For some basic video tracking I created a extended version of the original Quartz Composer Image Pixel patch.
This plugin has almost the same functionality, but for more than one pixel. It returns a structure with the color info of all the pixels from the image you feed it.
I also made a slightly modified version of the plugin (PixelS2) together with an OpenCL Patch with the same functionality. The OpenCL patch is much faster! Xcode project of the plugin included.