length < chunkSize) {
if(chunks.front().stype != "IDAT") {
buffer ~= cast(ubyte[]) decompressor.flush();
if(buffer.length != 0)
goto stillMore;
current = null;
buffer = null;
return;
}
buffer ~= cast(ubyte[])
decompressor.uncompress(chunks.front().payload);
chunks.popFront();
}
stillMore:
current = buffer[0 .. chunkSize];
buffer = buffer[chunkSize .. $];
}
bool empty() {
return (current.length == 0);
}
}
auto range = DatastreamByChunk!(typeof(chunks))(chunkSize, chunks);
return range;
}
auto byRgbaScanline() {
static struct ByRgbaScanline {
ReturnType!(rawDatastreamByChunk) datastream;
RgbaScanline current;
PngHeader header;
int bpp;
Color[] palette;
bool isEmpty = false;
bool empty() {
return isEmpty;
}
int length() {
return header.height;
}
// This is needed for the filter algorithms
immutable(ubyte)[] previousLine;
// FIXME: I think my range logic got screwed somewhere
// in the stack... this is messed up.
void popFront() {
assert(!empty());
if(datastream.empty()) {
isEmpty = true;
return;
}
current.pixels.length = header.width;
// ensure it is primed
if(datastream.front.length == 0)
datastream.popFront;
auto rawData = datastream.front();
auto filter = rawData[0];
auto data = unfilter(filter, rawData[1 .. $], previousLine, bpp);
if(data.length == 0) {
isEmpty = true;
return;
}
assert(data.length);
previousLine = data;
// FIXME: if it's rgba, this could probably be faster
assert(header.depth == 8,
"Sorry, depths other than 8 aren't implemented yet.");
auto offset = 0;
foreach(i; 0 .. header.width) {
switch(header.type) {
case 0: // greyscale
case 4: // grey with alpha
auto value = data[offset++];
current.pixels[i] = Color(
value,
value,
value,
(header.type == 4)
? data[offset++] : 255
);
break;
case 3: // indexed
current.pixels[i] = palette[data[offset++]];
break;
case 2: // truecolor
case 6: // true with alpha
current.pixels[i] = Color(
data[offset++],
data[offset++],
data[offset++],
(header.type == 6)
? data[offset++] : 255
);
break;
default:
throw new Exception("invalid png file");
}
}
assert(offset == data.length);
if(!datastream.empty())
datastream.popFront();
}
RgbaScanline front() {
return current;
}
}
assert(chunks.front.stype == "IDAT");
ByRgbaScanline range;
range.header = header;
range.bpp = bytesPerPixel;
range.palette = palette;
range.datastream = rawDatastreamByChunk(bytesPerLine());
range.popFront();
return range;
}
int bytesPerPixel() {
immutable bitsPerChannel = header.depth;
int bitsPerPixel = bitsPerChannel;
if(header.type & 2 && !(header.type & 1)) // in color, but no palette
bitsPerPixel *= 3;
if(header.type & 4) // has alpha channel
bitsPerPixel += bitsPerChannel;
return (bitsPerPixel + 7) / 8;
}
// FIXME: doesn't handle interlacing... I think
int bytesPerLine() {
immutable bitsPerChannel = header.depth;
int bitsPerPixel = bitsPerChannel;
if(header.type & 2 && !(header.type & 1)) // in color, but no palette
bitsPerPixel *= 3;
if(header.type & 4) // has alpha channel
bitsPerPixel += bitsPerChannel;
immutable int sizeInBits = header.width * bitsPerPixel;
// need to round up to the nearest byte
int sizeInBytes = (sizeInBits + 7) / 8;
return sizeInBytes + 1; // the +1 is for the filter byte that precedes all lines
}
PngHeader header;
Color[] palette;
}
/**************************************************
* Buffered input range - generic, non-image code
***************************************************/
/// Is the given range a buffered input range? That is, an input range
/// that also provides consumeFromFront(int) and appendToFront()
template isBufferedInputRange(R) {
enum bool isBufferedInputRange =
isInputRange!(R) && is(typeof(
{
R r;
r.consumeFromFront(0);
r.appendToFront();
}()));
}
/// Allows appending to front on a regular input range, if that range is
/// an array. It appends to the array rather than creating an array of
/// arrays; it's meant to make the illusion of one continuous front rather
/// than simply adding capability to walk backward to an existing input range.
///
/// I think something like this should be standard; I find File.byChunk
/// to be almost useless without this capability.
// FIXME: what if Range is actually an array itself? We should just use
// slices right into it... I guess maybe r.front() would be the whole
// thing in that case though, so we would indeed be slicing in right now.
