ref: c116550e6a41572796e4db65e4f6acbcb3d9d6f8
dir: /appl/lib/writegif.b/
implement WImagefile;
include "sys.m";
sys: Sys;
include "draw.m";
draw: Draw;
Chans, Display, Image, Rect: import draw;
include "bufio.m";
bufio: Bufio;
Iobuf: import bufio;
include "imagefile.m";
Nhash: con 4001;
Entry: adt
{
index: int;
prefix: int;
exten: int;
next: cyclic ref Entry;
};
IO: adt
{
fd: ref Iobuf;
buf: array of byte;
i: int;
nbits: int; # bits in right side of shift register
sreg: int; # shift register
};
tbl: array of ref Entry;
colormap: array of array of byte;
log2 := array[] of {1 => 0, 2 => 1, 4 => 2, 8 => 3, * => -1};
init(iomod: Bufio)
{
if(sys == nil){
sys = load Sys Sys->PATH;
draw = load Draw Draw->PATH;
}
bufio = iomod;
}
writeimage(fd: ref Iobuf, image: ref Image): string
{
case image.chans.desc {
(Draw->GREY1).desc or (Draw->GREY2).desc or
(Draw->GREY4).desc or (Draw->GREY8).desc or
(Draw->CMAP8).desc =>
if(image.depth > 8 || (image.depth&(image.depth-1)) != 0)
return "inconsistent depth";
* =>
return "unsupported channel type";
}
inittbl();
writeheader(fd, image);
writedescriptor(fd, image);
err := writedata(fd, image);
if(err != nil)
return err;
writetrailer(fd);
fd.flush();
return err;
}
inittbl()
{
tbl = array[4096] of ref Entry;
for(i:=0; i<len tbl; i++)
tbl[i] = ref Entry(i, -1, i, nil);
}
# Write header, logical screen descriptor, and color map
writeheader(fd: ref Iobuf, image: ref Image): string
{
# Header
fd.puts("GIF89a");
# Logical Screen Descriptor
put2(fd, image.r.dx());
put2(fd, image.r.dy());
# color table present, 4 bits per color (for RGBV best case), size of color map
fd.putb(byte ((1<<7)|(3<<4)|(image.depth-1)));
fd.putb(byte 0); # white background (doesn't matter anyway)
fd.putb(byte 0); # pixel aspect ratio - unused
# Global Color Table
getcolormap(image);
ldepth := log2[image.depth];
if(image.chans.eq(Draw->GREY8))
ldepth = 4;
fd.write(colormap[ldepth], len colormap[ldepth]);
return nil;
}
# Write image descriptor
writedescriptor(fd: ref Iobuf, image: ref Image)
{
# Image Separator
fd.putb(byte 16r2C);
# Left, top, width, height
put2(fd, 0);
put2(fd, 0);
put2(fd, image.r.dx());
put2(fd, image.r.dy());
# no special processing
fd.putb(byte 0);
}
# Write data
writedata(fd: ref Iobuf, image: ref Image): string
{
# LZW Minimum code size
if(image.depth == 1)
fd.putb(byte 2);
else
fd.putb(byte image.depth);
# Encode and emit the data
err := encode(fd, image);
if(err != nil)
return err;
# Block Terminator
fd.putb(byte 0);
return nil;
}
# Write data
writetrailer(fd: ref Iobuf)
{
fd.putb(byte 16r3B);
}
# Write little-endian 16-bit integer
put2(fd: ref Iobuf, i: int)
{
fd.putb(byte i);
fd.putb(byte (i>>8));
}
# Get color map for all ldepths, in format suitable for writing out
getcolormap(image: ref Draw->Image)
{
if(colormap != nil)
return;
colormap = array[5] of array of byte;
display := image.display;
colormap[4] = array[3*256] of byte;
colormap[3] = array[3*256] of byte;
colormap[2] = array[3*16] of byte;
colormap[1] = array[3*4] of byte;
colormap[0] = array[3*2] of byte;
c := colormap[4];
for(i:=0; i<256; i++){
c[3*i+0] = byte i;
c[3*i+1] = byte i;
c[3*i+2] = byte i;
}
c = colormap[3];
for(i=0; i<256; i++){
(r, g, b) := display.cmap2rgb(i);
c[3*i+0] = byte r;
c[3*i+1] = byte g;
c[3*i+2] = byte b;
}
c = colormap[2];
