qt3/src/kernel/qpngio.cpp

/****************************************************************************
**
** Implementation of PNG QImage IOHandler
**
** Created : 970521
**
** Copyright (C) 1992-2008 Trolltech ASA.  All rights reserved.
**
** This file is part of the kernel module of the Qt GUI Toolkit.
**
** This file may be used under the terms of the GNU General
** Public License versions 2.0 or 3.0 as published by the Free
** Software Foundation and appearing in the files LICENSE.GPL2
** and LICENSE.GPL3 included in the packaging of this file.
** Alternatively you may (at your option) use any later version
** of the GNU General Public License if such license has been
** publicly approved by Trolltech ASA (or its successors, if any)
** and the KDE Free Qt Foundation.
**
** Please review the following information to ensure GNU General
** Public Licensing requirements will be met:
** http://trolltech.com/products/qt/licenses/licensing/opensource/.
** If you are unsure which license is appropriate for your use, please
** review the following information:
** http://trolltech.com/products/qt/licenses/licensing/licensingoverview
** or contact the sales department at sales@trolltech.com.
**
** This file may be used under the terms of the Q Public License as
** defined by Trolltech ASA and appearing in the file LICENSE.QPL
** included in the packaging of this file.  Licensees holding valid Qt
** Commercial licenses may use this file in accordance with the Qt
** Commercial License Agreement provided with the Software.
**
** This file is provided "AS IS" with NO WARRANTY OF ANY KIND,
** INCLUDING THE WARRANTIES OF DESIGN, MERCHANTABILITY AND FITNESS FOR
** A PARTICULAR PURPOSE. Trolltech reserves all rights not granted
** herein.
**
**********************************************************************/

#include "qpngio.h"

#ifndef QT_NO_IMAGEIO_PNG

#include "qasyncimageio.h"
#include "qiodevice.h"

#include <png.h>


#ifdef Q_OS_TEMP
#define CALLBACK_CALL_TYPE	__cdecl
#else
#define CALLBACK_CALL_TYPE
#endif


/*
  All PNG files load to the minimal QImage equivalent.

  All QImage formats output to reasonably efficient PNG equivalents.
  Never to grayscale.
*/

#if defined(Q_C_CALLBACKS)
extern "C" {
#endif

static
void CALLBACK_CALL_TYPE iod_read_fn(png_structp png_ptr, png_bytep data, png_size_t length)
{
    QImageIO* iio = (QImageIO*)png_get_io_ptr(png_ptr);
    QIODevice* in = iio->ioDevice();

    while (length) {
	int nr = in->readBlock((char*)data, length);
	if (nr <= 0) {
	    png_error(png_ptr, "Read Error");
	    return;
	}
	length -= nr;
    }
}


static
void CALLBACK_CALL_TYPE qpiw_write_fn( png_structp png_ptr, png_bytep data, png_size_t length )
{
    QPNGImageWriter* qpiw = (QPNGImageWriter*)png_get_io_ptr( png_ptr );
    QIODevice* out = qpiw->device();

    uint nr = out->writeBlock( (char*)data, length );
    if ( nr != length ) {
	png_error( png_ptr, "Write Error" );
	return;
    }
}


static
void CALLBACK_CALL_TYPE qpiw_flush_fn( png_structp png_ptr )
{
    QPNGImageWriter* qpiw = (QPNGImageWriter*)png_get_io_ptr( png_ptr );
    QIODevice* out = qpiw->device();

    out->flush();
}

#if defined(Q_C_CALLBACKS)
}
#endif

static
void setup_qt( QImage& image, png_structp png_ptr, png_infop info_ptr, float screen_gamma=0.0 )
{
    if ( screen_gamma != 0.0 && png_get_valid(png_ptr, info_ptr, PNG_INFO_gAMA) ) {
	double file_gamma;
	png_get_gAMA(png_ptr, info_ptr, &file_gamma);
	png_set_gamma( png_ptr, screen_gamma, file_gamma );
    }

    png_uint_32 width;
    png_uint_32 height;
    int bit_depth;
    int color_type;
    png_get_IHDR(png_ptr, info_ptr, &width, &height, &bit_depth, &color_type,
	0, 0, 0);

    if ( color_type == PNG_COLOR_TYPE_GRAY ) {
	// Black & White or 8-bit grayscale
	if ( bit_depth == 1 && info_ptr->channels == 1 ) {
	    png_set_invert_mono( png_ptr );
	    png_read_update_info( png_ptr, info_ptr );
	    if (!image.create( width, height, 1, 2, QImage::BigEndian ))
		return;
	    image.setColor( 1, qRgb(0,0,0) );
	    image.setColor( 0, qRgb(255,255,255) );
	} else if (bit_depth == 16 && png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS)) {
	    png_set_expand(png_ptr);
	    png_set_strip_16(png_ptr);
	    png_set_gray_to_rgb(png_ptr);

