Problem using drawPixmap() to do scrolling effect in QPixmap

I'm trying to create a scrolling effect for painting done in a QPixmap, using a function that takes a pixmap and paints it into itself with the required deltas:

Code:

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void scrollGraphics(int dx, int dy, QPixmap *thePixmap)
{
  QPainter p;
  p.begin(thePixmap);
  p.drawPixmap(dx, dy, *thePixmap);
  p.end();
}

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If dx and/or dy < 0, the graphics of thePixmap appear shifted by the correct amount. However, if I pass a value greater than 0 for dx and/or dy, drawPixmap() paints into thePixmap multiple shifted copies of itself.

Is this a Qt bug? Or am I missing something?

You're missing a fact that you are trying to overdraw something with something that is currently being overdrawn... Paint a picture, let it dry and try painting this picture again shifted 1/3 to the right only by looking at the picture. You'll soon reach a stage when what you should paint now has just been overpainted with what is 1/3 of the image to the left. I assume that's exactly what you're observing. And I'm assuming you're doing that on Windows as on Unix you'd probably just get garbage.

Anyway... remove the pointers, work on copies - QPixmap is shared anyway.

Dziękuję serdecznie. :) I now understand why I can't paint a pixmap directly into itself.

However, I'm not completely clear on what you mean here:

Quote:

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Originally Posted by wysota

remove the pointers, work on copies - QPixmap is shared anyway.

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I understand that I need to make a copy of the pixmap, then paint the copy back into the original. But why are pointers bad in this case, and what do you mean by shared?

Thanks.

Quote:

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Originally Posted by ajoffe

Dziękuję serdecznie. :)

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Proszę uprzejmie.

Quote:

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I understand that I need to make a copy of the pixmap, then paint the copy back into the original. But why are pointers bad in this case, and what do you mean by shared

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Try something like this:

Code:

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QPixmap scrollGraphics(int dx, int dy, const QPixmap &thePixmap)
{
  QPixmap result(thePixmap.size());
  result.fill(Qt::transparent); // optionally
  QPainter p(&result);
  p.drawPixmap(dx, dy, thePixmap);
  p.end();
  return result;
}

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And read about "implicit sharing".

Quote:

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Originally Posted by wysota

Proszę uprzejmie.

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hmmmm... :)

Quote:

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Originally Posted by MrDeath

hmmmm... :)

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It means "you're welcome" in Polish. It's not a multi-post foreign language conversation :)

Quote:

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Originally Posted by wysota

It means "you're welcome" in Polish. It's not a multi-post foreign language conversation :)

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did i complaint? :). Anyways i learned one sentence in polish though:)