Image processing with using QByteArray

[loading...]

Hi all.
i have to process image in Qt, and i want do it with using qbytearray. I know how to convert QPixmap to QByteArray. For example:
QPixmap pixmap = OriginalImg.copy(0,0,x,y);
QByteArray ba;
QBuffer buffer(&ba);
buffer.open(QIODevice::WriteOnly);
pixmap.save(&buffer, "PNG");

PNG is a compressed format, so if the size of pixmap is 860x860, the size of ba is only 128518 (not 860*860*4). And i don't know how to change the color of pixel(i,j) on image using qbytearray ba. Can anyone teach me how to achieve it.
thanks