	    if (!image.create(width, height, 32))
		return;
	    image.setAlphaBuffer(TRUE);

	    if (QImage::systemByteOrder() == QImage::BigEndian)
		png_set_swap_alpha(png_ptr);

	    png_read_update_info(png_ptr, info_ptr);
	} else {
	    if ( bit_depth == 16 )
		png_set_strip_16(png_ptr);
	    else if ( bit_depth < 8 )
		png_set_packing(png_ptr);
	    int ncols = bit_depth < 8 ? 1 << bit_depth : 256;
	    png_read_update_info(png_ptr, info_ptr);
	    if (!image.create(width, height, 8, ncols))
		return;
	    for (int i=0; i<ncols; i++) {
		int c = i*255/(ncols-1);
		image.setColor( i, qRgba(c,c,c,0xff) );
	    }
	    if ( png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS) ) {
		const int g = info_ptr->trans_values.gray;
		if (g < ncols) {
		    image.setAlphaBuffer(TRUE);
		    image.setColor(g, image.color(g) & RGB_MASK);
		}
	    }
	}
    } else if ( color_type == PNG_COLOR_TYPE_PALETTE
     && png_get_valid(png_ptr, info_ptr, PNG_INFO_PLTE)
     && info_ptr->num_palette <= 256 )
    {
	// 1-bit and 8-bit color
	if ( bit_depth != 1 )
	    png_set_packing( png_ptr );
	png_read_update_info( png_ptr, info_ptr );
	png_get_IHDR(png_ptr, info_ptr,
	    &width, &height, &bit_depth, &color_type, 0, 0, 0);
	if (!image.create(width, height, bit_depth, info_ptr->num_palette,
	    QImage::BigEndian))
	    return;
	int i = 0;
	if ( png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS) ) {
	    image.setAlphaBuffer( TRUE );
	    while ( i < info_ptr->num_trans ) {
		image.setColor(i, qRgba(
		    info_ptr->palette[i].red,
		    info_ptr->palette[i].green,
		    info_ptr->palette[i].blue,
		    info_ptr->trans[i]
		    )
		);
		i++;
	    }
	}
	while ( i < info_ptr->num_palette ) {
	    image.setColor(i, qRgba(
		info_ptr->palette[i].red,
		info_ptr->palette[i].green,
		info_ptr->palette[i].blue,
		0xff
		)
	    );
	    i++;
	}
    } else {
	// 32-bit
	if ( bit_depth == 16 )
	    png_set_strip_16(png_ptr);

	png_set_expand(png_ptr);

	if ( color_type == PNG_COLOR_TYPE_GRAY_ALPHA )
	    png_set_gray_to_rgb(png_ptr);

	if (!image.create(width, height, 32))
	    return;

	// Only add filler if no alpha, or we can get 5 channel data.
	if (!(color_type & PNG_COLOR_MASK_ALPHA)
	   && !png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS)) {
	    png_set_filler(png_ptr, 0xff,
		QImage::systemByteOrder() == QImage::BigEndian ?
		    PNG_FILLER_BEFORE : PNG_FILLER_AFTER);
	    // We want 4 bytes, but it isn't an alpha channel
	} else {
	    image.setAlphaBuffer(TRUE);
	}

	if ( QImage::systemByteOrder() == QImage::BigEndian ) {
	    png_set_swap_alpha(png_ptr);
	}

	png_read_update_info(png_ptr, info_ptr);
    }

    // Qt==ARGB==Big(ARGB)==Little(BGRA)
    if ( QImage::systemByteOrder() == QImage::LittleEndian ) {
	png_set_bgr(png_ptr);
    }
}


#if defined(Q_C_CALLBACKS)
extern "C" {
#endif
static void CALLBACK_CALL_TYPE qt_png_warning(png_structp /*png_ptr*/, png_const_charp message)
{
    qWarning("libpng warning: %s", message);
}

#if defined(Q_C_CALLBACKS)
}
#endif


static
void read_png_image(QImageIO* iio)
{
    png_structp png_ptr;
    png_infop info_ptr;
    png_infop end_info;
    png_bytep* row_pointers;

    png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING,0,0,0);
    if (!png_ptr) {
	iio->setStatus(-1);
	return;
    }

    png_set_error_fn(png_ptr, 0, 0, qt_png_warning);

    info_ptr = png_create_info_struct(png_ptr);
    if (!info_ptr) {
	png_destroy_read_struct(&png_ptr, 0, 0);
	iio->setStatus(-2);
	return;
    }

    end_info = png_create_info_struct(png_ptr);
    if (!end_info) {
	png_destroy_read_struct(&png_ptr, &info_ptr, 0);
	iio->setStatus(-3);
	return;
    }

    if (setjmp(png_ptr->jmpbuf)) {
	png_destroy_read_struct(&png_ptr, &info_ptr, &end_info);
	iio->setStatus(-4);
	return;
    }

    png_set_read_fn(png_ptr, (void*)iio, iod_read_fn);
    png_read_info(png_ptr, info_ptr);

