Giving a QGLWidget a transparent background

WillWorkForCoffee

Hello!

If anyone saw my last thread, then you probably already know that I have been piddling around with QGLWidgets in my program.
I finally got my 3d drawing code to work, only to hit another problem. From what I have tried, I have been unable to successfully give my widget transparency on the form.
This includes setting the qglClearColor to a color with 0 for the alpha value, and using widget properties to apply the trait. These and other methods I have tried only resulted in a black background.

Anyone out there have any experience in this? I had seen suggestions that involved using Qpixmaps to make the background "look" transparent, but I needed some other opinions.

-Kaleb

WillWorkForCoffee

-bump-
Surely someone out there has an answer

WillWorkForCoffee

Really? No one?!

john_god

This works for me, in the widget constructor:

@setAttribute(Qt::WA_TranslucentBackground,true);
setWindowFlags(Qt::FramelessWindowHint | Qt::WindowStaysOnTopHint );
setWindowOpacity(0.5);
bool t=testAttribute ( Qt::WA_TranslucentBackground);
qDebug()<<"transparency"<<t;@

WillWorkForCoffee

I tried adding that code into the constructor, but unfortunately I can still see the qglclearcolor rather than the transparency. Am I doing something wrong??

Chris Kawa

Which OS are you working on? Either way this is a complex task and can't be done simply by setting some flags etc and has little to do with Qt unfortunately.

Don't know much about linux so I'll stick to windows.
On XP there was no window compositing so you're out of luck there. Beginning with Vista desktop compositing is done with DirectX. Windows draw on plain basic DirectX surfaces so you just set up some windows flags and gain direct acces to the window buffer and can efficiently draw to it whatever you want (including alpha channel).

With OpenGL this is an entirely different story. Windows does not allow so called layered OpenGL contexts(windows). Long story short -there's no simple way, and the hard ones have some drawbacks too (usually performance wise).
One option is to draw with OpenGL to some offscreen buffer and then copy it to the displayed qpixmap. With some amount of tinkering you can get it pretty efficient, but of course this is still too slow for some serious hi-frame animations. Take a look "here":http://www.dhpoware.com/demos/glLayeredWindows.html

On the other hand default Qt5 package uses now ANGLE on windows, which is an OpenGL-> DirectX translation layer, so maybe there is a way to get a hand on the native window buffer, but this obviously would be very difficult and hackish to say the least.

Edit: I played around and it seems that using DwmExtendFrameIntoClientArea with negative margin instead of DwmEnableBlurBehindWindow actually achieves what you want, but again - this requires Windows Vista or higher and platform specific dwmapi.h. I'm also not exactly sure what are the performance penalties with this.

john_god

The code I paste should work on Vista and above, on linux make sure your window manager has transparency enabled. Please lets us know your OS, Qt version, and small code snippet of what you did, and how are you displaying your widget.

WillWorkForCoffee

john_god:
So here is how I have my current form set up. I have a parent form with a few buttons on it, and a QGLWidget is placed on top. I then assign the class "GLWidget" to the QGLWidget. This class is defined in the code and handles drawing the basic cube. Here is where I added your code:
@
GLWidget::GLWidget(QWidget *parent) :
QGLWidget(parent),
timer(new QBasicTimer),
program(new QGLShaderProgram),
geometries(new GeometryEngine),
angularSpeed(0)

{
//setAutoBufferSwap(true);
setAttribute(Qt::WA_TranslucentBackground,true);
setWindowFlags(Qt::FramelessWindowHint | Qt::WindowStaysOnTopHint );
setWindowOpacity(0.5);
bool t=testAttribute ( Qt::WA_TranslucentBackground);
qDebug()<<"transparency"<<t;
}
@

As for the platform: I am using Windows 7 Professional edition, and my QT version is Qt5.

Krzysztof Kawa: Thanks for your feedback. So what you are essentially stating here is that I could hack together a solution, but it will have performance issues? I took a look at the link you provided, it has some promise if I can find a way to make it somewhat efficient.

john_god

What happens if you set the transparency to the parent form ?

WillWorkForCoffee

If I place the same code into the parent form's code, it simply does not show up. So clearly the transparency code works for the parent form, but the QGLWidget seems unaltered by it.

john_god

Try this on the MainWidget constructor

    @setAttribute(Qt::WA_TranslucentBackground,true);
    setWindowOpacity(0.5);
    bool t=testAttribute ( Qt::WA_TranslucentBackground);
    qDebug()<<"transparency"<<t;@

I just tested in the cube demo and it works great on linux.

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.