Documentation/admin-guide/media/raspberrypi-pisp-be.rst
.. SPDX-License-Identifier: GPL-2.0
The PiSP Back End is a memory-to-memory Image Signal Processor (ISP) which reads image data from DRAM memory and performs image processing as specified by the application through the parameters in a configuration buffer, before writing pixel data back to memory through two distinct output channels.
The ISP registers and programming model are documented in the Raspberry Pi Image Signal Processor (PiSP) Specification document_
The PiSP Back End ISP processes images in tiles. The handling of image
tessellation and the computation of low-level configuration parameters is
realized by a free software library called libpisp <https://github.com/raspberrypi/libpisp>_.
The full image processing pipeline, which involves capturing RAW Bayer data from
an image sensor through a MIPI CSI-2 compatible capture interface, storing them
in DRAM memory and processing them in the PiSP Back End to obtain images usable
by an application is implemented in libcamera <https://libcamera.org>_ as
part of the Raspberry Pi platform support.
The Raspberry Pi PiSP Back End (pisp-be) driver is located under
drivers/media/platform/raspberrypi/pisp-be. It uses the V4L2 API to register
a number of video capture and output devices, the V4L2 subdev API to register
a subdevice for the ISP that connects the video devices in a single media graph
realized using the Media Controller (MC) API.
The media topology registered by the pisp-be driver is represented below:
.. _pips-be-topology:
.. kernel-figure:: raspberrypi-pisp-be.dot :alt: Diagram of the default media pipeline topology :align: center
The media graph registers the following video device nodes:
Images to be processed by the ISP are queued to the pispbe-input output device
node. For a list of image formats supported as input to the ISP refer to the
Raspberry Pi Image Signal Processor (PiSP) Specification document_.
The pispbe-tdn_input output video device receives images to be processed by
the temporal denoise block which are captured from the pispbe-tdn_output
capture video device. Userspace is responsible for maintaining queues on both
devices, and ensuring that buffers completed on the output are queued to the
input.
To realize HDR (high dynamic range) image processing the image stitching and
tonemapping blocks are used. The pispbe-stitch_output writes images to memory
and the pispbe-stitch_input receives the previously written frame to process
it along with the current input image. Userspace is responsible for maintaining
queues on both devices, and ensuring that buffers completed on the output are
queued to the input.
The two capture devices write to memory the pixel data as processed by the ISP.
The pispbe-config output video devices receives a buffer of configuration
parameters that define the desired image processing to be performed by the ISP.
The format of the ISP configuration parameter is defined by
:c:type:pisp_be_tiles_config C structure and the meaning of each parameter is
described in the Raspberry Pi Image Signal Processor (PiSP) Specification document_.
The ISP configuration is described solely by the content of the parameters buffer. The only parameter that userspace needs to configure using the V4L2 API is the image format on the output and capture video devices for validation of the content of the parameters buffer.
.. _Raspberry Pi Image Signal Processor (PiSP) Specification document: https://datasheets.raspberrypi.com/camera/raspberry-pi-image-signal-processor-specification.pdf