Color channels are changing using Qt QImage with data and I don't know why - qt

Question

I have written a function in Qt to extract the data out of an image for the purpose of manipulating it.
I then have another function to reinsert the data back to an image and display it. The problem I am having is that even if I do no manipulation on the pixel data other than extract and reinsert, it is still changing the data. On a yellow image it changes it to turquoise blue when it should remain yellow.
I am including the function code to extract and reinsert as specimen code. I can include more if it is needed such as the display function etc...Does anyone know if I am doing something wrong?
#include "mainwindow.h"
#include "ui_mainwindow.h"
MainWindow::MainWindow(QWidget *parent)
: QMainWindow(parent)
, ui(new Ui::MainWindow)
{
ui->setupUi(this);
filter = "All Picture Files (*.png *.jpg *.jpeg *.bmp *.tif *.tiff)"
";; Bitmap Files (*.bmp) ;; JPEG (*.jpg *.jpeg) ;; PNG (*.png) ;; TIFF (*.tif *.tiff)";
}
MainWindow::~MainWindow()
{
delete ui;
}
void MainWindow::importImage()
{
importCancelled = false;
QString filename = QFileDialog::getOpenFileName(nullptr, QObject::tr("Import Image"), "", filter);
if(!filename.isEmpty()){
image.load(filename);
image = image.convertToFormat(QImage::Format_RGBA8888);
}
else {
importCancelled = true;
if(importCancelled){
QString cleanPlateCancelled = "Operation Cancelled";
ui->statusBar->showMessage(cleanPlateCancelled,5000);
return;
}
}
}
void MainWindow::scaleImage()
{
if (image.isNull()){
return;
}
else {
image = image.scaledToHeight(ui->view->height(), Qt::TransformationMode::SmoothTransformation);
}
}
void MainWindow::displayImage()
{
if (image.isNull()){
return;
}
else {
scene = new QGraphicsScene;
showImage = new QGraphicsPixmapItem(QPixmap::fromImage(image));
scene->addItem(showImage);
ui->view->setScene(scene);
}
}
void MainWindow::rgbaExtraction()
{
numberOfBytes = static_cast<uint>(image.sizeInBytes());
auto const imageData = image.bits();
rgba = std::vector<uchar>(numberOfBytes,0);
rgbaReset = std::vector<uchar>(numberOfBytes,0);
for (uint i{0}; i < numberOfBytes; ++i) {
rgbaReset[i] = rgba[i] = imageData[i];
}
}
void MainWindow::rgbaInsertion()
{
auto *imageData = new uchar[numberOfBytes];
for (uint i{0};i < numberOfBytes;++i) {
imageData[i] = rgba[i];
}
image = QImage(imageData, image.width(), image.height(), QImage::Format_RGBA8888);
}
void MainWindow::on_importButton_clicked()
{
importImage();
scaleImage();
displayImage();
rgbaExtraction();
}
void MainWindow::on_quitButton_clicked()
{
QApplication::quit();
}
void MainWindow::sceneUpdater()
{
showImage->setPixmap(QPixmap::fromImage(image));
scene->update();
ui->view->update();
}
void MainWindow::on_redSlider_valueChanged(int value)
{
QString redString = QString::number(value);
ui->redLabel->setText(redString);
redDelta = value;
colorRed();
rgbaInsertion();
sceneUpdater();
}
mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include <QDebug>
#include <QImage>
#include <QFileDialog>
#include <string>
#include <QGraphicsScene>
#include <QGraphicsPixmapItem>
QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
MainWindow(QWidget *parent = nullptr);
~MainWindow();
private slots:
void on_importButton_clicked();
void on_quitButton_clicked();
void on_redSlider_valueChanged(int value);
private:
QGraphicsPixmapItem *showImage;
QGraphicsScene *scene;
QString filter;
QImage image;
bool importCancelled;
QStatusBar *statusBar;
uint numberOfBytes;
std::vector<uchar> rgba;
std::vector<uchar> rgbaReset;
int redDelta{0};
int greenDelta{0};
int blueDelta{0};
int opacityDelta{0};
void importImage();
void scaleImage();
void displayImage();
void rgbaExtraction();
void rgbaInsertion();
void sceneUpdater();
void colorRed(); // Implemented in color.cpp
Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H
color.cpp
#include <mainwindow.h>
void MainWindow::colorRed()
{
for (uint i{0}; i < rgba.size()*sizeof (rgba[i]);i+=4) {
if(rgbaReset[i] + static_cast<uchar>(redDelta)>=255){
rgba[i] = 255;
}
else {
rgba[i] = rgbaReset[i];// + static_cast<uchar>(redDelta);
}
}
}
main.cpp
#include "mainwindow.h"
#include <QApplication>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}

Answer 1

The problem is in scaleImage(), because scaledToHeight() returns another image format. You may omit the scale operation, or convert the returned image to Format_RGBA8888:
void MainWindow::scaleImage()
{
if (image.isNull()){
return;
}
else {
image = image.scaledToHeight(ui->view->height(), Qt::TransformationMode::SmoothTransformation)
.convertToFormat(QImage::Format_RGBA8888);
}
}
My recomendation is to add some instrumentation after each image manipulation to check that it has the expected format:
qDebug() << Q_FUNC_INFO << "image format:" << image.format();