    QImage image;
    setup_qt(image, png_ptr, info_ptr, iio->gamma());
    if (image.isNull()) {
	png_destroy_read_struct(&png_ptr, &info_ptr, &end_info);
	iio->setStatus(-5);
	return;
    }

    png_uint_32 width;
    png_uint_32 height;
    int bit_depth;
    int color_type;
    png_get_IHDR(png_ptr, info_ptr, &width, &height, &bit_depth, &color_type,
	0, 0, 0);

    uchar** jt = image.jumpTable();
    row_pointers=new png_bytep[height];

    for (uint y=0; y<height; y++) {
	row_pointers[y]=jt[y];
    }

    png_read_image(png_ptr, row_pointers);

#if 0 // libpng takes care of this.
png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS)
    if (image.depth()==32 && png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS)) {
	QRgb trans = 0xFF000000 | qRgb(
	      (info_ptr->trans_values.red << 8 >> bit_depth)&0xff,
	      (info_ptr->trans_values.green << 8 >> bit_depth)&0xff,
	      (info_ptr->trans_values.blue << 8 >> bit_depth)&0xff);
	for (uint y=0; y<height; y++) {
	    for (uint x=0; x<info_ptr->width; x++) {
		if (((uint**)jt)[y][x] == trans) {
		    ((uint**)jt)[y][x] &= 0x00FFFFFF;
		} else {
		}
	    }
	}
    }
#endif

    image.setDotsPerMeterX(png_get_x_pixels_per_meter(png_ptr,info_ptr));
    image.setDotsPerMeterY(png_get_y_pixels_per_meter(png_ptr,info_ptr));

#ifndef QT_NO_IMAGE_TEXT
    png_textp text_ptr;
    int num_text=0;
    png_get_text(png_ptr,info_ptr,&text_ptr,&num_text);
    while (num_text--) {
	image.setText(text_ptr->key,0,text_ptr->text);
	text_ptr++;
    }
#endif

    delete [] row_pointers;

    if ( image.hasAlphaBuffer() ) {
	// Many PNG files lie (eg. from PhotoShop). Fortunately this loop will
	// usually be quick to find those that tell the truth.
	QRgb* c;
	int n;
	if (image.depth()==32) {
	    c = (QRgb*)image.bits();
	    n = image.bytesPerLine() * image.height() / 4;
	} else {
	    c = image.colorTable();
	    n = image.numColors();
	}
	while ( n-- && qAlpha(*c++)==0xff )
	    ;
	if ( n<0 ) // LIAR!
	    image.setAlphaBuffer(FALSE);
    }

    iio->setImage(image);

    png_read_end(png_ptr, end_info);
    png_destroy_read_struct(&png_ptr, &info_ptr, &end_info);

    iio->setStatus(0);
}

QPNGImageWriter::QPNGImageWriter(QIODevice* iod) :
    dev(iod),
    frames_written(0),
    disposal(Unspecified),
    looping(-1),
    ms_delay(-1),
    gamma(0.0)
{
}

QPNGImageWriter::~QPNGImageWriter()
{
}

void QPNGImageWriter::setDisposalMethod(DisposalMethod dm)
{
    disposal = dm;
}

void QPNGImageWriter::setLooping(int loops)
{
    looping = loops;
}

void QPNGImageWriter::setFrameDelay(int msecs)
{
    ms_delay = msecs;
}

void QPNGImageWriter::setGamma(float g)
{
    gamma = g;
}


#ifndef QT_NO_IMAGE_TEXT
static void set_text(const QImage& image, png_structp png_ptr, png_infop info_ptr, bool short_not_long)
{
    QValueList<QImageTextKeyLang> keys = image.textList();
    if ( keys.count() ) {
	png_textp text_ptr = new png_text[keys.count()];
	int i=0;
	for (QValueList<QImageTextKeyLang>::Iterator it=keys.begin();
		it != keys.end(); ++it)
	{
	    QString t = image.text(*it);
	    if ( (t.length() <= 200) == short_not_long ) {
		if ( t.length() < 40 )
		    text_ptr[i].compression = PNG_TEXT_COMPRESSION_NONE;
		else
		    text_ptr[i].compression = PNG_TEXT_COMPRESSION_zTXt;
		text_ptr[i].key = (png_charp)(*it).key.data();
		text_ptr[i].text = (png_charp)t.latin1();
		//text_ptr[i].text = qstrdup(t.latin1());
		i++;
	    }
	}
	png_set_text(png_ptr, info_ptr, text_ptr, i);
	//for (int j=0; j<i; j++)
	    //free(text_ptr[i].text);
	delete [] text_ptr;
    }
}
#endif