// Gotta check it though.
struct BufferedInputRange(Range)
if(isInputRange!(Range) && isArray!(ElementType!(Range)))
{
private Range underlyingRange;
private ElementType!(Range) buffer;
/// Creates a buffer for the given range. You probably shouldn't
/// keep using the underlying range directly.
///
/// It assumes the underlying range has already been primed.
this(Range r) {
underlyingRange = r;
// Is this really correct? Want to make sure r.front
// is valid but it doesn't necessarily need to have
// more elements...
enforce(!r.empty());
buffer = r.front();
usingUnderlyingBuffer = true;
}
/// Forwards to the underlying range's empty function
bool empty() {
return underlyingRange.empty();
}
/// Returns the current buffer
ElementType!(Range) front() {
return buffer;
}
// actually, not terribly useful IMO. appendToFront calls it
// implicitly when necessary
/// Discard the current buffer and get the next item off the
/// underlying range. Be sure to call at least once to prime
/// the range (after checking if it is empty, of course)
void popFront() {
enforce(!empty());
underlyingRange.popFront();
buffer = underlyingRange.front();
usingUnderlyingBuffer = true;
}
bool usingUnderlyingBuffer = false;
/// Remove the first count items from the buffer
void consumeFromFront(int count) {
buffer = buffer[count .. $];
}
/// Append the next item available on the underlying range to
/// our buffer.
void appendToFront() {
if(buffer.length == 0) {
// may let us reuse the underlying range's buffer,
// hopefully avoiding an extra allocation
popFront();
} else {
enforce(!underlyingRange.empty());
// need to make sure underlyingRange.popFront doesn't overwrite any
// of our buffer...
if(usingUnderlyingBuffer) {
buffer = buffer.dup;
usingUnderlyingBuffer = false;
}
underlyingRange.popFront();
buffer ~= underlyingRange.front();
}
}
}
/**************************************************
* Lower level implementations of image formats.
* and associated helper functions.
*
* Related to the module, but not particularly
* interesting, so it's at the bottom.
***************************************************/
/* PNG file format implementation */
import std.zlib;
import std.math;
/// All PNG files are supposed to open with these bytes according to the spec
enum immutable(ubyte[]) PNG_MAGIC_NUMBER = [0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a];
/// A PNG file consists of the magic number then a stream of chunks. This
/// struct represents those chunks.
struct Chunk {
uint size;
ubyte[4] type;
ubyte[] payload;
uint checksum;
/// returns the type as a string for easier comparison with literals
string stype() const {
return cast(string) type;
}
static Chunk* create(string type, ubyte[] payload)
in {
assert(type.length == 4);
}
body {
Chunk* c = new Chunk;
c.size = payload.length;
c.type = cast(ubyte[]) type;
c.payload = payload;
c.checksum = crcPng(type, payload);
return c;
}
/// Puts it into the format for outputting to a file
ubyte[] toArray() {
ubyte[] a;
a.length = size + 12;
int pos = 0;
a[pos++] = (size & 0xff000000) >> 24;
a[pos++] = (size & 0x00ff0000) >> 16;
a[pos++] = (size & 0x0000ff00) >> 8;
a[pos++] = (size & 0x000000ff) >> 0;
a[pos .. pos + 4] = type[0 .. 4];
pos += 4;
a[pos .. pos + size] = payload[0 .. size];
pos += size;
assert(checksum);
a[pos++] = (checksum & 0xff000000) >> 24;
a[pos++] = (checksum & 0x00ff0000) >> 16;
a[pos++] = (checksum & 0x0000ff00) >> 8;
a[pos++] = (checksum & 0x000000ff) >> 0;
return a;
}
}
/// The first chunk in a PNG file is a header that contains this info
struct PngHeader {
/// Width of the image, in pixels.
uint width;
/// Height of the image, in pixels.
uint height;
/**
This is bits per channel - per color for truecolor or grey
and per pixel for palette.
Indexed ones can have depth of 1,2,4, or 8,
Greyscale can be 1,2,4,8,16
Everything else must be 8 or 16.
*/
ubyte depth = 8;
/** Types from the PNG spec:
0 - greyscale
2 - truecolor
3 - indexed color
4 - grey with alpha
6 - true with alpha
1, 5, and 7 are invalid.
There's a kind of bitmask going on here:
If type&1, it has a palette.
If type&2, it is in color.
If type&4, it has an alpha channel in the datastream.
