创建好QT工程后，重载绘制画面函数。

思路：

1.重新定义QT的绘制函数

2.根据void TestRGB::paintEvent(QPaintEvent* ev)函数里的进行思路进行编程

main.cpp

#include "testrgb.h"
#include <QtWidgets/QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    TestRGB w;
    w.show();
    return a.exec();
}

testrgb.cpp

#include "testrgb.h"
#include <QPainter>
#include <QImage>
static int w = 1280;
static int h = 720;

TestRGB::TestRGB(QWidget *parent)
    : QWidget(parent)
{
    ui.setupUi(this);
	resize(w,h);//窗口大小
}

void TestRGB::paintEvent(QPaintEvent* ev)
{
	QImage img(w, h, QImage::Format_RGB888);
	auto d = img.bits();
	unsigned char r = 255;
	for (int j = 0; j < h; j++)
	{
		r--;
		int b = j * w * 3;
		for (int i = 0; i < w * 3; i += 3)
		{
			d[b + i] = r;//R
			d[b + i + 1] = 0;//G
			d[b + i + 2] = 0;//B
		}
	}
	QPainter p;
	p.begin(this);

	p.drawImage(0, 0, img);

	p.end();

}

testrgb.h

#pragma once

#include <QtWidgets/QWidget>
#include "ui_testrgb.h"

class TestRGB : public QWidget
{
    Q_OBJECT

public:
    TestRGB(QWidget *parent = Q_NULLPTR);
    //重载绘制画面函数
    void paintEvent(QPaintEvent* ev) override;


private:
    Ui::TestRGBClass ui;
};