OpenGL depth test works but not as expected - c++

Question

Using Windows7, VS2013, Qt 5.6, OpenGL 4.4
The code in question is as follows
void TestClass::paintGL()
{
/*
* Clear the screen
*/
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glLoadIdentity();
glClearColor(0, 0.25, 1, 1);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glEnable(GL_DEPTH_TEST);
glDepthMask(GL_TRUE);
glClearDepth(0.0);
glDepthFunc(GL_LEQUAL);
GLenum error = glGetError();
if (error != GL_NO_ERROR)
{
int i = 0;
i++;
}
/*
*/
//*************************************************
//double xx = mRotation.x() * mRotation.x();
//double xy = mRotation.x() * mRotation.y();
//double xz = mRotation.x() * mRotation.z();
//double xw = mRotation.x() * mRotation.scalar();
//double yy = mRotation.y() * mRotation.y();
//double yz = mRotation.y() * mRotation.z();
//double yw = mRotation.y() * mRotation.scalar();
//double zz = mRotation.z() * mRotation.z();
//double zw = mRotation.z() * mRotation.scalar();
//double m00 = 1 - 2 * (yy + zz);
//double m01 = 2 * (xy - zw);
//double m02 = 2 * (xz + yw);
//double m03 = 0;
//double m10 = 2 * (xy + zw);
//double m11 = 1 - 2 * (xx + zz);
//double m12 = 2 * (yz - xw);
//double m13 = 0;
//double m20 = 2 * (xz - yw);
//double m21 = 2 * (yz + xw);
//double m22 = 1 - 2 * (xx + yy);
//double m23 = 0;
//double m30 = 0;
//double m31 = 0;
//double m32 = -30;
//double m33 = 1;
//double blarg[] = {m00, m01, m02, m03, m10, m11, m12, m13, m20, m21, m22, m23, m30, m31, m32, m33};
//glLoadMatrixd(&blarg[0]);
//*************************************************
QMatrix4x4 yaw;
yaw.rotate(mYPR.x(), QVector3D(0, -1, 0));
QMatrix4x4 pitch;
pitch.rotate(mYPR.y(), QVector3D(1, 0, 0));
QMatrix4x4 roll;
roll.rotate(mYPR.z(), QVector3D(0, 0, -1));
QMatrix4x4 translate;
translate.translate(0, 0, -30);
//QMatrix4x4 vnToOpenGL(0, 0, -1, 0,
// 1, 0, 0, 0,
// 0, -1, 0, 0,
// 0, 0, 0, 1);
QMatrix4x4 vnToOpenGL(1, 0, 0, 0,
0, 1, 0, 0,
0, 0, 1, 0,
0, 0, 0, 1);
QMatrix4x4 openGLMatrix = vnToOpenGL * translate * yaw * pitch * roll * vnToOpenGL.transposed();
glLoadMatrixf(reinterpret_cast<const float*>(openGLMatrix.constData()));
int depth;
glGetIntegerv(GL_DEPTH_BITS, &depth);
QString ytext(QString("yaw : ") + QString::number(mYPR.x()));
QString ptext(QString("pitch : ") + QString::number(mYPR.y()));
QString rtext(QString("roll : ") + QString::number(mYPR.z()));
QString dtext(QString("depth : ") + QString::number(depth));
QPainter painter(this);
painter.drawText(10, 10, ytext);
painter.drawText(10, 25, ptext);
painter.drawText(10, 40, rtext);
painter.drawText(10, 55, dtext);
painter.end();
//*************************************************
//double scalar = std::acos(mRotation.scalar()) * 2;
//double X = std::asin(mRotation.x()) * 2;
//double Y = std::asin(mRotation.y()) * 2;
//double Z = std::asin(mRotation.z()) * 2;
//glRotated(scalar * radToDeg, X * radToDeg, Y * radToDeg, Z * radToDeg);
glScaled(mScale, mScale, mScale);
