Qt motion (linear) blur

Question

Is there simple solution to add motion blur to an image in Qt? Haven't found any good tutorial about blur. I need something really simple, that I could understand, and would really good if I could change blur angles.

Answer 1

Qt hasn't a motion blur filter. I looked at the code of QGraphicsBlurEffect; it uses QPixmapBlurEffect which itself uses an internal helper method called expblur (exponential blur).

expblur itself uses a one-dimensional blur effect (motion blur in X direction, method qt_blurrow) twice. It rotates the image between the two blurs by 90 degrees and afterwards rotates it back.

So actually, Qt has a motion blur effect but this is internal only. So you need to write your own effect. To do so, have a look at the code of qt_blurrrow, which you can find in the Qt sources at src/gui/qpixmapfilter.cpp. This will give you a good quality for the filter, since it is an exponential blur filter instead of a linear one.

If you don't want to go so deep into the Qt source code, take this pseudo-code as a start:

foreach pixel (x, y) in image {
    for dx = -r to r with x+dx within image {
        for dy = -r to r with y+dy within image {
            Add pixel (x+dx, y+dy) of the source image with ↩
            factor (matrix[dx, dy]) to the target image.
        }
    }
}

where matrix might be defined like this (for horizontal motion blur with radius 2):

0   0   0   0   0
0   0   0   0   0
0.1 0.2 0.4 0.2 0.1
0   0   0   0   0
0   0   0   0   0

Notice that the sum of all entries has to be 1, or you have to divide the colors by the sum of the matrix entries.

Building this matrix for a given radius r and angle α is difficult when you want to allow arbitrary angles for α (not only 90 degrees steps). [EDIT: See comment 3 for an easy generation of such a matrix.]