readjpg.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

				if(nzeros >= 0){
					if(acc[k] != 0)
						increment(h, acc, k, qt[k]<<Al);
					else if(nzeros-- == 0)
						acc[k] = pending;
					k++;
					continue;
				}
				rs = decode(h, acht);
				nibbles(rs, &rrrr, &ssss);
				if(ssss == 0){
					if(rrrr < 15){
						eobrun = 0;
						if(rrrr > 0)
							eobrun = receiveEOB(h, rrrr)-1;
						while(k <= Se){
							increment(h, acc, k, qt[k]<<Al);
							k++;
						}
						break;
					}
					for(i=0; i<16; k++){
						increment(h, acc, k, qt[k]<<Al);
						if(acc[k] == 0)
							i++;
					}
					continue;
				}else if(ssss != 1)
					jpgerror(h, "ReadJPG: ssss!=1 in progressive increment");
				nzeros = rrrr;
				pending = receivebit(h);
				if(pending == 0)
					pending = -1;
				pending *= qt[k]<<Al;
			}
		}

		/* process restart marker, if present */
		mcu++;
		if(ri>0 && mcu<nmcu && mcu%ri==0){
			restart(h, mcu);
			eobrun = 0;
			nzeros = -1;
		}
	}
}

static
void
progressivescan(Header *h, int colorspace)
{
	uchar *ss;
	int Ns, Ss, Ah, Al, c, comp, i;

	if(h->dccoeff[0] == nil)
		progressiveinit(h, colorspace);

	ss = h->ss;
	Ns = ss[0];
	Ss = ss[1+2*Ns];
	nibbles(ss[3+2*Ns], &Ah, &Al);
	c = ss[1];
	comp = -1;
	for(i=0; i<h->Nf; i++)
		if(h->comp[i].C == c)
			comp = i;
	if(comp == -1)
		jpgerror(h, "ReadJPG: bad component index in scan header");

	if(Ss == 0){
		progressivedc(h, comp, Ah, Al);
		return;
	}
	if(Ah == 0){
		progressiveac(h, comp, Al);
		return;
	}
	progressiveacinc(h, comp, Al);
}

enum {
	c1 = 2871,	/* 1.402 * 2048 */
	c2 = 705,		/* 0.34414 * 2048 */
	c3 = 1463,	/* 0.71414 * 2048 */
	c4 = 3629,	/* 1.772 * 2048 */
};

static
void
colormap1(Header *h, int colorspace, Rawimage *image, int data[8*8], int mcu, int nacross)
{
	uchar *pic;
	int x, y, dx, dy, minx, miny;
	int r, k, pici;

	USED(colorspace);
	pic = image->chans[0];
	minx = 8*(mcu%nacross);
	dx = 8;
	if(minx+dx > h->X)
		dx = h->X-minx;
	miny = 8*(mcu/nacross);
	dy = 8;
	if(miny+dy > h->Y)
		dy = h->Y-miny;
	pici = miny*h->X+minx;
	k = 0;
	for(y=0; y<dy; y++){
		for(x=0; x<dx; x++){
			r = clamp[(data[k+x]+128)+CLAMPOFF];
			pic[pici+x] = r;
		}
		pici += h->X;
		k += 8;
	}
}

static
void
colormapall1(Header *h, int colorspace, Rawimage *image, int data0[8*8], int data1[8*8], int data2[8*8], int mcu, int nacross)
{
	uchar *rpic, *gpic, *bpic, *rp, *gp, *bp;
	int *p0, *p1, *p2;
	int x, y, dx, dy, minx, miny;
	int r, g, b, k, pici;
	int Y, Cr, Cb;

