Why when drawBackground is called does it override the setBackgroundBrush colour? - c++

Question

I'm following this series on youtube Node Editor. I'm attempting to learn both Qt and C++ at the same time, possibly a stupid idea, but "hello world" tutorials don't do much for me.
Episode 1 We are to create a grid using the drawBackground method, however before this in the class constructor we have set the background to a light grey colour. When I run the code without the drawBackground the scene shows the light grey background. However when I run it with the drawBackground method uncommented the grid is drawn but the background is now white.
Why does the drawBackground() method override the QDMGraphicsScene() setBackgroundBrush?
qdmgraphicsscene.h
#ifndef QDMGRAPHICSSCENE_H
#define QDMGRAPHICSSCENE_H
#include <QGraphicsScene>
#include <QtGui>
#include <QtCore>
#include "math.h"
class QDMGraphicsScene : public QGraphicsScene
{
public:
QDMGraphicsScene();
//Settings
QColor _colour_background, _colour_light, _colour_dark;
QPen _pen_light, _pen_dark;
int scene_width, scene_height, gridSize, gridSquares;
QVector<QLine> lines_light, lines_dark;
void drawBackground(QPainter *painter, const QRectF &rect);
};
#endif // QDMGRAPHICSSCENE_H
qdmgraphicsscene.cpp
#include "qdmgraphicsscene.h"
QDMGraphicsScene::QDMGraphicsScene()
{
this->gridSize = 60;
this->gridSquares = 3;
this->_colour_background = QColor(57,57,57);
this->_colour_light = QColor(47,47,47);
this->_colour_dark = QColor(41, 41, 41);
this->_pen_light = QPen(this->_colour_light);
this->_pen_light.setWidth(1);
this->_pen_dark = QPen(this->_colour_dark);
this->_pen_dark.setWidth(2);
this->scene_width = 64000;
this->scene_height = 64000;
this->setSceneRect(-this->scene_width/2, -this->scene_height/2, this->scene_width, this->scene_height);
this->setBackgroundBrush(this->_colour_background);
}
void QDMGraphicsScene::drawBackground(QPainter *painter, const QRectF &rect)
{
//create grid
int left = (int)floor(rect.left());
int right = (int)ceil(rect.right());
int top = (int)floor(rect.top());
int bottom = (int)ceil(rect.bottom());
int first_left = left - (left % this->gridSize);
int first_top = top - (top % this->gridSize);
//compute lines
for(int x = first_left; x < right; x=x+this->gridSize){
if(x % (this->gridSize * this->gridSquares) != 0){
this->lines_light.append(QLine(x,top,x,bottom));
} else {
this->lines_dark.append(QLine(x,top,x,bottom));
}
}
for(int y = first_top; y < bottom; y=y+this->gridSize){
if(y % (this->gridSize * this->gridSquares) != 0){
this->lines_light.append(QLine(left,y,right,y));
} else {
this->lines_dark.append(QLine(left,y,right,y));
}
}
//draw lines
painter->setPen(this->_pen_light);
painter->drawLines(this->lines_light);
painter->setPen(this->_pen_dark);
painter->drawLines(this->lines_dark);
}

Answer 1

I suspect the problem is simply that your drawBackground override doesn't use the background brush -- you just set the QPainter pen and use it to draw lines.
I think the simplest solution would be to call the base class implementation of drawBackground at the top of your own, so...
void QDMGraphicsScene::drawBackground (QPainter *painter, const QRectF &rect)
{
/*
* Call base class drawBackground implementation to draw background.
*/
QGraphicsScene::drawBackground(painter, rect);
//create grid
.
.
.