Insane CPU usage in QT 5.0
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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);
}
