QtOpenGLWidget drawing triangle - c++

Question

How can I change the following code so that it actually draws the triangle?
First is the shader, then the implementation of the glwiedget class which is derived from QOpenglWidget.
// shaders here
static const char* vertexShaderSource =
"#version 330 core\n"
"in vec3 posAttr;\n"
//"attribute lowp vec3 colAttr;\n"
//"varying lowp vec4 col;\n"
//"uniform highp mat4 matrix;\n"
"void main() {\n"
//" col = colAttr;\n"
" gl_Position = vec4(posAttr, 1) ;\n"
"}\n";
// fragment shader
static const char* fragmentShaderSource =
"#version 330 core\n"
//"varying lowp vec4 col;\n"
"void main() {\n"
"gl_FragColor = vec4(1.0, 0.0, 1.0, 0.0);\n"
"}\n";
Glwidget::Glwidget(QWidget* parent):QOpenGLWidget(parent)
{
//
}
Glwidget::~Glwidget()
{
cleanup();
}
void Glwidget::initializeGL()
{
connect(context(), &QOpenGLContext::aboutToBeDestroyed, this, &Glwidget::cleanup);
initializeOpenGLFunctions();
glClearColor(.0f, .0f, .0f, 1.0f);
shader = new QOpenGLShaderProgram(this);
shader->addShaderFromSourceCode(QOpenGLShader::Vertex, vertexShaderSource);
shader->addShaderFromSourceCode(QOpenGLShader::Fragment, fragmentShaderSource);
// posAttribute = shader->attributeLocation("posAttr");
//colAttribute = shader->attributeLocation("colAttr");
//matrixAttribute = shader->uniformLocation("matrix");
Q_ASSERT(shader->link());
Q_ASSERT(shader->bind());
//shader->release();
glEnable(GL_DEPTH_TEST);
glEnable(GL_CULL_FACE);
}
void Glwidget::paintGL()
{
glClear(GL_COLOR_BUFFER_BIT);
makeCurrent();
matrix.perspective(60.0, 4.0f/3.0f, 0.1f, 10.0f);
matrix.translate(0, 0, -2);
matrix.rotate(100.0f, 0, 1, 0);
//shader->setUniformValue(matrixAttribute, matrix);
// shader->bind();
GLfloat vertices[] = {
0.0f, 0.707f, 1.0f,
-0.5f, -0.5f, 1.0f,
0.5f, -0.5f, 1.0f
};
shader->setAttributeArray(posAttribute,vertices, 3);
GLfloat colors[] = {
1.0f, 0.0f, 0.0f,
0.0f, 1.0f, 0.0f,
0.0f, 0.0f, 1.0f
};
glVertexAttribPointer(posAttribute, 3, GL_FLOAT, GL_FALSE, 0, vertices);
//glVertexAttribPointer(colAttribute, 3, GL_FLOAT, GL_FALSE, 0, colors);
glEnableVertexAttribArray(posAttribute);
//glEnableVertexAttribArray(colAttribute);
glDrawArrays(GL_TRIANGLES, 0, 1);
glDisableVertexAttribArray(posAttribute);
//glDisableVertexAttribArray(colAttribute);
//shader->release();
}
void Glwidget::resizeGL(int w, int h)
{
matrix.setToIdentity();
matrix.perspective(45.0f, w / float(h), 0.01f, 1000.0f);
//glViewport(0, 0, w, h);
}
void Glwidget::mousePressEvent(QMouseEvent *event)
{
Q_UNUSED(event);
}
void Glwidget::mouseMoveEvent(QMouseEvent *event)
{
Q_UNUSED(event);
}
void Glwidget::cleanup()
{
if (shader == nullptr)
return;
makeCurrent();
delete shader;
shader = 0;
doneCurrent();
}

Answer 1

You have to determine the attribute index of posAttr, after the program is linked:
Q_ASSERT(shader->link());
posAttribute = shader->attributeLocation("posAttr");
Since the depth test is enabled, you have to clear the depth buffer. See glClear:
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
You do not use any model, view or projection matrix, so the coordinates have to be set in normalized device space. This means all the coordinates have to be in the range [-1.0, 1.0], especially the near plane and far plane of -1.0 and 1.0 have to be considered. By default the depth test function is GL_LESS, so you tringle is clipped by the far plane, because a z coordinate of 1.0 is not less than the far plane of 1.0. Use a z coordinate of 0.0, for the vertices (of course somthing like 0.99 would work, too):
GLfloat vertices[] = { 0.0f, 0.707f, 0.0f, -0.5f, -0.5f, 0.0f, 0.5f, -0.5f, 0.0f };
shader->setAttributeArray(posAttribute,vertices, 3);
The 3d paramter of glDrawArrays has to be the number of vertices and not the number of primitives:
glDrawArrays(GL_TRIANGLES, 0, 3);
The relevant code parts may look like this:
void Glwidget::initializeGL()
{
connect(context(), &QOpenGLContext::aboutToBeDestroyed, this, &Glwidget::cleanup);
initializeOpenGLFunctions();
glClearColor(.0f, .0f, .0f, 1.0f);
shader = new QOpenGLShaderProgram(this);
shader->addShaderFromSourceCode(QOpenGLShader::Vertex, vertexShaderSource);
shader->addShaderFromSourceCode(QOpenGLShader::Fragment, fragmentShaderSource);
Q_ASSERT(shader->link());
posAttribute = shader->attributeLocation("posAttr");
Q_ASSERT(shader->bind());
glEnable(GL_DEPTH_TEST);
glEnable(GL_CULL_FACE);
}
void Glwidget::paintGL()
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
GLfloat vertices[] = { 0.0f, 0.707f, 0.0f, -0.5f, -0.5f, 0.0f, 0.5f, -0.5f, 0.0f };
shader->setAttributeArray(posAttribute,vertices, 3);
glEnableVertexAttribArray(posAttribute);
glDrawArrays(GL_TRIANGLES, 0, 3);
glDisableVertexAttribArray(posAttribute);
}
Note, since Face Culling is enabled, you have to respect the winding order of the primitives (this is the case, in your code).
Furthermore, as mentioned by #derhass, in the comments, you have to change the fragment shader. You have to define an Output Variable.
(Side note, use Raw string literals):
static const char* fragmentShaderSource = R"(
#version 330 core
out vec4 fragColor;
void main() {
fragColor = vec4(1.0, 0.0, 1.0, 0.0);
}
)";
See the preview:
If you want to draw lines instead of polygons, then you can do this by GL_LINE_LOOP. See Line primitives.
glDrawArrays(GL_LINE_LOOP, 0, 3);