Shifting a QImage up by 1 pixel row

[MSUdom5]

I was wondering if someone could suggest an efficient way to do this:

I have a QImage of an arbitrary size. I want to very quickly transform this to a QImage of the same size with the entire image shifted up by one row of pixels. The bottom row of pixels will be the same color. In other words, I want to shift the top row of pixels out and shift in a new row at the bottom of pixels with the same color.

I can think of ways to do this, but I need an extremely fast and efficient solution. Thoughts?

Thanks in advance,
MSUdom5

[wysota]

The fastest way would probably be this:

QImage shiftUp(const QImage &original) {
  QImage shifted(original.size(), original.format());
  if(!original.colorTable().isEmpty())
    shifted.setColorTable(original.colorTable());
  uchar *data = original.bits();
  uchar *dest = shifted.bits();
  memcpy(dest, data+original.bytesPerLine(), original.numBytes()-original.bytesPerLine());
  return shifted;
}

QImage shiftUp(const QImage &original) {
  QImage shifted(original.size(), original.format());
  if(!original.colorTable().isEmpty())
    shifted.setColorTable(original.colorTable());
  uchar *data = original.bits();
  uchar *dest = shifted.bits();
  memcpy(dest, data+original.bytesPerLine(), original.numBytes()-original.bytesPerLine());
  return shifted;
}

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Then just fill the last row with your colour (you can probably use memset() to do that quickly).

[hickscorp]

If you're trying to do it without duplicating your image (e.g. working directly in your original image), you can use a convolution matrix of this vector:
0 0 0
0 0 0
0 1 0
Or more simply:
0
0
1

Pierre.