# Operating Modes

If you specify a 5x5 kernel, for example, the following figure shows how an output pixel relies on the values of specific pixels in the input image:

When calculating output pixels near the boundary of the image, not all the pixels inside the kernel can necessarily be mapped to the underlying input image. The following figure shows pixels in a 5x5 kernel that do not map to the underlying input image:

The NxM Convolve operator generates an output image that is smaller than the input image because the 3x3 kernel must always be fully contained within the input image as the operator executes, so rows and columns along the edge of the input image are discarded. To preserve the size of the output image, especially in cases where you might specify a large NxM kernel, the NxM Convolution interface supports different operating modes that control how kernel pixels that lie outside the input image are handled.