How to draw to NSOpenGLView from a view controller object?

Question

In IB I placed an NSOpenGLView instance into the window.

I have all of my drawing code inside a custom NSViewController like so:

// MyOpenGLViewController.h

@interface MyOpenGLViewController : NSViewController 
{
    IBOutlet NSOpenGLView *glView;  
}

// MyOpenGLViewController.m

-(void)awakeFromNib
{
    [self drawFrame];
}

-(void)drawFrame
{
    [[glView openGLContext] makeCurrentContext];
    glClearColor(0, 0, 0, 1);
}

Everything is linked up and "drawFrame" gets called but I don't get anything on the screen. It's always white. What should I be checking for?

Mark · Accepted Answer · 2011-02-14T22:57:08.663

4

Typically one subclasses NSOpenGLView and installs a timer to update your data model and if anything changes then draw the changes. The controller shouldn't be doing the drawing. Apple has some good documentation on getting something working. Start with the OpenGl Programming Guide for Mac OS X.

I seem to remember a bug with using the NSOpenGLView object in Interface Builder. What I always do is add a generic NSObject where you want the view and set its class to be your NSOpenGLView subclass.

edited Feb 14 '11 at 22:57

answered Feb 14 '11 at 22:51

Mark

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Ok, that appears to be the case—I can't do the drawing from the controller. I found some sample code where they had it setup that way though, so I thought it should work. However, I wasn't able to init my shaders in the view subclass. So I init them in my controller and then passed the "program" to the view. Seems to be working. Thanks. – anna Feb 14 '11 at 23:43
Drawing from the controller should work, maybe there was something else going on, but it breaks the MVC paradigm. Also, initing the shaders should work in the view subclass. I took a class in OpenGL a few years ago and all of the platform code was isolated from the OpenGL code. All of the OpenGL code was old fashioned C with global state. The C code was called through an Objective-C layer and the controller would only call the objective C if the message existed. The resulting OpenGL code should be cross platform, although I never tried it. Unfortunately I can't post any code from that class. – Mark Feb 15 '11 at 12:30

score 2 · Answer 2 · answered Feb 14 '11 at 21:40

2

Not sure about cocoa, but glClearColor() just sets some state. glClear(GL_COLOR_BUFFER_BIT) will actually clear the framebuffer.

answered Feb 14 '11 at 21:40

genpfault

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Good observation. Although I do have that in the actual code. I only posted one line here. Still not working. None of the drawing code gets output to the screen and I'm not sure where to look.. – anna Feb 14 '11 at 22:39

score 1 · Answer 3 · answered Jan 18 '12 at 18:54

A bit late to answer, but as I was wondering the same thing and found out this thread I thought I should update it with some missing bits :P

As someone else mentioned glClearColor() only set an attribute, the actual clear must be done with glClear (note that in addition to the color buffer it can also clear the Z-Buffer, accumulator & stencil).

Finally your OpenGL commands stream need to be actually flushed. This can be done with either glFlush(), if you are single buffered, or glSwapAPPLE() if you are double-buffered.

How to draw to NSOpenGLView from a view controller object?

3 Answers3