bool QPNGImageWriter::writeImage(const QImage& image, int off_x, int off_y)
{
    return writeImage(image, -1, off_x, off_y);
}

bool QPNGImageWriter::writeImage(const QImage& image, int quality_in, int off_x_in, int off_y_in)
{
    QPoint offset = image.offset();
    int off_x = off_x_in + offset.x();
    int off_y = off_y_in + offset.y();

    png_structp png_ptr;
    png_infop info_ptr;
    png_bytep* row_pointers;

    png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING,0,0,0);
    if (!png_ptr) {
	return FALSE;
    }

    png_set_error_fn(png_ptr, 0, 0, qt_png_warning);

    info_ptr = png_create_info_struct(png_ptr);
    if (!info_ptr) {
	png_destroy_write_struct(&png_ptr, 0);
	return FALSE;
    }

    if (setjmp(png_ptr->jmpbuf)) {
	png_destroy_write_struct(&png_ptr, &info_ptr);
	return FALSE;
    }

    int quality = quality_in;
    if (quality >= 0) {
	if (quality > 9) {
#if defined(QT_CHECK_RANGE)
	    qWarning( "PNG: Quality %d out of range", quality );
#endif
	    quality = 9;
	}
	png_set_compression_level(png_ptr, quality);
    }

    if (gamma != 0.0) {
	png_set_gAMA(png_ptr, info_ptr, 1.0/gamma);
    }

    png_set_write_fn(png_ptr, (void*)this, qpiw_write_fn, qpiw_flush_fn);

    info_ptr->channels =
	(image.depth() == 32)
	    ? (image.hasAlphaBuffer() ? 4 : 3)
	    : 1;

    png_set_IHDR(png_ptr, info_ptr, image.width(), image.height(),
	image.depth() == 1 ? 1 : 8 /* per channel */,
	image.depth() == 32
	    ? image.hasAlphaBuffer()
		? PNG_COLOR_TYPE_RGB_ALPHA
		: PNG_COLOR_TYPE_RGB
	    : PNG_COLOR_TYPE_PALETTE, 0, 0, 0);


    //png_set_sBIT(png_ptr, info_ptr, 8);
    info_ptr->sig_bit.red = 8;
    info_ptr->sig_bit.green = 8;
    info_ptr->sig_bit.blue = 8;

    if (image.depth() == 1 && image.bitOrder() == QImage::LittleEndian)
       png_set_packswap(png_ptr);

    png_colorp palette = 0;
    png_bytep copy_trans = 0;
    if (image.numColors()) {
	// Paletted
        int num_palette = image.numColors();
	palette = new png_color[num_palette];
	png_set_PLTE(png_ptr, info_ptr, palette, num_palette);
	int* trans = new int[num_palette];
	int num_trans = 0;
	for (int i=0; i<num_palette; i++) {
	    QRgb rgb=image.color(i);
	    info_ptr->palette[i].red = qRed(rgb);
	    info_ptr->palette[i].green = qGreen(rgb);
	    info_ptr->palette[i].blue = qBlue(rgb);
	    if (image.hasAlphaBuffer()) {
		trans[i] = rgb >> 24;
		if (trans[i] < 255) {
		    num_trans = i+1;
		}
	    }
	}
	if (num_trans) {
	    copy_trans = new png_byte[num_trans];
	    for (int i=0; i<num_trans; i++)
		copy_trans[i] = trans[i];
	    png_set_tRNS(png_ptr, info_ptr, copy_trans, num_trans, 0);
	}
	delete [] trans;
    }

    if ( image.hasAlphaBuffer() ) {
	info_ptr->sig_bit.alpha = 8;
    }

    // Swap ARGB to RGBA (normal PNG format) before saving on
    // BigEndian machines
    if ( QImage::systemByteOrder() == QImage::BigEndian ) {
	png_set_swap_alpha(png_ptr);
    }

    // Qt==ARGB==Big(ARGB)==Little(BGRA)
    if ( QImage::systemByteOrder() == QImage::LittleEndian ) {
	png_set_bgr(png_ptr);
    }

    if (off_x || off_y) {
	png_set_oFFs(png_ptr, info_ptr, off_x, off_y, PNG_OFFSET_PIXEL);
    }

    if ( frames_written > 0 )
	png_set_sig_bytes(png_ptr, 8);

    if ( image.dotsPerMeterX() > 0 || image.dotsPerMeterY() > 0 ) {
	png_set_pHYs(png_ptr, info_ptr,
		image.dotsPerMeterX(), image.dotsPerMeterY(),
		PNG_RESOLUTION_METER);
    }

#ifndef QT_NO_IMAGE_TEXT
    // Write short texts early.
    set_text(image,png_ptr,info_ptr,TRUE);
#endif

    png_write_info(png_ptr, info_ptr);

