OpenGL texture not rendering if it's currently active - qt

Question

I was using QOpenGLWidget to render textured triangle, the code was looking good but the triangle was always rendering black i had problem with it for two days until i accidentally found out what the title says.
This is the code, the texture gets loaded to default location of GL_TEXTURE0 and the code will not work unless i call glActiveTexture(GL_TEXTURE1) at the end, GL_TEXTURE1 is just an example it can be any other texture slot except the one where texture actually is. Without the call the object will be black.
QImage ready;
QImage image("C:/Users/Gamer/Desktop/New folder/ring.jpg");
ready = image.convertToFormat(QImage::Format_RGBA8888);
glGenTextures(1, &texture);
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, texture);
glUniform1i(glGetUniformLocation(program.programId(), "samp"), 0);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, ready.width(), ready.height(), 0, GL_RGBA, GL_UNSIGNED_BYTE, ready.constBits());
glGenerateMipmap(GL_TEXTURE_2D);
glActiveTexture(GL_TEXTURE1)
I've tried some tests, creating multiple textures and displaying them all at once, the last active texture was always black unless i activate some other unoccupied slot.
I don't know what to make of this, i'm begginer in OpenGL and Qt but this doesn't sound right.
EDIT:
Main function
#include "mainwindow.h"
#include <QApplication>
#include <QSurfaceFormat>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QSurfaceFormat format;
format.setVersion(3, 3);
format.setProfile(QSurfaceFormat::CoreProfile);
format.setDepthBufferSize(24);
format.setStencilBufferSize(8);
format.setSamples(4);
format.setSwapInterval(0);
QSurfaceFormat::setDefaultFormat(format);
MainWindow w;
w.show();
return a.exec();
}
Widget code
#include "openglwidget.h"
#include <QOpenGLShaderProgram>
#include <QImage>
#include <QDebug>
OpenGLWidget::OpenGLWidget(QWidget *parent) :
QOpenGLWidget(parent)
{
}
OpenGLWidget::~OpenGLWidget()
{
glDeleteBuffers(1, &vbo);
glDeleteVertexArrays(1, &vao);
glDeleteTextures(1, &texture);
}
void OpenGLWidget::initializeGL()
{
QOpenGLFunctions_3_3_Core::initializeOpenGLFunctions();
GLfloat vertices[] = {
0.0f, 0.75f, 0.0f,
-0.75f, -0.75f, 0.0f,
0.75f, -0.75f, 0.0f,
0.5f, 0.0f,
0.0f, 1.0f,
1.0f, 1.0f
};
glGenVertexArrays(1, &vao);
glBindVertexArray(vao);
program.addShaderFromSourceFile(QOpenGLShader::Vertex, "C:/Users/Gamer/Desktop/New folder/vertex.vert");
program.addShaderFromSourceFile(QOpenGLShader::Fragment, "C:/Users/Gamer/Desktop/New folder/fragment.frag");
program.link();
program.bind();
glGenBuffers(1, &vbo);
glBindBuffer(GL_ARRAY_BUFFER, vbo);
glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, (void*)0);
glEnableVertexAttribArray(0);
glVertexAttribPointer(1, 2, GL_FLOAT, GL_FALSE, 0, (void*)36);
glEnableVertexAttribArray(1);
QImage ready;
QImage image("C:/Users/Gamer/Desktop/New folder/ring.jpg");
ready = image.convertToFormat(QImage::Format_RGBA8888);
glGenTextures(1, &texture);
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, texture);
glUniform1i(glGetUniformLocation(program.programId(), "samp"), 0);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, ready.width(), ready.height(), 0, GL_RGBA, GL_UNSIGNED_BYTE, ready.constBits());
glGenerateMipmap(GL_TEXTURE_2D);
// glActiveTexture(GL_TEXTURE1);
}
void OpenGLWidget::paintGL()
{
GLfloat yellow[] = {1.0, 1.0, 0.0, 0.0};
glClearBufferfv(GL_COLOR, 0, yellow);
glDrawArrays(GL_TRIANGLES, 0, 3);
}
void OpenGLWidget::resizeGL(int w, int h)
{
glViewport(0, 0, w, h);
}
And shaders
#version 330 core
layout(location = 0) in vec3 pos;
layout(location = 1) in vec2 coord;
out vec2 tc;
void main(void)
{
tc = coord;
gl_Position = vec4(pos, 1.0);
}
#version 330 core
uniform sampler2D samp;
in vec2 tc;
out vec4 color;
void main(void)
{
color = texture(samp, tc);
}

Answer 1

QOpenGLWidget is a rather complex abstraction which has some side effects which you might not expect. Quoting from the Qt5 docs:
All rendering happens into an OpenGL framebuffer object. makeCurrent() ensure that it is bound in the context. Keep this in mind when creating and binding additional framebuffer objects in the rendering code in paintGL(). Never re-bind the framebuffer with ID 0. Instead, call defaultFramebufferObject() to get the ID that should be bound.
Now, this in itself isn't an issue. However, looking at the description for the initializeGL() method (my emphasis):
There is no need to call makeCurrent() because this has already been done when this function is called. Note however that the framebuffer is not yet available at this stage, so avoid issuing draw calls from here. Defer such calls to paintGL() instead.
Now, this in itself still is not the issue. But: it means that Qt will create the FBO in-between initializeGL and the first paintGL. Since Qt creates a texture as the color buffer for the FBO, this means it will re-use the currently active texture unit, and change the texture binding you did establish in initializeGL.
If you, on the other hand set glActiveTexture to something other than unit 0, Qt will screw up the binding of that unit, but since you only use unit 0, it will not have any negative effects in your example.

Answer 2

You need to bind the texture to the texture unit before drawing. Texture unit state is not part of program state, unlike uniforms. It is unusual to try and set texture unit state during program startup, that would require allocating different texture units to each program (not out of the question, it's just not the way things are normally done).
Add the following line to paintGL, before the draw call:
glBindTexture(GL_TEXTURE_2D, texture);