6. Camera Hardware¶
This chapter attempts to provide an overview of the operation of the camera under various conditions, as well as to provide an introduction to the low level software interface that picamera utilizes.
6.1. Camera Modes¶
The Pi’s camera has a discrete set of input modes which are as follows:
This table is accurate as of firmware revision #656. Firmwares prior to this had a more restricted set of modes, and all video modes had partial FoV. Please use sudo rpi-update to upgrade to the latest firmware.
Modes with full field of view (FoV) capture from the whole area of the camera’s sensor (2592x1944 pixels). Modes with partial FoV only capture from the center 1920x1080 pixels. The difference between these areas is shown in the illustration below:
- The mode must be acceptable. Video modes can be used for video recording, or for image captures from the video port (i.e. when use_video_port is True in calls to the various capture methods). Image captures when use_video_port is False must use an image mode (of which only one currently exists).
- The closer the requested resolution is to the mode’s resolution the better, but downscaling from a higher input resolution is preferable to upscaling from a lower input resolution.
- The requested framerate should be within the range of the input mode. Note that this is not a hard restriction (it is possible, but unlikely, for the camera to select a mode that does not support the requested framerate).
- The closer the aspect ratio of the requested resolution is to the mode’s resolution, the better. Attempts to set resolutions with aspect ratios other than 4:3 or 16:9 (which are the only ratios directly supported by the modes in the table above) will choose the mode which maximizes the resulting FoV.
A few examples are given below to clarify the operation of this heuristic:
- If you set the resolution to 1024x768 (a 4:3 aspect ratio), and framerate to anything less than 42fps, the 1296x976 mode will be selected, and the camera will downscale the result to 1024x768.
- If you set the resolution to 1280x720 (a 16:9 wide-screen aspect ratio), and framerate to anything less than 49fps, the 1296x730 mode will be selected and downscaled appropriately.
- Setting resolution to 1920x1080 and framerate to 30fps exceeds the resolution of both the 1296x730 and 1296x976 modes (i.e. they would require upscaling), so the 1920x1080 mode is selected instead, although it has a reduced FoV.
- A resolution of 800x600 and a framerate of 60fps will select the 640x480 60fps mode, even though it requires upscaling because the algorithm considers the framerate to take precedence in this case.
- Any attempt to capture an image without using the video port will (temporarily) select the 2592x1944 mode while the capture is performed (this is what causes the flicker you sometimes see when a preview is running while a still image is captured).
6.2. Under the Hood¶
This section attempts to provide detail of what picamera is doing “under the hood” in response to various method calls.
The Pi’s camera has three ports, the still port, the video port, and the preview port. The following sections describe how these ports are used by picamera and how they influence the camera’s resolutions.
6.2.1. The Still Port¶
Firstly, the still port. Whenever this is used to capture images, it (briefly) forces the camera’s mode to the only supported still mode (see Camera Modes) so that images are captured using the full area of the sensor. It also appears to perform a considerable amount of post-processing on captured images so that they appear higher quality.
The still port is used by the various capture() methods when their use_video_port parameter is False (which it is by default).
6.2.2. The Video Port¶
The video port is somewhat simpler in that it never changes the camera’s mode. The video port is used by the start_recording() method (for recording video), and is also used by the various capture() methods when their use_video_port parameter is True. Images captured from the video port tend to have a “grainy” appearance, much more akin to a video frame than the images captured by the still port (the author suspects the still port may be taking an average of several frames).
6.2.3. The Preview Port¶
The preview port operates more or less identically to the video port. As the preview port is never used for encoding we won’t mention it further in this section.
The camera provides various encoders which can be attached to the still and video ports for the purpose of producing output (e.g. JPEG images or H.264 encoded video). A port can have a single encoder attached to it at any given time (or nothing if the port is not in use).
Encoders are connected directly to the still port. For example, when capturing a picture using the still port, the camera’s state conceptually moves through these states:
As you have probably noticed in the diagram above, the video port is a little more complex. In order to permit simultaneous video recording and image capture via the video port, a “splitter” component is permanently connected to the video port by picamera, and encoders are in turn attached to one of its four output ports (numbered 0, 1, 2, and 3). Hence, when recording video the camera’s setup looks like this:
And when simultaneously capturing images via the video port whilst recording, the camera’s configuration moves through the following states:
When the resize parameter is passed to one of the aforementioned methods, a resizer component is placed between the camera’s ports and the encoder, causing the output to be resized before it reaches the encoder. This is particularly useful for video recording, as the H.264 encoder cannot cope with full resolution input. Hence, when performing full frame video recording, the camera’s setup looks like this:
Finally, when performing raw captures an encoder is (naturally) not required. Instead data is taken directly from the camera’s ports. When raw YUV format is requested no components are attached to the ports at all (as the ports are configured for YUV output at all times). When another raw format like RGBA is requested, a resizer is used (with its output resolution set to the input resolution, unless the resize option is specified with something different), and its output format is set to the requested raw format:
Please note that even the description above is almost certainly far removed from what actually happens at the camera’s ISP level. Rather, what has been described in this section is how the MMAL library exposes the camera to applications which utilize it (these include the picamera library, along with the official raspistill and raspivid applications).
In other words, by using picamera you are passing through (at least) two abstraction layers which necessarily obscure (but hopefully simplify) the “true” operation of the camera.