#ifndef QT_NO_IMAGE_TEXT
    // Write long texts later.
    set_text(image,png_ptr,info_ptr,FALSE);
#endif

    if ( image.depth() != 1 )
	png_set_packing(png_ptr);

    if ( image.depth() == 32 && !image.hasAlphaBuffer() )
	png_set_filler(png_ptr, 0,
	    QImage::systemByteOrder() == QImage::BigEndian ?
		PNG_FILLER_BEFORE : PNG_FILLER_AFTER);

    if ( looping >= 0 && frames_written == 0 ) {
	uchar data[13] = "NETSCAPE2.0";
	//                0123456789aBC
	data[0xB] = looping%0x100;
	data[0xC] = looping/0x100;
	png_write_chunk(png_ptr, (png_byte*)"gIFx", data, 13);
    }
    if ( ms_delay >= 0 || disposal!=Unspecified ) {
	uchar data[4];
	data[0] = disposal;
	data[1] = 0;
	data[2] = (ms_delay/10)/0x100; // hundredths
	data[3] = (ms_delay/10)%0x100;
	png_write_chunk(png_ptr, (png_byte*)"gIFg", data, 4);
    }

    png_uint_32 width;
    png_uint_32 height;
    int bit_depth;
    int color_type;
    png_get_IHDR(png_ptr, info_ptr, &width, &height, &bit_depth, &color_type,
	0, 0, 0);

    uchar** jt = image.jumpTable();
    row_pointers=new png_bytep[height];
    uint y;
    for (y=0; y<height; y++) {
	row_pointers[y]=jt[y];
    }
    png_write_image(png_ptr, row_pointers);
    delete [] row_pointers;

    png_write_end(png_ptr, info_ptr);
    frames_written++;

    if ( palette )
	delete [] palette;
    if ( copy_trans )
	delete [] copy_trans;

    png_destroy_write_struct(&png_ptr, &info_ptr);

    return TRUE;
}

static
void write_png_image(QImageIO* iio)
{
    QPNGImageWriter writer(iio->ioDevice());
    int quality = iio->quality();
    if ( quality >= 0 ) {
	quality = QMIN( quality, 100 );
	quality = (100-quality) * 9 / 91; // map [0,100] -> [9,0]
    }
    writer.setGamma(iio->gamma());
    bool ok = writer.writeImage( iio->image(), quality );
    iio->setStatus( ok ? 0 : -1 );
}

/*!
    \class QPNGImagePacker qpngio.h
    \brief The QPNGImagePacker class creates well-compressed PNG animations.

    \ingroup images
    \ingroup graphics

    By using transparency, QPNGImagePacker allows you to build a PNG
    image from a sequence of QImages.

    Images are added using packImage().
*/


/*!
    Creates an image packer that writes PNG data to IO device \a iod
    using a \a storage_depth bit encoding (use 8 or 32, depending on
    the desired quality and compression requirements).

    If the image needs to be modified to fit in a lower-resolution
    result (e.g. converting from 32-bit to 8-bit), use the \a
    conversionflags to specify how you'd prefer this to happen.

    \sa Qt::ImageConversionFlags
*/
QPNGImagePacker::QPNGImagePacker(QIODevice* iod, int storage_depth,
	int conversionflags) :
    QPNGImageWriter(iod),
    depth(storage_depth),
    convflags(conversionflags),
    alignx(1)
{
}

/*!
    Aligns pixel differences to \a x pixels. For example, using 8 can
    improve playback on certain hardware. Normally the default of
    1-pixel alignment (i.e. no alignment) gives better compression and
    performance.
*/
void QPNGImagePacker::setPixelAlignment(int x)
{
    alignx = x;
}

/*!
    Adds the image \a img to the PNG animation, analyzing the
    differences between this and the previous image to improve
    compression.
*/
bool QPNGImagePacker::packImage(const QImage& img)
{
    QImage image = img.convertDepth(32);
    if ( previous.isNull() ) {
	// First image
	writeImage(image.convertDepth(depth,convflags));
    } else {
	bool done;
	int minx, maxx, miny, maxy;
	int w = image.width();
	int h = image.height();

	QRgb** jt = (QRgb**)image.jumpTable();
	QRgb** pjt = (QRgb**)previous.jumpTable();

	// Find left edge of change
	done = FALSE;
	for (minx = 0; minx < w && !done; minx++) {
	    for (int ty = 0; ty < h; ty++) {
		if ( jt[ty][minx] != pjt[ty][minx] ) {
		    done = TRUE;
		    break;
		}
	    }
	}
	minx--;

	// Find right edge of change
	done = FALSE;
	for (maxx = w-1; maxx >= 0 && !done; maxx--) {
	    for (int ty = 0; ty < h; ty++) {
		if ( jt[ty][maxx] != pjt[ty][maxx] ) {
		    done = TRUE;
		    break;
		}
	    }
	}
	maxx++;

