Apparatus for data communications includes a host interface, which is configured to be connected to a bus of a host computer having a processor and a memory. Processing circuitry, which is coupled to the host interface, is configured to receive video data with respect to a sequence of pixels, the video data including data words of more than eight bits per pixel for at least one pixel component of the pixels, and to write the video data, via the host interface, to at least one buffer in the memory while justifying the video data in the memory so that the successive pixels in the sequence are byte-aligned in the at least one buffer.