transparency for an image?

[Darkware]

I have spent about an hour and a half trying to work with masks, whether it be under the QPixmap, QImage, or QColor classes and haven't gotten anywhere. So, I decided to try to just manipulate the individual pixels one by one as a last resort.

The objective is to extract the rgb from each pixel, then set that pixel to the same rgb values, but use a different alpha value. So essentially, just make the 50x50 section of the image more see-through or opaque.

My image is just a red circle. When I run this block of code, this is what it looks like: http://imgur.com/Rrn5sSn It's successfully attempting to manipulate the 50x50 pixel box of the image I'm wanting to modify, it's just not modifying them correctly.

	QImage p2("myBG.png");
	QColor apixel;
	QRgb value;
	for(int i=1; i<50; i++){
		for(int j=1; j<50; j++){
			apixel = p2.pixel(i,j);
			value = qRgba(apixel.red(), apixel.green(), apixel.blue(), 200);
			p2.setPixel(i, j, value);
		}
	}

The end goal is for this silly red circle to be a button (with a much cleaner looking graphic) where it fades in a little when you hover over it and fades out when you un-hover it.

Side note: I'd rather manage the animation myself using time() or something rather than use the animation machine stuff. I don't like that code at all and refuse to use it just so you know.

[Sam]

I use the following code to create a transparent image , give it a try

 QPixmap temp(mainPixmap.size());
    temp.fill(Qt::transparent);

    QPainter p(&temp);
    p.setCompositionMode(QPainter::CompositionMode_Source);
    p.drawPixmap(0, 0, mainPixmap);
    p.setCompositionMode(QPainter::CompositionMode_DestinationIn);
    p.fillRect(temp.rect(), QColor(0, 0, 0, 200));   
    p.end();

    mainPixmap = temp;

[Darkware]

This works great. It must be something to do with the composition modes. I will read the docs on those. Thanks for the quick reply. Very much appreciated.

[onek24]

@Darkware
If you still want to chance alpha by modifying pixel by pixel, you might try it that way:

QImage image = ...;
for ( int y = image.height(); y--; ) {
	QRgb *rgbLine = (QRgb*)image.scanLine(y);
	for ( int x = image.width(); x--; ) {
		int red = qRed(*rgbLine);
		int green = qGreen(*rgbLine);
		int blue = qBlue(*rgbLine);
		int alpha = qAlpha(*rgbLine)

		// modify your values here

		*rgbLine = qRgba(red, green, blue, alpha);
		++rgbLine;
	}
}

[Darkware]

So I have one more question regarding this image code. The code works if it's all within main(), but the moment I try to separate the pixmap loading code from main() by putting it into a function and just calling that function, it ceases to work.

The strangest thing is that I can get the graphics to appear if I put a QMessageBox in the function, but you can only see the graphics as long as the QMessageBox is there. Once you close out of it, the graphics disappear.

Where am I doing wrong?

#include <QtCore>
#include <QtWidgets>
#include <QApplication>

void startscreen(QGraphicsScene *scene);

int main(int argc, char **argv)
{
	QApplication app(argc, argv);

	//make the scene
	QGraphicsScene* scene = new QGraphicsScene;
	scene->setBackgroundBrush(Qt::white);

	//make the view for the scene
	QGraphicsView window(scene);
	window.setFrameStyle(0);
	window.setAlignment(Qt::AlignLeft | Qt::AlignTop);
	window.setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
	window.setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
	window.resize(300, 300);
	window.show();

	//load graphics
	startscreen(scene); //If you take this function out and
					//just put the graphics code here
					//instead of calling them through
					//a function, the graphics load.
					//Through this function, they don't
					//though.

	return app.exec();
}

void startscreen(QGraphicsScene *scene)
{
	//Make a QPixmap from "myBG.png"
	//and make it more transparent with a QPainter
	QPixmap p9("myBG.png");
	QPixmap temp(p9.size());
	temp.fill(Qt::transparent);
	QPainter p(&temp);
	p.setCompositionMode(QPainter::CompositionMode_Source);
	p.drawPixmap(0, 0, p9);
	p.setCompositionMode(QPainter::CompositionMode_DestinationIn);
	p.fillRect(temp.rect(), QColor(0, 0, 0, 50));
	p.end();
	p9 = temp;

	//Make a QGraphicsPixmapItem from p9 above
	//so it can be added to the scene.
	QGraphicsPixmapItem gfxPixItemP2( p9 );
	QPointF mpoint2(100,100);
	gfxPixItemP2.setPos(mpoint2);
	scene->addItem(&gfxPixItemP2);

	//Graphics only show up if this message box is here.
	//Also, you have to keep the message box open for the graphics
	//to appear. Once you close the box, the graphics disappear.
	QMessageBox msgBox;
	msgBox.setText("My graphics only show up if I exec() this QMessageBox");
	msgBox.exec();
}

#include "main.moc"

[mrjj]

@Darkware said:
Hi the differnce between having the code directly in main and
using void startscreen(QGraphicsScene *scene)
is that when using the function, variables can run out of scope and get deleted.

When u then put in messagebox and use exec() u block/stop it from running out of
scope until u press ok.

I didnt look through all the code but you clearly have some classes u allocated in the function that need to have longer life than the function call.
Eaxmple
addItem(QGraphicsItem * item)
expects pointer to the element.
You give it address of element
QGraphicsPixmapItem gfxPixItemP2( p9 );
scene->addItem(&gfxPixItemP2);

gfxPixItemP2 will ONLY exists as long as inside function.
You need to new it
QGraphicsPixmapItem *gfxPixItemP2=new QGraphicsPixmapItem ( p9 );
scene->addItem(gfxPixItemP2);

[Darkware]

Thanks. I got it working.

Sam, onek24, mrjj - thank you for all for the help you have given me.