	// Find top edge of change
	done = FALSE;
	for (miny = 0; miny < h && !done; miny++) {
	    for (int tx = 0; tx < w; tx++) {
		if ( jt[miny][tx] != pjt[miny][tx] ) {
		    done = TRUE;
		    break;
		}
	    }
	}
	miny--;

	// Find right edge of change
	done = FALSE;
	for (maxy = h-1; maxy >= 0 && !done; maxy--) {
	    for (int tx = 0; tx < w; tx++) {
		if ( jt[maxy][tx] != pjt[maxy][tx] ) {
		    done = TRUE;
		    break;
		}
	    }
	}
	maxy++;

	if ( minx > maxx ) minx=maxx=0;
	if ( miny > maxy ) miny=maxy=0;

	if ( alignx > 1 ) {
	    minx -= minx % alignx;
	    maxx = maxx - maxx % alignx + alignx - 1;
	}

	int dw = maxx-minx+1;
	int dh = maxy-miny+1;

	QImage diff(dw, dh, 32);

	diff.setAlphaBuffer(TRUE);
	int x, y;
	if ( alignx < 1 )
	    alignx = 1;
	for (y = 0; y < dh; y++) {
	    QRgb* li = (QRgb*)image.scanLine(y+miny)+minx;
	    QRgb* lp = (QRgb*)previous.scanLine(y+miny)+minx;
	    QRgb* ld = (QRgb*)diff.scanLine(y);
	    if ( alignx ) {
		for (x = 0; x < dw; x+=alignx) {
		    int i;
		    for (i=0; i<alignx; i++) {
			if ( li[x+i] != lp[x+i] )
			    break;
		    }
		    if ( i == alignx ) {
			// All the same
			for (i=0; i<alignx; i++) {
			    ld[x+i] = qRgba(0,0,0,0);
			}
		    } else {
			// Some different
			for (i=0; i<alignx; i++) {
			    ld[x+i] = 0xff000000 | li[x+i];
			}
		    }
		}
	    } else {
		for (x = 0; x < dw; x++) {
		    if ( li[x] != lp[x] )
			ld[x] = 0xff000000 | li[x];
		    else
			ld[x] = qRgba(0,0,0,0);
		}
	    }
	}

	diff = diff.convertDepth(depth,convflags);
	if ( !writeImage(diff, minx, miny) )
	    return FALSE;
    }
    previous = image;
    return TRUE;
}


#ifndef QT_NO_ASYNC_IMAGE_IO

class QPNGFormat : public QImageFormat {
public:
    QPNGFormat();
    virtual ~QPNGFormat();

    int decode(QImage& img, QImageConsumer* consumer,
	    const uchar* buffer, int length);

    void info(png_structp png_ptr, png_infop info);
    void row(png_structp png_ptr, png_bytep new_row,
		png_uint_32 row_num, int pass);
    void end(png_structp png_ptr, png_infop info);
#ifdef PNG_USER_CHUNKS_SUPPORTED
    int user_chunk(png_structp png_ptr,
	    png_bytep data, png_uint_32 length);
#endif

private:
    // Animation-level information
    enum { MovieStart, FrameStart, Inside, End } state;
    int first_frame;
    int base_offx;
    int base_offy;

    // Image-level information
    png_structp png_ptr;
    png_infop info_ptr;

    // Temporary locals during single data-chunk processing
    QImageConsumer* consumer;
    QImage* image;
    int unused_data;
};

class QPNGFormatType : public QImageFormatType
{
    QImageFormat* decoderFor(const uchar* buffer, int length);
    const char* formatName() const;
};


/*
  \class QPNGFormat qpngio.h
  \brief The QPNGFormat class provides an incremental image decoder for PNG
  image format.

  \ingroup images
  \ingroup graphics

  This subclass of QImageFormat decodes PNG format images,
  including animated PNGs.

  Animated PNG images are standard PNG images. The PNG standard
  defines two extension chunks that are useful for animations:

  \list
    \i gIFg - GIF-like Graphic Control Extension.
     This includes frame disposal, user input flag (we ignore this),
	    and inter-frame delay.
   \i gIFx - GIF-like Application Extension.
    This is multi-purpose, but we just use the Netscape extension
	    which specifies looping.
  \endlist

  The subimages usually contain a offset chunk (oFFs) but need not.

  The first image defines the "screen" size. Any subsequent image that
  doesn't fit is clipped.
*/
/* ###TODO: decide on this point. gIFg gives disposal types, so it can be done.
  All images paste (\e not composite, just place all-channel copying)
  over the previous image to produce a subsequent frame.
*/

/*
  \class QPNGFormatType qasyncimageio.h
  \brief The QPNGFormatType class provides an incremental image decoder
  for PNG image format.

