Giving a QGLWidget a transparent background

Chris Kawa

Yeah, this works for regular widgets, but doesn't seem to for QGLwidget on Windows.

As I said you can try to put this in your constructor as a platform dependent solution:
@
#include <dwmapi.h>
#pragma comment(lib, "dwmapi")

//in the constructor
setWindowFlags(Qt::FramelessWindowHint);
MARGINS m;
m.cxLeftWidth = -1;
m.cxRightWidth = -1;
m.cyBottomHeight = -1;
m.cyTopHeight = -1;
DwmExtendFrameIntoClientArea((HWND)winId(), &m);
@
Seems to be working, but check if it meets your requirements.

WillWorkForCoffee

That somewhat accomplished what I wanted, actually!
However it gave the entire window transparency, is there a way to only give the background of the QGLWidget transparency?

Thanks again!

Chris Kawa

The opacity is not the same as full alpha channel transparency. Opacity makes entire window and its contents transparent and doesn't give you per pixel control (eg. only background transparent and the drawings fully opaque).

WillWorkForCoffee

Sorry Krzysztof Kawa, ignore my last post, it was aimed at john_god.
I am trying out your code at the moment.

WillWorkForCoffee

So, after trying your code Krzysztof Kawa, I did not see a difference.

Here is the constructor of the widget:
@GLWidget::GLWidget(QWidget *parent) :
QGLWidget(parent),
timer(new QBasicTimer),
program(new QGLShaderProgram),
geometries(new GeometryEngine),
angularSpeed(0)

{
setWindowFlags(Qt::FramelessWindowHint);
MARGINS m;
m.cxLeftWidth = -1;
m.cxRightWidth = -1;
m.cyBottomHeight = -1;
m.cyTopHeight = -1;
DwmExtendFrameIntoClientArea((HWND)winId(), &m);
}@

And a screenshot:
This one with no qglclearcolor
!http://img13.imageshack.us/img13/3069/noclear.png(No clear color)!

john_god

bq. That somewhat accomplished what I wanted, actually!
However it gave the entire window transparency, is there a way to only give the background of the QGLWidget transparency?
Thanks again!

I don't know. If you find let us know :) I will try other ways.

Chris Kawa

The DWM thingie worked for me. I got this triangle with transparent background:
!http://img801.imageshack.us/img801/7412/transparency.jpg(triangle with transparent background)!

Here's the widget code I used:
@
#include "myglwidget.h"
#include <dwmapi.h>

#pragma comment(lib, "dwmapi")

MyGLWidget::MyGLWidget(QWidget *parent) :
QGLWidget(parent)
{
setWindowFlags(Qt::FramelessWindowHint | Qt::WindowStaysOnTopHint);

MARGINS m;
m.cxLeftWidth = -1;
m.cxRightWidth = -1;
m.cyBottomHeight = -1;
m.cyTopHeight = -1;
DwmExtendFrameIntoClientArea((HWND)winId(), &m);

}

void MyGLWidget::initializeGL()
{
glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
}

void MyGLWidget::resizeGL(int w, int h)
{
glViewport(0, 0, w, h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
GLfloat x = GLfloat(w) / h;
glFrustum(-x, x, -1.0, 1.0, 1.0, 10.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}

void MyGLWidget::paintGL()
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

glBegin(GL_TRIANGLES);
glColor3f(1.0, 0.0, 0.0);
glVertex3f(-1.0, 1.0, -3.0);
glColor3f(0.0, 1.0, 0.0);
glVertex3f(1.0, 1.0, -3.0);
glColor3f(0.0, 0.0, 1.0);
glVertex3f(0.0, -1.0, -3.0);
glEnd();