	rpic = image->chans[0];
	gpic = image->chans[1];
	bpic = image->chans[2];
	minx = 8*(mcu%nacross);
	dx = 8;
	if(minx+dx > h->X)
		dx = h->X-minx;
	miny = 8*(mcu/nacross);
	dy = 8;
	if(miny+dy > h->Y)
		dy = h->Y-miny;
	pici = miny*h->X+minx;
	k = 0;
	for(y=0; y<dy; y++){
		p0 = data0+k;
		p1 = data1+k;
		p2 = data2+k;
		rp = rpic+pici;
		gp = gpic+pici;
		bp = bpic+pici;
		if(colorspace == CYCbCr)
			for(x=0; x<dx; x++){
				*rp++ = clamp[*p0++ + 128 + CLAMPOFF];
				*gp++ = clamp[*p1++ + 128 + CLAMPOFF];
				*bp++ = clamp[*p2++ + 128 + CLAMPOFF];
			}
		else
			for(x=0; x<dx; x++){
				Y = (*p0++ + 128) << 11;
				Cb = *p1++;
				Cr = *p2++;
				r = Y+c1*Cr;
				g = Y-c2*Cb-c3*Cr;
				b = Y+c4*Cb;
				*rp++ = clamp[(r>>11)+CLAMPOFF];
				*gp++ = clamp[(g>>11)+CLAMPOFF];
				*bp++ = clamp[(b>>11)+CLAMPOFF];
			}
		pici += h->X;
		k += 8;
	}
}

static
void
colormap(Header *h, int colorspace, Rawimage *image, int *data0[8*8], int *data1[8*8], int *data2[8*8], int mcu, int nacross, int Hmax, int Vmax,  int *H, int *V)
{
	uchar *rpic, *gpic, *bpic;
	int x, y, dx, dy, minx, miny;
	int r, g, b, pici, H0, H1, H2;
	int t, b0, b1, b2, y0, y1, y2, x0, x1, x2;
	int Y, Cr, Cb;

	rpic = image->chans[0];
	gpic = image->chans[1];
	bpic = image->chans[2];
	minx = 8*Hmax*(mcu%nacross);
	dx = 8*Hmax;
	if(minx+dx > h->X)
		dx = h->X-minx;
	miny = 8*Vmax*(mcu/nacross);
	dy = 8*Vmax;
	if(miny+dy > h->Y)
		dy = h->Y-miny;
	pici = miny*h->X+minx;
	H0 = H[0];
	H1 = H[1];
	H2 = H[2];
	for(y=0; y<dy; y++){
		t = y*V[0];
		b0 = H0*(t/(8*Vmax));
		y0 = 8*((t/Vmax)&7);
		t = y*V[1];
		b1 = H1*(t/(8*Vmax));
		y1 = 8*((t/Vmax)&7);
		t = y*V[2];
		b2 = H2*(t/(8*Vmax));
		y2 = 8*((t/Vmax)&7);
		x0 = 0;
		x1 = 0;
		x2 = 0;
		for(x=0; x<dx; x++){
			if(colorspace == CYCbCr){
				rpic[pici+x] = clamp[data0[b0][y0+x0++*H0/Hmax] + 128 + CLAMPOFF];
				gpic[pici+x] = clamp[data1[b1][y1+x1++*H1/Hmax] + 128 + CLAMPOFF];
				bpic[pici+x] = clamp[data2[b2][y2+x2++*H2/Hmax] + 128 + CLAMPOFF];
			}else{
				Y = (data0[b0][y0+x0++*H0/Hmax]+128)<<11;
				Cb = data1[b1][y1+x1++*H1/Hmax];
				Cr = data2[b2][y2+x2++*H2/Hmax];
				r = Y+c1*Cr;
				g = Y-c2*Cb-c3*Cr;
				b = Y+c4*Cb;
				rpic[pici+x] = clamp[(r>>11)+CLAMPOFF];
				gpic[pici+x] = clamp[(g>>11)+CLAMPOFF];
				bpic[pici+x] = clamp[(b>>11)+CLAMPOFF];
			}
			if(x0*H0/Hmax >= 8){
				x0 = 0;
				b0++;
			}
			if(x1*H1/Hmax >= 8){
				x1 = 0;
				b1++;
			}
			if(x2*H2/Hmax >= 8){
				x2 = 0;
				b2++;
			}
		}
		pici += h->X;
	}
}