  \ingroup images
  \ingroup graphics
  \ingroup io

  This subclass of QImageFormatType recognizes PNG format images, creating
  a QPNGFormat when required. An instance of this class is created
  automatically before any other factories, so you should have no need for
  such objects.
*/

QImageFormat* QPNGFormatType::decoderFor(
    const uchar* buffer, int length)
{
    if (length < 8) return 0;
    if (buffer[0]==137
     && buffer[1]=='P'
     && buffer[2]=='N'
     && buffer[3]=='G'
     && buffer[4]==13
     && buffer[5]==10
     && buffer[6]==26
     && buffer[7]==10)
	return new QPNGFormat;
    return 0;
}

const char* QPNGFormatType::formatName() const
{
    return "PNG";
}

#if defined(Q_C_CALLBACKS)
extern "C" {
#endif

static void
CALLBACK_CALL_TYPE info_callback(png_structp png_ptr, png_infop info)
{
    QPNGFormat* that = (QPNGFormat*)png_get_progressive_ptr(png_ptr);
    that->info(png_ptr,info);
}

static void
CALLBACK_CALL_TYPE row_callback(png_structp png_ptr, png_bytep new_row,
   png_uint_32 row_num, int pass)
{
    QPNGFormat* that = (QPNGFormat*)png_get_progressive_ptr(png_ptr);
    that->row(png_ptr,new_row,row_num,pass);
}

static void
CALLBACK_CALL_TYPE end_callback(png_structp png_ptr, png_infop info)
{
    QPNGFormat* that = (QPNGFormat*)png_get_progressive_ptr(png_ptr);
    that->end(png_ptr,info);
}

#if 0
#ifdef PNG_USER_CHUNKS_SUPPORTED
static int
CALLBACK_CALL_TYPE user_chunk_callback(png_structp png_ptr,
         png_unknown_chunkp chunk)
{
    QPNGFormat* that = (QPNGFormat*)png_get_progressive_ptr(png_ptr);
    return that->user_chunk(png_ptr,chunk->data,chunk->size);
}
#endif
#endif

#if defined(Q_C_CALLBACKS)
}
#endif


/*!
    Constructs a QPNGFormat object.
*/
QPNGFormat::QPNGFormat()
{
    state = MovieStart;
    first_frame = 1;
    base_offx = 0;
    base_offy = 0;
    png_ptr = 0;
    info_ptr = 0;
}


/*!
    Destroys a QPNGFormat object.
*/
QPNGFormat::~QPNGFormat()
{
    if ( png_ptr )
        png_destroy_read_struct(&png_ptr, &info_ptr, 0);
}


/*!
    This function decodes some data into image changes.

    Returns the number of bytes consumed.
*/
int QPNGFormat::decode(QImage& img, QImageConsumer* cons,
	const uchar* buffer, int length)
{
    consumer = cons;
    image = &img;

    if ( state != Inside ) {
	png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, 0, 0, 0);
	if (!png_ptr) {
	    info_ptr = 0;
	    image = 0;
		return -1;
	}

	png_set_error_fn(png_ptr, 0, 0, qt_png_warning);
	png_set_compression_level(png_ptr, 9);

	info_ptr = png_create_info_struct(png_ptr);
	if (!info_ptr) {
	    png_destroy_read_struct(&png_ptr, &info_ptr, 0);
	    image = 0;
	    return -1;
	}

	if (setjmp((png_ptr)->jmpbuf)) {
	    png_destroy_read_struct(&png_ptr, &info_ptr, 0);
	    image = 0;
	    return -1;
	}

	png_set_progressive_read_fn(png_ptr, (void *)this,
	                            info_callback, row_callback, end_callback);

#ifdef PNG_USER_CHUNKS_SUPPORTED
	// Can't do this yet. libpng has a crash bug with unknown (user) chunks.
	// Warwick has sent them a patch.
	// png_set_read_user_chunk_fn(png_ptr, 0, user_chunk_callback);
	// png_set_keep_unknown_chunks(png_ptr, 2/*HANDLE_CHUNK_IF_SAFE*/, 0, 0);
#endif

	if ( state != MovieStart && *buffer != 0211 ) {
	    // Good, no signature - the preferred way to concat PNG images.
	    // Skip them.
	    png_set_sig_bytes(png_ptr, 8);
	}

	state = Inside;
    }

    if ( !png_ptr ) return 0;

    if (setjmp(png_ptr->jmpbuf)) {
	png_destroy_read_struct(&png_ptr, &info_ptr, 0);
	image = 0;
	state = MovieStart;
	return -1;
    }
    unused_data = 0;
    png_process_data(png_ptr, info_ptr, (png_bytep)buffer, length);
    int l = length - unused_data;