}
@

WillWorkForCoffee

Oh man. I placed your code into my GLWidget's code, just to see if it would work. But I am now getting errors that functions like glVertex3f are undefined. Is it a possibility that something with my .lib files is preventing this from working?
Let me give you a lowdown of my environment so you aren't in the dark on this.
I'm running Visual Studio 2010, with the QT 5.0 addin. Here are the additional dependencies in my linker:
@qtmaind.lib
Qt5Cored.lib
Qt5Guid.lib
Qt5Widgetsd.lib
msvcrtd.lib
Qt5OpenGLd.lib
libEGLd.lib
libGLESv2d.lib
gdi32.lib
user32.lib
dwmapi.lib@

The includes I have at the top of the GLWidget:
@#include "GLWidget.h"
#include "geometryengine.h"
#include <QtOpenGL/QGLShaderProgram>
#include <QBasicTimer>
#include <QMouseEvent>
#include <QDebug>
#include <math.h>
#include <locale.h>
#include <dwmapi.h>
#pragma comment(lib, "dwmapi")@

I feel like I'm missing something here. Is there something up with my dependencies or includes?
Thanks again for the help though. I'm confident that your code works flawlessly, my compiler just doesn't like the thought of success lol.

Chris Kawa

Hm, I might be wrong but it looks like yet another type of issue.

The thing is I'm using Qt5 I compiled myself with the -opengl desktop switch, which means I get to use the full desktop OpenGl along with the fixed pipeline (glMatrixMode, glBegin, glVertex etc.).

I'm guessing you are using the default pre-compiled Qt5 package available from download page, which, as I said in one of the earlier post, uses ANGLE - an OpenGL ES implementation on top of DirectX.
I think the fixed pipeline is not available on OpenGL ES. You need to use VBOs, shaders and your own matrix transformations there so the code I pasted is not valid in that scenario. Sorry for the confusion.

WillWorkForCoffee

Ah alright, that is definitely a possibility. I am, in fact, using the pre-compiled Qt5 package as you said. Is there any way I can recompile it with the -opengl switch?
I feel as though that may be the simpler solution, but I honestly have no clue.

Chris Kawa

Well yes, you can download the sources from "here":http://releases.qt-project.org/qt5/5.0.1/single/qt-everywhere-opensource-src-5.0.1.zip or from git and compile it with whatever options you might need, but I would reserve myself from calling it a simler solution. The process is described "here":http://qt-project.org/wiki/Building_Qt_5_from_Git and takes some nerve, some luck, a lot of fiddling and a few hours at least :)

The easier solution would be to ignore the opengl part of my code and stick to the OpenGL ES subset when drawing (which the cube demo does). My triangle there is just to show anything on the screen, the really important parts are the window flags and the DWM thing.

On the other hand it might well be the case that the ANGLE port has some problems with transparency, but I have no way to check that at the moment as I don't have the prebuilt package setup, sorry.

Halftux

Hi,

I compiled this tranparent qglwidget and it is working as expected. Now I wanted to make the widget transparent for mouse events by setting an attribute "setAttribute( Qt::WA_TransparentForMouseEvents, true );".
Sadly this has no effect. When I have a normal Widget with this attribute which holds a qglwidget both ingnore the mouse events.
However then the qglwidget is no more transparent.
Is there a way to get a standalone transparent qglwidget to ignore mouse events?

? Offline

@Halftux Hi. If everything else doesn't work you can still install an eventfilter on said widget.

Halftux

@Wieland is it possible to route the caught events by the eventfilter to the window system (windows 7)? With WA_TransparentForMouseEvents mouse over get ignored and when you click on the widget the window below will get active (or get the event). So in principle the qapplication is invisible for all mouse events and other application or desktop will see the events.

? Offline

@Halftux 1) The WA_TransparentForMouseEvents flags works only for widgets inside your application. 2) If you're fine with a Windows-only solution, then this should do the trick.

Halftux

@Wieland Thank you now I got it.

WS_EX_TRANSPARENT was not working with the qglwidget. However I used now a window flag.

setWindowFlags(Qt::FramelessWindowHint | Qt::WindowStaysOnTopHint | Qt::WindowTransparentForInput);
in the constructor did the right thing.