How to draw 2d text in a QGLWidget

julio jerez

Hi I am trying to make a QGLWidget for a 3d viewer,
At the very basic level my class looks like this.

@DemoEntityManager::DemoEntityManager(QWidget* const parent)
:QGLWidget (parent)
,m_font()
//,m_font("Arial")
{
// Create the main Camera
m_camera = new DemoCamera();

// initialized the physics world for the new scene
Cleanup ();

// Start rendering after the system is set properly
ContinueExecution();
}

void DemoEntityManager::paintGL()
{
glClearColor (0.5f, 0.5f, 0.5f, 0.0f );
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
renderText (4, 4, "This is a test", m_font);
glFlush();
}@

However no mater what I do I can no get the ext to show on the screen, what I am doin wrong.

I windows the way I do this is by creation and array display List wih on for diaply lest mtaching ever font, windo come with spacil funtion to do that,
At typical implemantaion will look like this

@GLuint GLContext::BuildFont()
{
HFONT font = CreateFont(
20, // nHeight
0, // nWidth
0, // nEscapement
0, // nOrientation
FW_BOLD, // nWeight
FALSE, // bItalic
FALSE, // bUnderline
0, // cStrikeOut
ANSI_CHARSET, // nCharSet
OUT_DEFAULT_PRECIS, // nOutPrecision
CLIP_DEFAULT_PRECIS, // nClipPrecision
DEFAULT_QUALITY, // nQuality
DEFAULT_PITCH | FF_SWISS, // nPitchAndFamily
_T("Arial")); // lpszFacename

// only 96 usable ascii for performance in some low end system
GLuint displayLists = glGenLists(96);

// remember the old font
HFONT oldfont = (HFONT)SelectObject (m_hDC, font);
wglUseFontBitmaps(m_hDC, 32, 96, displayLists);
SelectObject(m_hDC, oldfont);
DeleteObject(font);

return displayLists;
}@

Then to print any text, is just a matter to call the display list with the array on text as an array of display list starting from the base,
I assuming Qt does something similar but I cannot figure it out,
Can someone help me?

Franzk

The advice from "QGLWidget":http://doc.trolltech.com/latest/qglwidget.html#renderText docs:

[quote]"Overpaint":http://doc.trolltech.com/latest/opengl-overpainting.html with QPainter::drawText() instead.[/quote]

julio jerez

I go it, thank you.

I have anothe quetsion related to this and peroformance.
do you know if the 2d text is drawn using Open Gl command or is by using the drawing funtionality.

one of the big problems in many graphics apps is that bitmap fonts becomes the slower part of the application, you can see that for example when ising SDL ot GLUT.
It is always better to make fonts by a using a true font and building a display List for each character.

baysmith

Another way is using a QPainter to draw to a texture which is displayed with quads. This is the approach I use in MIFit. You can see the code here:

http://gitorious.org/mifit/mifit/blobs/master/libs/opengl/Text.h
http://gitorious.org/mifit/mifit/blobs/master/libs/opengl/Text.cpp

Franzk

QPainter uses openGL calls to render the text. (It's a gl widget after all.)

julio jerez

Thanks I saw that, and I have what I want aleady.
QPaint provides lots of 2d functionality, it is awesome.

Thanks you for the help