| .. SPDX-License-Identifier: GPL-2.0 | |
| ========================================================= | |
| Raspberry Pi PiSP Back End Memory-to-Memory ISP (pisp-be) | |
| ========================================================= | |
| The PiSP Back End | |
| ================= | |
| 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 pisp-be driver | |
| ================== | |
| 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: | |
| - pispbe-input: output device for images to be submitted to the ISP for | |
| processing. | |
| - pispbe-tdn_input: output device for temporal denoise. | |
| - pispbe-stitch_input: output device for image stitching (HDR). | |
| - pispbe-output0: first capture device for processed images. | |
| - pispbe-output1: second capture device for processed images. | |
| - pispbe-tdn_output: capture device for temporal denoise. | |
| - pispbe-stitch_output: capture device for image stitching (HDR). | |
| - pispbe-config: output device for ISP configuration parameters. | |
| pispbe-input | |
| ------------ | |
| 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`_. | |
| pispbe-tdn_input, pispbe-tdn_output | |
| ----------------------------------- | |
| 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. | |
| pispbe-stitch_input, pispbe-stitch_output | |
| ----------------------------------------- | |
| 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. | |
| pispbe-output0, pispbe-output1 | |
| ------------------------------ | |
| The two capture devices write to memory the pixel data as processed by the ISP. | |
| pispbe-config | |
| ------------- | |
| 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`_. | |
| ISP configuration | |
| ================= | |
| 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 |