/*
 * decode next 8-bit value from entropy-coded input.  chart F-26
 */
static
int
decode(Header *h, Huffman *t)
{
	int code, v, cnt, m, sr, i;
	int *maxcode;
	static int badcode;

	maxcode = t->maxcode;
	if(h->cnt < 8)
		nextbyte(h, 0);
	/* fast lookup */
	code = (h->sr>>(h->cnt-8))&0xFF;
	v = t->value[code];
	if(v >= 0){
		h->cnt -= t->shift[code];
		return v;
	}

	h->cnt -= 8;
	if(h->cnt == 0)
		nextbyte(h, 0);
	h->cnt--;
	cnt = h->cnt;
	m = 1<<cnt;
	sr = h->sr;
	code <<= 1;
	i = 9;
	for(;;i++){
		if(sr & m)
			code |= 1;
		if(code <= maxcode[i])
			break;
		code <<= 1;
		m >>= 1;
		if(m == 0){
			sr = nextbyte(h, 0);
			m = 0x80;
			cnt = 8;
		}
		cnt--;
	}
	if(i >= 17){
		if(badcode == 0)
			fprint(2, "badly encoded %dx%d JPEG file; ignoring bad value\n", h->X, h->Y);
		badcode = 1;
		i = 0;
	}
	h->cnt = cnt;
	return t->val[t->valptr[i]+(code-t->mincode[i])];
}

/*
 * load next byte of input
 */
static
int
nextbyte(Header *h, int marker)
{
	int b, b2;

	if(h->peek >= 0){
		b = h->peek;
		h->peek = -1;
	}else{
		b = Bgetc(h->fd);
		if(b == Beof)
			jpgerror(h, "truncated file");
		b &= 0xFF;
	}

	if(b == 0xFF){
		if(marker)
			return b;
		b2 = Bgetc(h->fd);
		if(b2 != 0){
			if(b2 == Beof)
				jpgerror(h, "truncated file");
			b2 &= 0xFF;
			if(b2 == DNL)
				jpgerror(h, "ReadJPG: DNL marker unimplemented");
			/* decoder is reading into marker; satisfy it and restore state */
			Bungetc(h->fd);
			h->peek = b;
		}
	}
	h->cnt += 8;
	h->sr = (h->sr<<8) | b;
	return b;
}

/*
 * return next s bits of input, MSB first, and level shift it
 */
static
int
receive(Header *h, int s)
{
	int v, m;

	while(h->cnt < s)
		nextbyte(h, 0);
	h->cnt -= s;
	v = h->sr >> h->cnt;
	m = (1<<s);
	v &= m-1;
	/* level shift */
	if(v < (m>>1))
		v += ~(m-1)+1;
	return v;
}

/*
 * return next s bits of input, decode as EOB
 */
static
int
receiveEOB(Header *h, int s)
{
	int v, m;

	while(h->cnt < s)
		nextbyte(h, 0);
	h->cnt -= s;
	v = h->sr >> h->cnt;
	m = (1<<s);
	v &= m-1;
	/* level shift */
	v += m;
	return v;
}

/* 
 * return next bit of input
 */
static
int
receivebit(Header *h)
{
	if(h->cnt < 1)
		nextbyte(h, 0);
	h->cnt--;
	return (h->sr >> h->cnt) & 1;
}

/*
 *  Scaled integer implementation.
 *  inverse two dimensional DCT, Chen-Wang algorithm
 * (IEEE ASSP-32, pp. 803-816, Aug. 1984)
 * 32-bit integer arithmetic (8 bit coefficients)
 * 11 mults, 29 adds per DCT
 *
 * coefficients extended to 12 bit for IEEE1180-1990 compliance
 */

enum {
	W1		= 2841,	/* 2048*sqrt(2)*cos(1*pi/16)*/
	W2		= 2676,	/* 2048*sqrt(2)*cos(2*pi/16)*/
	W3		= 2408,	/* 2048*sqrt(2)*cos(3*pi/16)*/
	W5		= 1609,	/* 2048*sqrt(2)*cos(5*pi/16)*/
	W6		= 1108,	/* 2048*sqrt(2)*cos(6*pi/16)*/
	W7		= 565,	/* 2048*sqrt(2)*cos(7*pi/16)*/

	W1pW7	= 3406,	/* W1+W7*/
	W1mW7	= 2276,	/* W1-W7*/
	W3pW5	= 4017,	/* W3+W5*/
	W3mW5	= 799,	/* W3-W5*/
	W2pW6	= 3784,	/* W2+W6*/
	W2mW6	= 1567,	/* W2-W6*/

	R2		= 181	/* 256/sqrt(2)*/
};

static
void
idct(int b[8*8])
{
	int x, y, eighty, v;
	int x0, x1, x2, x3, x4, x5, x6, x7, x8;
	int *p;

