cannot display OpenGL texture [closed] - c++

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I have been trying to use OpenGL to render an image to a screen and in order to learn more about texturing etc., my approach was to do map the image to the texture and then draw it using GL_QUADS. However, all I get is a blank screen.
So, the texture was initialised as follows:
class Texture
{
public:
void init_texture(int rows, int cols);
void load_texture(const Image * frame);
GLuint _texture_obj;
};
void Texture::init_texture(int rows, int cols)
{
if (_texture_obj) glDeleteTextures(1, &_texture_obj);
_texture_target = GL_TEXTURE_2D;
glGenTextures(1, &_texture_obj);
if (_texture_obj) {
glBindTexture(_texture_target, _texture_obj);
glTexImage2D(_texture_target, 0, GL_BGRA, cols, rows, 0,
GL_BGRA, GL_UNSIGNED_BYTE, (GLvoid *)NULL);
glTexParameterf(_texture_target, GL_TEXTURE_MIN_FILTER,
GL_LINEAR);
glTexParameterf(_texture_target, GL_TEXTURE_MAG_FILTER,
GL_LINEAR);
}
else {
throw std::runtime_error("Could not create the OpenGL texture");
}
}
void Texture::load_texture(const Image *frame)
{
glBindTexture(GL_TEXTURE_2D, _texture_obj);
glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, frame->cols(),
frame->rows(), GL_BGRA, GL_UNSIGNED_BYTE,
frame->data());
}
The code above initializes and loads the texture.
To display the texture, I use a QOpenGLWidget derived object where the initialization and the paint method looks like this:
class GLWidget: public QOpenGLWidget
{
public:
GLWidget(Image *image);
void initializeGL();
void resizeGL(int w, int h);
void paintGL();
private:
Texture texture;
};
void GLWidget::GLWidget(Image * image)
{
texture.init(image->rows, image->cols);
texture.load(image);
}
void GLWidget::initializeGL()
{
glEnable(GL_DEPTH_TEST);
glEnable(GL_CULL_FACE);
}
void GLWidget::resizeGL(int w, int h)
{}
void GLWidget::paintGL()
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glLoadIdentity();
glEnable(GL_TEXTURE_2D);
glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE,GL_REPLACE);
glBindTexture(GL_TEXTURE_2D, texture._tex_object);
glBegin(GL_QUADS);
// width(), height() gives the component width and height
glTexCoord2i(0, 0); glVertex2i(0, 0);
glTexCoord2i(0, 1); glVertex2i(0, this->width());
glTexCoord2i(1, 1); glVertex2i(this->height(), this->width());
glTexCoord2i(1, 0); glVertex2i(this->height(), 0);
glEnd();
glDisable(GL_TEXTURE_2D);
}
However, this only shows me a blank screen. Is there some viewport that I need to setup? I only started using OpenGL a few days ago, so am quite green in that aspect. I am using qt version 5.4 and OpenGL 4.3 on linux.

Answer 1

The call
glTexImage2D(_texture_target, 0, GL_BGRA, cols, rows, 0, GL_BGRA, GL_UNSIGNED_BYTE, (GLvoid *)NULL);
is invalid, because GL_BGRA is not a valid internalFormat. You should use GL_RGBA for that. Note that the internalFormat just defines the basic data type and the number of channels, the actual layout is totally implementation-specific. The format parameter on the other hand tells the GL how to interpret the data in client memory, so formats like GL_RGBA and GL_BGRA are actually defined.
I'm not really familiar with QOpenGLWidget. However, I think that creating your texture in the constructor might be a bad idea, as the GL context might not be available at that point (or at least not made current to the thread). You should move that stuff into initializeGL().