*/
ubyte type = 6;
ubyte compressionMethod = 0; /// should be zero
ubyte filterMethod = 0; /// should be zero
/// 0 is non interlaced, 1 if Adam7. No more are defined in the spec
ubyte interlaceMethod = 0;
static PngHeader fromChunk(in Chunk c) {
enforce(c.stype == "IHDR",
"The chunk is not an image header");
PngHeader h;
auto data = c.payload;
int pos = 0;
enforce(data.length == 13,
"Malformed PNG file - the IHDR is the wrong size");
h.width |= data[pos++] << 24;
h.width |= data[pos++] << 16;
h.width |= data[pos++] << 8;
h.width |= data[pos++] << 0;
h.height |= data[pos++] << 24;
h.height |= data[pos++] << 16;
h.height |= data[pos++] << 8;
h.height |= data[pos++] << 0;
h.depth = data[pos++];
h.type = data[pos++];
h.compressionMethod = data[pos++];
h.filterMethod = data[pos++];
h.interlaceMethod = data[pos++];
return h;
}
Chunk* toChunk() {
ubyte[] data;
data.length = 13;
int pos = 0;
data[pos++] = width >> 24;
data[pos++] = (width >> 16) & 0xff;
data[pos++] = (width >> 8) & 0xff;
data[pos++] = width & 0xff;
data[pos++] = height >> 24;
data[pos++] = (height >> 16) & 0xff;
data[pos++] = (height >> 8) & 0xff;
data[pos++] = height & 0xff;
data[pos++] = depth;
data[pos++] = type;
data[pos++] = compressionMethod;
data[pos++] = filterMethod;
data[pos++] = interlaceMethod;
assert(pos == 13);
return Chunk.create("IHDR", data);
}
}
void writePng(OutputRange, InputRange)(OutputRange where, InputRange image)
if(
isOutputRange!(OutputRange, ubyte[]) &&
isInputRange!(InputRange) &&
is(ElementType!InputRange == RgbaScanline))
{
where.put(PNG_MAGIC_NUMBER);
PngHeader header;
assert(!image.empty());
// using the default values for header here... FIXME not super clear
header.width = image.front.pixels.length;
header.height = image.length;
enforce(header.width > 0, "Image width <= 0");
enforce(header.height > 0, "Image height <= 0");
where.put(header.toChunk().toArray());
auto compressor = new std.zlib.Compress();
const(void)[] compressedData;
int cnt;
foreach(line; image) {
// YOU'VE GOT TO BE FUCKING KIDDING ME!
// I have to /cast/ to void[]!??!?
ubyte[] data;
data.length = 1 + header.width * 4;
data[0] = 0; // filter type
int offset = 1;
foreach(pixel; line.pixels) {
data[offset++] = pixel.r;
data[offset++] = pixel.g;
data[offset++] = pixel.b;
data[offset++] = pixel.a;
}
compressedData ~= compressor.compress(cast(void[])
data);
if(compressedData.length > 2_000) {
where.put(Chunk.create("IDAT", cast(ubyte[])
compressedData).toArray());
compressedData = null;
}
cnt++;
}
assert(cnt == header.height, format("Got %d lines instead of %d", cnt, header.height));
compressedData ~= compressor.flush();
if(compressedData.length)
where.put(Chunk.create("IDAT", cast(ubyte[])
compressedData).toArray());
where.put(Chunk.create("IEND", null).toArray());
}
// bKGD - palette entry for background or the RGB (16 bits each) for that. or 16 bits of grey
// Private: used for checking/updating PNG checksums
uint update_crc(in uint crc, in ubyte[] buf){
static const uint[256] crc_table = [0, 1996959894, 3993919788, 2567524794, 124634137, 1886057615, 3915621685, 2657392035, 249268274, 2044508324, 3772115230, 2547177864, 162941995, 2125561021, 3887607047, 2428444049, 498536548, 1789927666, 4089016648, 2227061214, 450548861, 1843258603, 4107580753, 2211677639, 325883990, 1684777152, 4251122042, 2321926636, 335633487, 1661365465, 4195302755, 2366115317, 997073096, 1281953886, 3579855332, 2724688242, 1006888145, 1258607687, 3524101629, 2768942443, 901097722, 1119000684, 3686517206, 2898065728, 853044451, 1172266101, 3705015759, 2882616665, 651767980, 1373503546, 3369554304, 3218104598, 565507253, 1454621731, 3485111705, 3099436303, 671266974, 1594198024, 3322730930, 2970347812, 795835527, 1483230225, 3244367275, 3060149565, 