Insane CPU usage in QT 5.0 - c++

Question

I'm having trouble using the QT framework, particularly with the paintEvent of QWidget. I have a QWidget set up, and am overriding the paintEvent of it. I need to render a bunch of rectangles (grid system), 51 by 19, leading to 969 rectangles being drawn. This is done in a for loop. Then I also need to draw an image on each on of these grids. The QWidget is added to a QMainWindow, which is shown.
This works nicely, but it's using up 47% of CPU per window open! And I want to allow the user to open multiple windows like this, likey having 3-4 open at a time, which puts the CPU close to 150%.
Why does this happen? Here is the paintEvent contents. The JNI calls don't cause the CPU usage, commenting them out doesn't lower it, but commenting out the p.fillRect and Renderer::renderString (which draws the image) lowers the CPU to about 5%.
// Background
QPainter p(this);
p.fillRect(0, 0, this->width(), this->height(), QBrush(QColor(0, 0, 0)));
// Lines
for (int y = 0; y < Global::terminalHeight; y++) {
// Line and color method ID
jmethodID lineid = Manager::jenv->GetMethodID(this->javaClass, "getLine", "(I)Ljava/lang/String;");
error();
jmethodID colorid = Manager::jenv->GetMethodID(this->javaClass, "getColorLine", "(I)Ljava/lang/String;");
error();
// Values
jstring jl = (jstring) Manager::jenv->CallObjectMethod(this->javaObject, lineid, jint(y));
error();
jstring cjl = (jstring) Manager::jenv->CallObjectMethod(this->javaObject, colorid, jint(y));
error();
// Convert to C values
const char *l = Manager::jenv->GetStringUTFChars(jl, 0);
const char *cl = Manager::jenv->GetStringUTFChars(cjl, 0);
QString line = QString(l);
QString color = QString(cl);
// Render line
for (int x = 0; x < Global::terminalWidth; x++) {
QColor bg = Renderer::colorForHex(color.mid(x + color.length() / 2, 1));
// Cell location on widget
int cellx = x * Global::cellWidth + Global::xoffset;
int celly = y * Global::cellHeight + Global::yoffset;
// Background
p.fillRect(cellx, celly, Global::cellWidth, Global::cellHeight, QBrush(bg));
// String
// Renders the image to the grid
Renderer::renderString(p, tc, text, cellx, celly);
}
// Release
Manager::jenv->ReleaseStringUTFChars(jl, l);
Manager::jenv->ReleaseStringUTFChars(cjl, cl);
}

Answer 1

The problem you're having is that every time you call a draw function on QPainter, it has an overhead in setting up the painter for what it needs to draw, especially when the pen and brush needs changing. As you're calling the function over 900 times, that's over 900 times that Painter needs to change its internal properties for rendering and would explain why commenting out the drawRect and drawString functions reduce the CPU usage.
To solve this problem you need to batch up all the same types of paint calls, where a type uses the same brush and pen. To do this you can use the class QPainterPath and add the objects that you need with functions such as addRect(..); ensuring you do this outside of the paint function!
If, for example, you were going to draw a chess board pattern, you would create two QPainterPath objects and add all the white squares to one and all the black to another. Then when it comes to drawing use the QPainter function drawPath. This would only require two calls to draw the entire board.
Of-course, if you need each square to be a different colour, then you're still going to have the same problem, in which case I suggest looking to generate an image of what you need and rendering that instead.