    // TODO: send incremental stuff to consumer (optional)

    if ( state != Inside ) {
	if ( png_ptr )
	    png_destroy_read_struct(&png_ptr, &info_ptr, 0);
    }

    image = 0;
    return l;
}

void QPNGFormat::info(png_structp png, png_infop)
{
    png_set_interlace_handling(png);
    setup_qt(*image, png, info_ptr);
}

void QPNGFormat::row(png_structp png, png_bytep new_row,
    png_uint_32 row_num, int)
{
    uchar* old_row = image->scanLine(row_num);
    png_progressive_combine_row(png, old_row, new_row);
}


void QPNGFormat::end(png_structp png, png_infop info)
{
    int offx = png_get_x_offset_pixels(png,info) - base_offx;
    int offy = png_get_y_offset_pixels(png,info) - base_offy;
    if ( first_frame ) {
	base_offx = offx;
	base_offy = offy;
	first_frame = 0;
    }
    image->setOffset(QPoint(offx,offy));
    image->setDotsPerMeterX(png_get_x_pixels_per_meter(png,info));
    image->setDotsPerMeterY(png_get_y_pixels_per_meter(png,info));
#ifndef QT_NO_IMAGE_TEXT
    png_textp text_ptr;
    int num_text=0;
    png_get_text(png,info,&text_ptr,&num_text);
    while (num_text--) {
	image->setText(text_ptr->key,0,text_ptr->text);
	text_ptr++;
    }
#endif
    QRect r(0,0,image->width(),image->height());
    consumer->frameDone(QPoint(offx,offy),r);
    consumer->end();
    state = FrameStart;
    unused_data = (int)png->buffer_size; // Since libpng doesn't tell us
}

#ifdef PNG_USER_CHUNKS_SUPPORTED

/*
#ifndef QT_NO_IMAGE_TEXT
static bool skip(png_uint_32& max, png_bytep& data)
{
    while (*data) {
	if ( !max ) return FALSE;
	max--;
	data++;
    }
    if ( !max ) return FALSE;
    max--;
    data++; // skip to after NUL
    return TRUE;
}
#endif
*/

int QPNGFormat::user_chunk(png_structp png,
	    png_bytep data, png_uint_32 length)
{
#if 0 // NOT SUPPORTED: experimental PNG animation.
    // qDebug("Got %ld-byte %s chunk", length, png->chunk_name);
    if ( 0==qstrcmp((char*)png->chunk_name, "gIFg")
	    && length == 4 ) {

	//QPNGImageWriter::DisposalMethod disposal =
	//  (QPNGImageWriter::DisposalMethod)data[0];
	// ### TODO: use the disposal method
	int ms_delay = ((data[2] << 8) | data[3])*10;
	consumer->setFramePeriod(ms_delay);
	return 1;
    } else if ( 0==qstrcmp((char*)png->chunk_name, "gIFx")
	    && length == 13 ) {
	if ( qstrncmp((char*)data,"NETSCAPE2.0",11)==0 ) {
	    int looping = (data[0xC]<<8)|data[0xB];
	    consumer->setLooping(looping);
	    return 1;
	}
    }
#else
    Q_UNUSED( png )
    Q_UNUSED( data )
    Q_UNUSED( length )
#endif

#ifndef QT_NO_IMAGE_TEXT
    /*

    libpng now supports this chunk.


    if ( 0==qstrcmp((char*)png->chunk_name, "iTXt") && length>=6 ) {
	const char* keyword = (const char*)data;
	if ( !skip(length,data) ) return 0;
	if ( length >= 4 ) {
	    char compression_flag = *data++;
	    char compression_method = *data++;
	    if ( compression_flag == compression_method ) {
		// fool the compiler into thinking they're used
	    }
	    const char* lang = (const char*)data;
	    if ( !skip(length,data) ) return 0;
	    // const char* keyword_utf8 = (const char*)data;
	    if ( !skip(length,data) ) return 0;
	    const char* text_utf8 = (const char*)data;
	    if ( !skip(length,data) ) return 0;
	    QString text = QString::fromUtf8(text_utf8);
	    image->setText(keyword,lang[0] ? lang : 0,text);
	    return 1;
	}
    }
    */
#endif

    return 0;
}
#endif


static QPNGFormatType* globalPngFormatTypeObject = 0;

#endif // QT_NO_ASYNC_IMAGE_IO

static bool done = FALSE;
void qCleanupPngIO()
{
#ifndef QT_NO_ASYNC_IMAGE_IO
    if ( globalPngFormatTypeObject ) {
	delete globalPngFormatTypeObject;
	globalPngFormatTypeObject = 0;
    }
#endif
    done = FALSE;
}

void qInitPngIO()
{
    if ( !done ) {
	done = TRUE;
	QImageIO::defineIOHandler( "PNG", "^.PNG\r", 0, read_png_image,
				   write_png_image);
#ifndef QT_NO_ASYNC_IMAGE_IO
	globalPngFormatTypeObject = new QPNGFormatType;
#endif
	qAddPostRoutine( qCleanupPngIO );
    }
}

void qt_zlib_compression_hack()
{
    compress(0,0,0,0);
    uncompress(0,0,0,0);
}

#endif // QT_NO_IMAGEIO_PNG