Instance Method


Overrides to perform your custom operations prior to or after rendering a frame of a composition.


func render(atTime time: TimeInterval, arguments: [AnyHashable : Any]!) -> Bool



The rendering time, in seconds, of the composition frame.


An optional dictionary that can contain QCRendererEventKey or QCRendererMouseLocationKey and the associated values. (See QCRenderer or more information.)

Return Value

false if your custom rendering fails, otherwise, true.


Do not call this method directly. You override this method only for subclasses of the QCView class and only if you want to perform custom operations or OpenGL rendering before and/or after Quartz Composer renders a frame of the composition.

The most common reasons to override this method are to:

  • synchronize communication with the composition. For example, you might want to set input parameters of the composition. By overriding this method, you can set parameters only when necessary and only at a specific time.

  • underlay or overlay custom OpenGL rendering.

To synchronize communication between a composition and another part of the application, the implementation looks similar to the following:

- (BOOL) renderAtTime:(NSTimeInterval)time
  // Your code to computer the value of myParameterValue
  [self setValue:myParameterValue forInputKey:@”myInput”];
  BOOL success = [super renderAtTime:time arguments:arguments];
  id result = [self valueForOutputKey:@”myOutput”];
  //Your code to perform some operation on the result
  return success;

To perform OpenGL drawing in a QCView object, follow these guidelines:

  • Use the OpenGL context of the QCView object to do drawing. You can retrieve the OpenGL context by calling [self openGLContext]. Note that this context won't necessarily be set as the current OpenGL context.

  • Use CGL macros instead of managing the current OpenGL context yourself.

    OpenGL performs a global context and renderer lookup for each command it executes to ensure that all OpenGL commands are issued to the correct rendering context and renderer. There is significant overhead associated with these lookups that can measurably affect performance. CGL macros let you provide a local context variable and cache the current renderer in that variable. They are simple to use, taking only a few lines of code to set up.

  • Save and restore all state changes except the ones that are part of GL_CURRENT_BIT (RGBA color, color index, normal vector, texture coordinates, and so forth).

  • Check for OpenGL errors with glGetError.

Here’s an example implementation of this method using OpenGL to draw an overlay:

#import <OpenGL/CGLMacro.h>  // Set up using macros
- (BOOL) renderAtTime:(NSTimeInterval)time
    BOOL success = [super renderAtTime:time arguments:arguments];
    // Use the OpenGL context of the view for drawing.
    CGLContextObj cgl_ctx = [[self openGLContext] CGLContextObj];
    // Save and set OpenGL states appropriately.
    glGetIntegerv(GL_MATRIX_MODE, &saveMode);
    glRotatef(45.0, 0.0, 0.0, 1.0);
    // The code that performs OpenGL drawing goes here.
    //After drawing, restore original OpenGL states.
    // Check for errors.
    return success;