how to modify pixels of QBitmap

Question

I want to work with masks on a QImage. To handle the mask, I have a QBitmap. Now I'm looking for a fast way to do this things:

1. Invert the Bits of the mask
2. Set pixels of the mask to a new value

is there a fast way to do this? Or can I only use a QPainter Object to modify the QBitmap?

Greetings

Answer 1

Your best bet is to use QImage with the format set to QImage::Format_Mono. This way you create a 1-bit per pixel image you can use as a mask.

1. For inverting the pixels, use the invertPixels method.
2. QImage bits can be accessed using the bits or scanLine methods.

To use the QImage as a mask, you'll have to convert it to a QPixmap first:

QPixmap mask = QPixmap::fromImage(img);
painter.setClipRegion(QRegion(mask));

Since QImage::Format_Mono encodes the pixels MSB first (means first pixel will be stored in most significant bit of the first byte) with 8 pixels/byte, you'll need some bit-magic to access the correct bit for the given x/y position:

int GetPixel(const QImage& img, const int x, const int y) const
{
    const uchar mask = 0x80 >> (x % 8);
    return img.scanLine(y)[x / 8] & mask ? 1 : 0;
}

void SetPixel(QImage& img, const int x, const int y, const int pixel)
{
    const uchar mask = 0x80 >> (x % 8);
    if (pixel)
        img.scanLine(y)[x / 8] |= mask;
    else
        img.scanLine(y)[x / 8] &= ~mask;
}

Of course, don't use a function like SetPixel when you're manipulating a lot of pixels on the same row, as you don't want to lookup scanLine(y) for each pixel. Be creative!