	/* transform horizontally*/
	for(y=0; y<8; y++){
		eighty = y<<3;
		/* if all non-DC components are zero, just propagate the DC term*/
		p = b+eighty;
		if(p[1]==0)
		if(p[2]==0 && p[3]==0)
		if(p[4]==0 && p[5]==0)
		if(p[6]==0 && p[7]==0){
			v = p[0]<<3;
			p[0] = v;
			p[1] = v;
			p[2] = v;
			p[3] = v;
			p[4] = v;
			p[5] = v;
			p[6] = v;
			p[7] = v;
			continue;
		}
		/* prescale*/
		x0 = (p[0]<<11)+128;
		x1 = p[4]<<11;
		x2 = p[6];
		x3 = p[2];
		x4 = p[1];
		x5 = p[7];
		x6 = p[5];
		x7 = p[3];
		/* first stage*/
		x8 = W7*(x4+x5);
		x4 = x8 + W1mW7*x4;
		x5 = x8 - W1pW7*x5;
		x8 = W3*(x6+x7);
		x6 = x8 - W3mW5*x6;
		x7 = x8 - W3pW5*x7;
		/* second stage*/
		x8 = x0 + x1;
		x0 -= x1;
		x1 = W6*(x3+x2);
		x2 = x1 - W2pW6*x2;
		x3 = x1 + W2mW6*x3;
		x1 = x4 + x6;
		x4 -= x6;
		x6 = x5 + x7;
		x5 -= x7;
		/* third stage*/
		x7 = x8 + x3;
		x8 -= x3;
		x3 = x0 + x2;
		x0 -= x2;
		x2 = (R2*(x4+x5)+128)>>8;
		x4 = (R2*(x4-x5)+128)>>8;
		/* fourth stage*/
		p[0] = (x7+x1)>>8;
		p[1] = (x3+x2)>>8;
		p[2] = (x0+x4)>>8;
		p[3] = (x8+x6)>>8;
		p[4] = (x8-x6)>>8;
		p[5] = (x0-x4)>>8;
		p[6] = (x3-x2)>>8;
		p[7] = (x7-x1)>>8;
	}
	/* transform vertically*/
	for(x=0; x<8; x++){
		/* if all non-DC components are zero, just propagate the DC term*/
		p = b+x;
		if(p[8*1]==0)
		if(p[8*2]==0 && p[8*3]==0)
		if(p[8*4]==0 && p[8*5]==0)
		if(p[8*6]==0 && p[8*7]==0){
			v = (p[8*0]+32)>>6;
			p[8*0] = v;
			p[8*1] = v;
			p[8*2] = v;
			p[8*3] = v;
			p[8*4] = v;
			p[8*5] = v;
			p[8*6] = v;
			p[8*7] = v;
			continue;
		}
		/* prescale*/
		x0 = (p[8*0]<<8)+8192;
		x1 = p[8*4]<<8;
		x2 = p[8*6];
		x3 = p[8*2];
		x4 = p[8*1];
		x5 = p[8*7];
		x6 = p[8*5];
		x7 = p[8*3];
		/* first stage*/
		x8 = W7*(x4+x5) + 4;
		x4 = (x8+W1mW7*x4)>>3;
		x5 = (x8-W1pW7*x5)>>3;
		x8 = W3*(x6+x7) + 4;
		x6 = (x8-W3mW5*x6)>>3;
		x7 = (x8-W3pW5*x7)>>3;
		/* second stage*/
		x8 = x0 + x1;
		x0 -= x1;
		x1 = W6*(x3+x2) + 4;
		x2 = (x1-W2pW6*x2)>>3;
		x3 = (x1+W2mW6*x3)>>3;
		x1 = x4 + x6;
		x4 -= x6;
		x6 = x5 + x7;
		x5 -= x7;
		/* third stage*/
		x7 = x8 + x3;
		x8 -= x3;
		x3 = x0 + x2;
		x0 -= x2;
		x2 = (R2*(x4+x5)+128)>>8;
		x4 = (R2*(x4-x5)+128)>>8;
		/* fourth stage*/
		p[8*0] = (x7+x1)>>14;
		p[8*1] = (x3+x2)>>14;
		p[8*2] = (x0+x4)>>14;
		p[8*3] = (x8+x6)>>14;
		p[8*4] = (x8-x6)>>14;
		p[8*5] = (x0-x4)>>14;
		p[8*6] = (x3-x2)>>14;
		p[8*7] = (x7-x1)>>14;
	}
}