Am I doing that right? Scaling in Qt - c++

Question

I wrote code to make a 2D transformation: scaling.
( value = variable from slider 1-10 )
int x=punktx.back();
int y=punkty.back();
DrawPixel(x*value/6.0,y*value/6.0,bits,255,255,255);
And I received that output:
As you can see I received a little breaks in that square. Is it okay or I have wrong code?

Answer 1

It's not how you scale things in Qt. Use QImage::scaled() or QPixmap::scaled() method instead.
Regarding your problem, the breaks are result of the fact that you use the same number of pixels for drawing the large square as for the small square - you would have to fill the gaps between the pixels, but scaling that way doesn't make sense anyway as mentioned above.

Answer 2

the problem is that if you iterate over an input image that's e.g. 10x10 pixels, and output the same number of pixels then you're only drawing 100 pixels no matter how much you "scale" it by. If you're scaling it to fill 20x20 pixels in size but you only draw your original 100 pixels, of course it will have holes in it.
If you want to implement a simple scaling function as a learning exercise, some approaches are:
instead of drawing 1 pixel per original pixel, draw a rectangle of the scaled size, in that pixel's color. This has the advantage that it's only a tiny change to your existing code, so it might be worth trying as a first attempt.
loop over the output pixels, then interpolate where that would be on the input image (reverse the scaling) then draw one pixel of the right color. This avoids some of the overheads with drawing lots of rectangles (which could paint the same output pixels more than once).
as above, but write the data into a bitmap stored in memory, then draw it all at once with a bitmap drawing command.
Also if you really want to get better results you can work out whether an output pixel crosses over several input pixels, then average out the colors, and so on. This will give smoother looking results but could blur things for solid color images.