I'm trying to update a texture with glTexSubImage2D - c++

Question

I started learning OpenGL recently and I've been messing with texture. I updated my texture using glTexImage2D but I've learned that it's better to use glTexSubImage2D, so I tried to change my code but i doesn't work.
Working code
void GLWidget::updateTextures(){
QImage t = img.mirrored();
glBindTexture(GL_TEXTURE_2D, tex);
glTexImage2D(GL_TEXTURE_2D, 0, 3, t.width(), t.height(), 0, GL_RGBA, GL_UNSIGNED_BYTE, t.bits());
glBindTexture( GL_TEXTURE_2D, 0);
}
Not working code
void GLWidget::updateTextures(){
QImage t = img.mirrored();
glBindTexture(GL_TEXTURE_2D, tex);
glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, t.width(), t.height(), GL_RGBA, GL_UNSIGNED_BYTE, t.bits());
glBindTexture( GL_TEXTURE_2D, 0);
}
All I have is a black screen.
Thanks.
EDIT :
Here is the initialization of my texture :
void GLWidget::initializeGL(){
...
LoadGLTextures();
...
}
void GLWidget::LoadGLTextures(){
QImage t = img.mirrored();
glGenTextures(1, &tex);
glBindTexture(GL_TEXTURE_2D, tex);
glTexImage2D(GL_TEXTURE_2D, 0, 3, t.width(), t.height(), 0, GL_RGBA, GL_UNSIGNED_BYTE, t.bits());
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glBindTexture( GL_TEXTURE_2D, 0);
}
img is a QImage variable containing the pixel data.
glGetError() Returns code 1281.

Answer 1

glTexSubImage2D updates the content of a previously allocated texture. glTexImage2D has to be called at least once to trigger the allocation:
void GLWidget::initializeGL(){
//...
QImage t = img.mirrored();
glBindTexture(GL_TEXTURE_2D, tex);
glTexImage2D(
GL_TEXTURE_2D,
0,
3,
t.width(),
t.height(),
0,
GL_RGBA,
GL_UNSIGNED_BYTE,
t.bits()
);
glBindTexture( GL_TEXTURE_2D, 0);
// ...
}
Update with glTexSubImage2D:
void GLWidget::updateTextures(){
QImage t = img.mirrored();
glBindTexture(GL_TEXTURE_2D, tex);
glTexSubImage2D(
GL_TEXTURE_2D,
0,
0,
0,
t.width(),
t.height(),
GL_RGBA,
GL_UNSIGNED_BYTE,
t.bits()
);
glBindTexture( GL_TEXTURE_2D, 0);
}
EDIT: the problem was glTexImage2D and glTexSubImage2D were called with different image sizes, generating the error GL_INVALID_VALUE (1281, 0x501) on glTexSubImage2D call.