writepng.c

这是一个同样来自贝尔实验室的和UNIX有着渊源的操作系统, 其简洁的设计和实现易于我们学习和理解

/*
 * See PNG 1.2 spec, also RFC 2083.
 */
#include <u.h>
#include <libc.h>
#include <draw.h>
#include <memdraw.h>
#include <ctype.h>
#include <bio.h>
#include <flate.h>
#include "imagefile.h"

enum
{
	IDATSIZE = 	20000,
	FilterNone =	0,
};

typedef struct ZlibR ZlibR;
typedef struct ZlibW ZlibW;

struct ZlibR
{
	uchar *data;
	int width;
	int dx;
	int dy;
	int x;
	int y;
	int pixwid;
};

struct ZlibW
{
	Biobuf *io;
	uchar *buf;
	uchar *b;
	uchar *e;
};

static ulong *crctab;
static uchar PNGmagic[] = { 137, 'P', 'N', 'G', '\r', '\n', 26, '\n'};

static void
put4(uchar *a, ulong v)
{
	a[0] = v>>24;
	a[1] = v>>16;
	a[2] = v>>8;
	a[3] = v;
}

static void
chunk(Biobuf *bo, char *type, uchar *d, int n)
{
	uchar buf[4];
	ulong crc = 0;

	if(strlen(type) != 4)
		return;
	put4(buf, n);
	Bwrite(bo, buf, 4);
	Bwrite(bo, type, 4);
	Bwrite(bo, d, n);
	crc = blockcrc(crctab, crc, type, 4);
	crc = blockcrc(crctab, crc, d, n);
	put4(buf, crc);
	Bwrite(bo, buf, 4);
}

static int
zread(void *va, void *buf, int n)
{
	int a, i, pixels, pixwid;
	uchar *b, *e, *img;
	ZlibR *z;

	z = va;
	pixwid = z->pixwid;
	b = buf;
	e = b+n;
	while(b+pixwid <= e){
		if(z->y >= z->dy)
			break;
		if(z->x == 0)
			*b++ = FilterNone;
		pixels = (e-b)/pixwid;
		if(pixels > z->dx - z->x)
			pixels = z->dx - z->x;
		img = z->data + z->width*z->y + pixwid*z->x;
		memmove(b, img, pixwid*pixels);
		if(pixwid == 4){
			/*
			 * Convert to non-premultiplied alpha.
			 */
			for(i=0; i<pixels; i++, b+=4){
				a = b[3];
				if(a == 0)
					b[0] = b[1] = b[2] = 0;
				else if(a != 255){
					if(b[0] >= a)
						b[0] = a;
					b[0] = (b[0]*255)/a;
					if(b[1] >= a)
						b[1] = a;
					b[1] = (b[1]*255)/a;
					if(b[2] >= a)
						b[2] = a;
					b[2] = (b[2]*255)/a;
				}
			}
		}else	
			b += pixwid*pixels;

		z->x += pixels;
		if(z->x >= z->dx){
			z->x = 0;
			z->y++;
		}
	}
	return b - (uchar*)buf;
}

static void
IDAT(ZlibW *z)
{
	chunk(z->io, "IDAT", z->buf, z->b - z->buf);
	z->b = z->buf;
}

static int
zwrite(void *va, void *buf, int n)
{
	int m;
	uchar *b, *e;
	ZlibW *z;

	z = va;
	b = buf;
	e = b+n;

	while(b < e){
		m = z->e - z->b;
		if(m > e - b)
			m = e - b;
		memmove(z->b, b, m);
		z->b += m;
		b += m;
		if(z->b >= z->e)
			IDAT(z);
	}
	return n;
}

static Memimage*
memRGBA(Memimage *i)
{
	Memimage *ni;
	char buf[32];
	ulong dst;
	
	/*
	 * [A]BGR because we want R,G,B,[A] in big-endian order.  Sigh.
	 */
	chantostr(buf, i->chan);
	if(strchr(buf, 'a'))
		dst = ABGR32;
	else
		dst = BGR24;
		
	if(i->chan == dst)
		return i;

	ni = allocmemimage(i->r, dst);
	if(ni == nil)
		return ni;
	memimagedraw(ni, ni->r, i, i->r.min, nil, i->r.min, S);
	return ni;
}

char*
memwritepng(Biobuf *io, Memimage *m, ImageInfo *II)
{
	int err, n;
	uchar buf[200], *h;
	ulong vgamma;
	Tm *tm;
	Memimage *rgb;
	ZlibR zr;
	ZlibW zw;

	crctab = mkcrctab(0xedb88320);
	if(crctab == nil)
		sysfatal("mkcrctab error");
	deflateinit();

	rgb = memRGBA(m);
	if(rgb == nil)
		return "allocmemimage nil";

	Bwrite(io, PNGmagic, sizeof PNGmagic);

	/* IHDR chunk */
	h = buf;
	put4(h, Dx(m->r)); h += 4;
	put4(h, Dy(m->r)); h += 4;
	*h++ = 8;	/* 8 bits per channel */
	if(rgb->chan == BGR24)
		*h++ = 2;		/* RGB */
	else
		*h++ = 6;		/* RGBA */
	*h++ = 0;	/* compression - deflate */
	*h++ = 0;	/* filter - none */
	*h++ = 0;	/* interlace - none */
	chunk(io, "IHDR", buf, h-buf);

	/* time - using now is suspect */
	tm = gmtime(time(0));
	h = buf;
	*h++ = (tm->year + 1900)>>8;
	*h++ = (tm->year + 1900)&0xff;
	*h++ = tm->mon + 1;
	*h++ = tm->mday;
	*h++ = tm->hour;
	*h++ = tm->min;
	*h++ = tm->sec;
	chunk(io, "tIME", buf, h-buf);

	/* gamma */
	if(II->fields_set & II_GAMMA){
		vgamma = II->gamma*100000;
		put4(buf, vgamma);
		chunk(io, "gAMA", buf, 4);
	}

	/* comment */
	if(II->fields_set & II_COMMENT){
		strncpy((char*)buf, "Comment", sizeof buf);
		n = strlen((char*)buf)+1; // leave null between Comment and text
		strncpy((char*)(buf+n), II->comment, sizeof buf - n);
		chunk(io, "tEXt", buf, n+strlen((char*)buf+n));
	}

	/* image data */
	zr.dx = Dx(m->r);
	zr.dy = Dy(m->r);
	zr.width = rgb->width * sizeof(ulong);
	zr.data = rgb->data->bdata;
	zr.x = 0;
	zr.y = 0;
	zr.pixwid = chantodepth(rgb->chan)/8;
	zw.io = io;
	zw.buf = malloc(IDATSIZE);
	if(zw.buf == nil)
		sysfatal("malloc: %r");
	zw.b = zw.buf;
	zw.e = zw.b + IDATSIZE;
	if((err=deflatezlib(&zw, zwrite, &zr, zread, 6, 0)) < 0)
		sysfatal("deflatezlib %s", flateerr(err));
	if(zw.b > zw.buf)
		IDAT(&zw);
	free(zw.buf);
	chunk(io, "IEND", nil, 0);
	if(m != rgb)
		freememimage(rgb);
	return nil;
}