QVector3D orthogonalPoint;
QVector3D orthogonalVector;
QVector3D currentNormal;
for (size_t index = 0; index < mSphere->getTriangles().size(); index++)
{
// BEGIN CODE IN QUESTION
glEnable(GL_DEPTH_TEST);
//glDepthMask(GL_TRUE);
//glClearDepth(0.0);
//glDepthFunc(GL_LEQUAL);
// END CODE IN QUESTION
currentNormal = mSphere->getTriangles()[index].getNormal();
orthogonalVector = QVector3D::crossProduct(QVector3D(currentNormal.x(), currentNormal.y(), currentNormal.z()),
QVector3D(currentNormal.x(), currentNormal.y(), 0));
orthogonalVector.normalize();
glLineWidth(1.0);
glColor3f(1.0, 1.0, 1.0);
glBegin(GL_LINES);
glVertex3f(0, 0, 0);
glVertex3f(currentNormal.x(), currentNormal.y(), currentNormal.z());
glEnd();
glLineWidth(3.0);
glColor3f(1.0, 0.0, 0.0);
glBegin(GL_LINE_STRIP);
glVertex3f(0, 0, 0);
glVertex3f(0, 0, -1);
glEnd();
glLineWidth(3.0);
glColor3f(0.0, 1.0, 0.0);
glBegin(GL_LINE_STRIP);
glVertex3f(0, 0, 0);
glVertex3f(1, 0, 0);
glEnd();
glLineWidth(3.0);
glColor3f(0.0, 1.0, 1.0);
glBegin(GL_LINE_STRIP);
glVertex3f(0, 0, 0);
glVertex3f(0, 1, 0);
glEnd();
glColor3f(0.0, 0.0, 0.0);
glBegin(GL_TRIANGLE_FAN);
glVertex3d(currentNormal.x(), currentNormal.y(), currentNormal.z());
for (size_t fanIndex = 0; fanIndex < 9; ++fanIndex)
{
double degrees = static_cast<double>(fanIndex) * (360.0 / mTargetTesselations);
QMatrix4x4 matrix;
matrix.rotate(degrees, currentNormal.x(), currentNormal.y(), currentNormal.z());
orthogonalPoint = (orthogonalVector * matrix) / 20.0;
glVertex3d(currentNormal.x() + orthogonalPoint.x(),
currentNormal.y() + orthogonalPoint.y(),
currentNormal.z() + orthogonalPoint.z());
}
glEnd();
glColor3f(1.0, 0.0, 0.0);
glBegin(GL_TRIANGLE_FAN);
glVertex3d(currentNormal.x(), currentNormal.y(), currentNormal.z());
for (size_t fanIndex = 0; fanIndex < 9; ++fanIndex)
{
double degrees = 360.0 - (static_cast<double>(fanIndex) * (360.0 / mTargetTesselations));
QMatrix4x4 matrix;
matrix.rotate(degrees, currentNormal.x(), currentNormal.y(), currentNormal.z());
orthogonalPoint = (orthogonalVector * matrix) / 20.0;
glVertex3d(currentNormal.x() + orthogonalPoint.x(),
currentNormal.y() + orthogonalPoint.y(),
currentNormal.z() + orthogonalPoint.z());
}
glEnd();
}
/*
* Don't forget about the model-view matrix
*/
glPopMatrix();
}
I had bee searching basically all night to figure out why the depth test was not working. Things in the distance were being drawn over the things in the foreground. Finally, after writing halfway through a question here on stackoverflow I placed the code
glEnable(GL_DEPTH_TEST);
//glDepthMask(GL_TRUE);
//glClearDepth(0.0);
//glDepthFunc(GL_LEQUAL);
at the start of my for loop in the paintGL function. Low and behold the depth was correct. So, now that I have it working I'd really like to know WHY it is working. Why does this work properly because I enabled 'GL_DEPTH_TEST' in the loop? If I don't manually change it shouldn't the state remain the same? I'd like to know because knowing is half the battle.