Apparatus and method for improving denoising of images generated by a rendering engine such as a ray tracing engine. For example, one embodiment renders a first plurality of images during runtime using a first sample count. Denoising is performed on the first plurality of images during runtime by a machine-learning engine. In addition, a reference region is generated from one or more of the first plurality of images at a second sample count which is greater than the first sample count. The reference region is then used to perform additional runtime training of the machine-learning engine.