for(i=0; i<16; i++){
col := (i<<4)|i;
(r, g, b) := display.cmap2rgb(col);
c[3*i+0] = byte r;
c[3*i+1] = byte g;
c[3*i+2] = byte b;
}
c = colormap[1];
for(i=0; i<4; i++){
col := (i<<6)|(i<<4)|(i<<2)|i;
(r, g, b) := display.cmap2rgb(col);
c[3*i+0] = byte r;
c[3*i+1] = byte g;
c[3*i+2] = byte b;
}
c = colormap[0];
for(i=0; i<2; i++){
if(i == 0)
col := 0;
else
col = 16rFF;
(r, g, b) := display.cmap2rgb(col);
c[3*i+0] = byte r;
c[3*i+1] = byte g;
c[3*i+2] = byte b;
}
}
# Put n bits of c into output at io.buf[i];
output(io: ref IO, c, n: int)
{
if(c < 0){
if(io.nbits != 0)
io.buf[io.i++] = byte io.sreg;
io.fd.putb(byte io.i);
io.fd.write(io.buf, io.i);
io.nbits = 0;
return;
}
if(io.nbits+n >= 31){
sys->print("panic: WriteGIF sr overflow\n");
exit;
}
io.sreg |= c<<io.nbits;
io.nbits += n;
while(io.nbits >= 8){
io.buf[io.i++] = byte io.sreg;
io.sreg >>= 8;
io.nbits -= 8;
}
if(io.i >= 255){
io.fd.putb(byte 255);
io.fd.write(io.buf, 255);
io.buf[0:] = io.buf[255:io.i];
io.i -= 255;
}
}
# LZW encoder
encode(fd: ref Iobuf, image: ref Image): string
{
c, h, csize, prefix: int;
e, oe: ref Entry;
first := 1;
ld := log2[image.depth];
# ldepth 0 must generate codesize 2 with values 0 and 1 (see the spec.)
ld0 := ld;
if(ld0 == 0)
ld0 = 1;
codesize := (1<<ld0);
CTM := 1<<codesize;
EOD := CTM+1;
io := ref IO (fd, array[300] of byte, 0, 0, 0);
sreg := 0;
nbits := 0;
bitsperpixel := 1<<ld;
pm := (1<<bitsperpixel)-1;
# Read image data into memory
# potentially one extra byte on each end of each scan line
data := array[image.r.dy()*(2+(image.r.dx()>>(3-log2[image.depth])))] of byte;
ndata := image.readpixels(image.r, data);
if(ndata < 0)
return sys->sprint("WriteGIF: readpixels: %r");
datai := 0;
x := image.r.min.x;
Init:
for(;;){
csize = codesize+1;
nentry := EOD+1;
maxentry := (1<<csize);
hash := array[Nhash] of ref Entry;
for(i := 0; i<nentry; i++){
e = tbl[i];
h = (e.prefix<<24) | (e.exten<<8);
h %= Nhash;
if(h < 0)
h += Nhash;
e.next = hash[h];
hash[h] = e;
}
prefix = -1;
if(first)
output(io, CTM, csize);
first = 0;
# Scan over pixels. Because of partially filled bytes on ends of scan lines,
# which must be ignored in the data stream passed to GIF, this is more
# complex than we'd like
Next:
for(;;){
if(ld != 3){
# beginning of scan line is difficult; prime the shift register
if(x == image.r.min.x){
if(datai == ndata)
break;
sreg = int data[datai++];
nbits = 8-((x&(7>>ld))<<ld);
}
x++;
if(x == image.r.max.x)
x = image.r.min.x;
}
if(nbits == 0){
if(datai == ndata)
break;
sreg = int data[datai++];
nbits = 8;
}
nbits -= bitsperpixel;
c = sreg>>nbits & pm;
h = prefix<<24 | c<<8;
h %= Nhash;
if(h < 0)
h += Nhash;
oe = nil;
for(e = hash[h]; e!=nil; e=e.next){
if(e.prefix == prefix && e.exten == c){
if(oe != nil){
oe.next = e.next;
e.next = hash[h];
hash[h] = e;
}
prefix = e.index;
continue Next;
}
oe = e;
}
output(io, prefix, csize);
early:=0; # peculiar tiff feature here for reference
if(nentry == maxentry-early){
if(csize == 12){
nbits += codesize; # unget pixel
x--;
output(io, CTM, csize);
continue Init;
}
csize++;
maxentry = (1<<csize);
}
e = tbl[nentry];
e.prefix = prefix;
e.exten = c;
e.next = hash[h];
hash[h] = e;
prefix = c;
nentry++;
}
break Init;
}
output(io, prefix, csize);
output(io, EOD, csize);
output(io, -1, csize);
return nil;
}