1994146192, 31158534, 2563907772, 4023717930, 1907459465, 112637215, 2680153253, 3904427059, 2013776290, 251722036, 2517215374, 3775830040, 2137656763, 141376813, 2439277719, 3865271297, 1802195444, 476864866, 2238001368, 4066508878, 1812370925, 453092731, 2181625025, 4111451223, 1706088902, 314042704, 2344532202, 4240017532, 1658658271, 366619977, 2362670323, 4224994405, 1303535960, 984961486, 2747007092, 3569037538, 1256170817, 1037604311, 2765210733, 3554079995, 1131014506, 879679996, 2909243462, 3663771856, 1141124467, 855842277, 2852801631, 3708648649, 1342533948, 654459306, 3188396048, 3373015174, 1466479909, 544179635, 3110523913, 3462522015, 1591671054, 702138776, 2966460450, 3352799412, 1504918807, 783551873, 3082640443, 3233442989, 3988292384, 2596254646, 62317068, 1957810842, 3939845945, 2647816111, 81470997, 1943803523, 3814918930, 2489596804, 225274430, 2053790376, 3826175755, 2466906013, 167816743, 2097651377, 4027552580, 2265490386, 503444072, 1762050814, 4150417245, 2154129355, 426522225, 1852507879, 4275313526, 2312317920, 282753626, 1742555852, 4189708143, 2394877945, 397917763, 1622183637, 3604390888, 2714866558, 953729732, 1340076626, 3518719985, 2797360999, 1068828381, 1219638859, 3624741850, 2936675148, 906185462, 1090812512, 3747672003, 2825379669, 829329135, 1181335161, 3412177804, 3160834842, 628085408, 1382605366, 3423369109, 3138078467, 570562233, 1426400815, 3317316542, 2998733608, 733239954, 1555261956, 3268935591, 3050360625, 752459403, 1541320221, 2607071920, 3965973030, 1969922972, 40735498, 2617837225, 3943577151, 1913087877, 83908371, 2512341634, 3803740692, 2075208622, 213261112, 2463272603, 3855990285, 2094854071, 198958881, 2262029012, 4057260610, 1759359992, 534414190, 2176718541, 4139329115, 1873836001, 414664567, 2282248934, 4279200368, 1711684554, 285281116, 2405801727, 4167216745, 1634467795, 376229701, 2685067896, 3608007406, 1308918612, 956543938, 2808555105, 3495958263, 1231636301, 1047427035, 2932959818, 3654703836, 1088359270, 936918000, 2847714899, 3736837829, 1202900863, 817233897, 3183342108, 3401237130, 1404277552, 615818150, 3134207493, 3453421203, 1423857449, 601450431, 3009837614, 3294710456, 1567103746, 711928724, 3020668471, 3272380065, 1510334235, 755167117];
uint c = crc;
foreach(b; buf)
c = crc_table[(c ^ b) & 0xff] ^ (c >> 8);
return c;
}
uint crcPng(in string chunkName, in ubyte[] buf){
uint c = update_crc(0xffffffffL, cast(ubyte[]) chunkName);
return update_crc(c, buf) ^ 0xffffffffL;
}
immutable(ubyte)[] unfilter(ubyte filterType, in ubyte[] data, in ubyte[] previousLine, int bpp) {
// Note: the overflow arithmetic on the ubytes in here is intentional
switch(filterType) {
case 0:
return data.idup; // FIXME is copying really necessary?
case 1:
auto arr = data.dup;
// first byte gets zero added to it so nothing special
foreach(i; bpp .. arr.length) {
arr[i] += arr[i - bpp];
}
return assumeUnique(arr);
case 2:
auto arr = data.dup;
foreach(i; 0 .. arr.length) {
arr[i] += previousLine[i];
}
return assumeUnique(arr);
case 3:
auto arr = data.dup;
foreach(i; 0 .. arr.length) {
auto prev = i < bpp ? 0 : arr[i - bpp];
arr[i] += cast(ubyte)
std.math.floor( cast(int) (prev + previousLine[i]) / 2);
}
return assumeUnique(arr);
case 4:
auto arr = data.dup;
foreach(i; 0 .. arr.length) {
ubyte prev = i < bpp ? 0 : arr[i - bpp];
ubyte prevLL = i < bpp ? 0 : previousLine[i - bpp];
arr[i] += PaethPredictor(prev, previousLine[i], prevLL);
}
return assumeUnique(arr);
default:
throw new Exception("invalid PNG file, bad filter type");
}
}
ubyte PaethPredictor(ubyte a, ubyte b, ubyte c) {
int p = cast(int) a + b - c;
auto pa = abs(p - a);
auto pb = abs(p - b);
auto pc = abs(p - c);
if(pa <= pb && pa <= pc)
return a;
if(pb <= pc)
return b;
return c;
}