QImage::save() 流程

qt-everywhere-opensource-src-4.8.1/src/gui/image/qimage.cpp

QImageData::doImageIO()

qimagewriter.cpp

QImageWriter::write()
{
    if (!d->handler->write(image))
}

handler is QPngHandler:

createWriteHandlerHelper()
{

#ifndef QT_NO_IMAGEFORMAT_PNG
        } else if (testFormat == "png") {
            handler = new QPngHandler;
#endif
}

qpnghandler.cpp

bool QPngHandler::write(const QImage &image)
{
    return write_png_image(image, device(), d->quality, d->gamma, d->description);
}

static bool write_png_image(const QImage &image, QIODevice *device,
                            int quality, float gamma, const QString &description)
{
    QPNGImageWriter writer(device);
    if (quality >= 0) {
        quality = qMin(quality, 100);
        quality = (100-quality) * 9 / 91; // map [0,100] -> [9,0]
    }
    writer.setGamma(gamma);
    return writer.writeImage(image, quality, description);
}
 

qpnghandler.cpp

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


bool Q_INTERNAL_WIN_NO_THROW QPNGImageWriter::writeImage(const QImage& image, 

    int quality_in, const QString &description, int off_x_in, int off_y_in)
{ 

... 

    int height = image.height();
    int width = image.width();
    switch (image.format()) {
    case QImage::Format_Mono:
    case QImage::Format_MonoLSB:
    case QImage::Format_Indexed8:
    case QImage::Format_RGB32:
    case QImage::Format_ARGB32:
    case QImage::Format_RGB888:
        {
            png_bytep* row_pointers = new png_bytep[height];
            for (int y=0; y<height; y++)
                row_pointers[y] = (png_bytep)image.constScanLine(y);
            png_write_image(png_ptr, row_pointers);
            delete [] row_pointers;
        }
        break;
    default:
        {
            QImage::Format fmt = image.hasAlphaChannel() ? QImage::Format_ARGB32 : 

                                 QImage::Format_RGB32;
            QImage row;
            png_bytep row_pointers[1];
            for (int y=0; y<height; y++) {
                row = image.copy(0, y, width, 1).convertToFormat(fmt);
                row_pointers[0] = png_bytep(row.constScanLine(0));
                png_write_rows(png_ptr, row_pointers, 1);
            }
        }
        break;
    } 

... 

}

最終呼叫 QPNGImageWriter::writeImage 來把 QImage 的內容存成 png 格式。