how to unbind glBindFramebuffer in c++ opengles ios - c++

Question

i am porting a program that run on windows and android to ios.
the following code works on both platform but on ios it stops rendering after that code is being executed, i suspect that the bind never gets unbinded, what is the proper way of doing it?
the objective of the code is to get the textures pixels.
this is the code:
void Texture::Bind()
{
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, mTextureID);
}
GLubyte* Texture::GetPixels()
{
Bind();
int data_size = mWidth * mHeight * 4;
GLubyte* pixels = new GLubyte[data_size];
#ifdef _WIN32
glGetTexImage(GL_TEXTURE_2D, 0, GL_RGBA, GL_UNSIGNED_BYTE, pixels);
#else
GLuint fbo;
glGenFramebuffers(1, &fbo);
glBindFramebuffer(GL_FRAMEBUFFER, fbo);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, mTextureID, 0);
glReadPixels(0, 0, mWidth, mHeight, GL_RGBA, GL_UNSIGNED_BYTE, pixels);
glBindFramebuffer(GL_FRAMEBUFFER, 0);
glDeleteFramebuffers(1, &fbo);
#endif
return pixels;
}

Answer 1

iOS does not work with default framebuffer indexed with 0. You need to bind the buffer you use as main. It depends on the tool you used but if you are using directly an UIView then you should find some code similar to the following:
- (instancetype)initWithView:(UIView *)view {
if((self = [super init])) {
{
GLuint bufferID = 0;
glGenFramebuffers(1, &bufferID);
glBindFramebuffer(GL_FRAMEBUFFER, bufferID);
self.frameBufferID = bufferID;
}
{
GLuint bufferID = 0;
glGenRenderbuffers(1, &bufferID);
glBindRenderbuffer(GL_RENDERBUFFER, bufferID);
view.layer.contentsScale = UIScreen.mainScreen.scale;
[[EAGLContext currentContext] renderbufferStorage:GL_RENDERBUFFER fromDrawable:(CAEAGLLayer *)view.layer];
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, bufferID);
self.colorBufferID = bufferID;
GLint width, height;
glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_WIDTH, &width);
glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_HEIGHT, &height);
self.bufferWidth = width;
self.bufferHeight = height;
}
}
return self;
}
You are looking for a call to renderbufferStorage:fromDrawable:. Near it a frame buffer should be created which is associated to this render buffer. The id of that frame buffer is what you need to bind.
So in the snipped above you would use self.frameBufferID.
As for the snippet I posted it is a part of a project which generates frame and render buffer from a given UIView. First it generates frame buffer and binds it. Next render buffer is created and bound. Render buffer is then setup through native iOS code with layer. We attach render buffer to frame buffer. At the end width and height are extracted.
Before drawing to this object the following bind method is called:
- (void)bind {
glBindFramebuffer(GL_FRAMEBUFFER, self.frameBufferID);
glBindRenderbuffer(GL_RENDERBUFFER, self.colorBufferID);
}