c++ - How to Reduce the noisy While Scale a Image/Picture with Qt -

i use code like:

void mylabel::paintevent(qpaintevent *event){     qpainter painters(this);     /*对img进行平滑缩放*/      int image_width,image_height;      image_width = width();      image_height = height();      qimage result = img.scaled(image_width<<2,          image_height<<2).scaled(image_width,                 image_height,qt::ignoreaspectratio,qt::smoothtransformation                 );     painters.drawpixmap(0,0,image_width,image_height,qpixmap::fromimage(img));     } 

i want scale image size want, when scale non equal proportion , terrible things happened ,noisy huge ,and image/picture this:

and want know how way scale image/picture instead:

i suspect because call qimage::scaled() twice, first upscale 4× desired size , second actual desired size. first call uses default value transformation mode (which qt::fasttransformation). you're combining non-smooth , smooth transformation. note first transformation should unnecessary anyway. scale desired size immediately, using qt::smoothtransformation.

qimage result = img.scaled(image_width, image_height,                            qt::ignoreaspectratio,                            qt::smoothtransformation); 

edit: noticed else. @ drawpixmap line:

painters.drawpixmap(0,0,image_width,image_height,qpixmap::fromimage(img)); 

you're drawing original img when meant draw result:

painters.drawpixmap(0,0,image_width,image_height,qpixmap::fromimage(result));