QGLWidget renders text at incorrect depth - qt

Question

I use this to render text in a QGLWidget
QGLWidget::renderText(x, y, z, text, font)
The string is rendered at a depth of ~0.5 (obtained via glReadPixel()).
However, in my case it should be closer to ~0.9.
When I convert the x,y,z coords into screen coords with the current matrices I also find a result of ~0.9.
Why such a difference? It makes the text appear always in front.
I created a simple QT project in visual studio to reproduce the issue.
It draws a green square with text in front and behind the square. Both text look in front of the square. And the depth of the pixel can be read by hovering the mouse.
I use Qt version 5.5 built for 64bit platform.
MyGLWidget.h
#include <QGLWidget>
#include <QMouseEvent>
class MyGLWidget : public QGLWidget
{
Q_OBJECT
private:
float _depth;
public:
MyGLWidget(QWidget * parent = 0);
virtual ~MyGLWidget();
virtual void initializeGL();
virtual void paintGL();
void mouseMoveEvent(QMouseEvent * event);
signals:
void depthRead(float);
};
MyGLWidget.cpp
#include "MyGLWidget.h"
#include <gl/GLU.h>
#include <Qfont>
MyGLWidget::MyGLWidget(QWidget * parent) : QGLWidget(parent)
{
}
MyGLWidget::~MyGLWidget()
{
}
void MyGLWidget::initializeGL()
{
}
void MyGLWidget::paintGL()
{
// set up projection
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
float width = this->width();
float height = this->height();
glViewport(0, 0, width, height);
gluPerspective(45, width / height, 1, 100);
// set up model view
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(0,0,5, // eye
0,0,0, // look at
0,1,0); // up
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glEnable(GL_DEPTH_TEST);
// draw a green square
glColor4f(0,1,0,1);
glBegin(GL_QUADS);
glVertex3f(-1,-1,0);
glVertex3f(1,-1,0);
glVertex3f(1,1,0);
glVertex3f(-1,1,0);
glEnd();
// render some blue text behind the square
QFont font;
font.setPointSize(20);
glColor4f(0,0,1,1);
renderText(-2,-0.5,-1, "BEHIND_BEHIND_BEHIND_BEHIND", font);
// render some red text in front of the square
glColor4f(1,0,0,1);
renderText(-2,0.5,+1, "IN_FRONT_IN_FRONT_IN_FRONT_I", font);
}
void MyGLWidget::mouseMoveEvent(QMouseEvent * event)
{
int x = event->x();
// flip y for QT origin is top left while OpenGL origin is bottom left
int y = this->height() - event->y();
// read pixel depth
glReadPixels(x, y, 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &_depth);
// update ui
emit depthRead(_depth);
}
rendertexttest.h
#ifndef RENDERTEXTTEST_H
#define RENDERTEXTTEST_H
#include <QtWidgets/QMainWindow>
#include "ui_rendertexttest.h"
#include "MyGLWidget.h"
class RenderTextTest : public QMainWindow
{
Q_OBJECT
public:
RenderTextTest(QWidget *parent = 0);
~RenderTextTest();
public slots:
void onDepthRead(float depth);
private:
Ui::RenderTextTestClass ui;
MyGLWidget * _glwidget;
};
#endif // RENDERTEXTTEST_H
rendertexttest.cpp
#include "rendertexttest.h"
RenderTextTest::RenderTextTest(QWidget *parent)
: QMainWindow(parent)
{
ui.setupUi(this);
_glwidget = new MyGLWidget(this);
_glwidget->setMouseTracking(true);
QObject::connect(_glwidget, SIGNAL(depthRead(float)),
this, SLOT(onDepthRead(float)));
ui._mainLayout->addWidget(_glwidget);
}
RenderTextTest::~RenderTextTest()
{
}
void RenderTextTest::onDepthRead(float depth)
{
ui._lblDepth->setText(QString::number(depth));
}
ui_rendertexttest.h
/********************************************************************************
** Form generated from reading UI file 'rendertexttest.ui'
**
** Created by: Qt User Interface Compiler version 5.3.1
**
** WARNING! All changes made in this file will be lost when recompiling UI file!
********************************************************************************/
#ifndef UI_RENDERTEXTTEST_H
#define UI_RENDERTEXTTEST_H
#include <QtCore/QVariant>
#include <QtWidgets/QAction>
#include <QtWidgets/QApplication>
#include <QtWidgets/QButtonGroup>
#include <QtWidgets/QHeaderView>
#include <QtWidgets/QLabel>
#include <QtWidgets/QMainWindow>
#include <QtWidgets/QVBoxLayout>
#include <QtWidgets/QWidget>
QT_BEGIN_NAMESPACE
class Ui_RenderTextTestClass
{
public:
QWidget *centralWidget;
QWidget *verticalLayoutWidget;
QVBoxLayout *_mainLayout;
QLabel *_lblDepth;
void setupUi(QMainWindow *RenderTextTestClass)
{
if (RenderTextTestClass->objectName().isEmpty())
RenderTextTestClass->setObjectName(QStringLiteral("RenderTextTestClass"));
RenderTextTestClass->resize(600, 438);
centralWidget = new QWidget(RenderTextTestClass);
centralWidget->setObjectName(QStringLiteral("centralWidget"));
verticalLayoutWidget = new QWidget(centralWidget);
verticalLayoutWidget->setObjectName(QStringLiteral("verticalLayoutWidget"));
verticalLayoutWidget->setGeometry(QRect(9, 9, 581, 381));
_mainLayout = new QVBoxLayout(verticalLayoutWidget);
_mainLayout->setSpacing(6);
_mainLayout->setContentsMargins(11, 11, 11, 11);
_mainLayout->setObjectName(QStringLiteral("_mainLayout"));
_mainLayout->setSizeConstraint(QLayout::SetDefaultConstraint);
_mainLayout->setContentsMargins(0, 0, 0, 0);
_lblDepth = new QLabel(centralWidget);
_lblDepth->setObjectName(QStringLiteral("_lblDepth"));
_lblDepth->setGeometry(QRect(10, 410, 581, 16));
RenderTextTestClass->setCentralWidget(centralWidget);
retranslateUi(RenderTextTestClass);
QMetaObject::connectSlotsByName(RenderTextTestClass);
} // setupUi
void retranslateUi(QMainWindow *RenderTextTestClass)
{
RenderTextTestClass->setWindowTitle(QApplication::translate("RenderTextTestClass", "RenderTextTest", 0));
_lblDepth->setText(QApplication::translate("RenderTextTestClass", "Depth:", 0));
} // retranslateUi
};
namespace Ui {
class RenderTextTestClass: public Ui_RenderTextTestClass {};
} // namespace Ui
QT_END_NAMESPACE
#endif // UI_RENDERTEXTTEST_H

