Game Programming using Qt 5 Beginner's Guide

上QQ阅读APP看书，第一时间看更新

Time for action – Adding path items to the scene

Let's paint a few objects consisting of a large number of lines:

static const int SIZE = 100;
static const int MARGIN = 10;
static const int FIGURE_COUNT = 5;
static const int LINE_COUNT = 500;
for(int figureNum = 0; figureNum < FIGURE_COUNT; ++figureNum) {
    QPainterPath path;
    path.moveTo(0, 0);
    for(int i = 0; i < LINE_COUNT; ++i) {
        path.lineTo(qrand() % SIZE, qrand() % SIZE);
    }
    QGraphicsPathItem *item = scene.addPath(path);
    item->setPos(figureNum * (SIZE + MARGIN), 0);
}

For each item, we first create a QPainterPath and set the current position to (0, 0). Then, we use the qrand() function to generate random numbers, apply the modulus operator (%) to produce a number from 0 to SIZE (excluding SIZE), and feed them to the lineTo() function that strokes a line from the current position to the given position and sets it as the new current position. Next, we use the addPath() convenience function that creates a QGraphicsPathItem object and adds it to the scene. Finally, we use setPos() to move each item to a different position in the scene. The result looks like this:

QPainterPath allows you to use practically every paint operation Qt supports. For example, QGraphicsPathItem is the only standard item able to draw Bezier curves in the scene, as QPainterPath supports them. Refer to the documentation of QPainterPath for more information.

Using painter paths in this example is very efficient, because we avoided creating thousands of individual line objects on the heap. However, putting a large part of a scene in a single item may reduce the performance. When parts of the scene are separate graphics items, Qt can efficiently determine which items are not visible and skip drawing them.