Answer 1

There might 3 issues
1- the target depth is computed correctly in QGLWidget::renderText() and passed to the paintEngine via setTranslateZ(). However the vertex shader does not set this value directly in the vertex clipped coordinates. Instead it translate by this value.
2- the sign of translateZ seems incorrect. That would explain why the depth of the text pixels increases when the text comes closer.
3- it seems clipped coordinates [0, 1] are mapped to range [0.5 , 1] Yet I did not see any call to glDepthRange() in Qt's source.
If we change the code of qglslComplexGeometryPositionOnlyVertexShader in qglengineshadersource_p.h with the following, it fixes the problem.
static const char* const qglslComplexGeometryPositionOnlyVertexShader = "\n\
uniform highp mat3 matrix; \n\
uniform highp float translateZ; \n\
attribute highp vec2 vertexCoordsArray; \n\
void setPosition(void) \n\
{ \n\
vec3 v = matrix * vec3(vertexCoordsArray, 1.0); \n\
v.z = (-translateZ - 0.5f) * 2.0f; \n\
gl_Position = vec4(v.xyz, 1.0);\n\
} \n";

Answer 2

I found that a nice workaround is to render the text into a texture. And then display that texture in the scene.
This works nicely if the text is rendered over a solid background color.
QGLWidget does not seem to write to the alpha channel.