6. Frequently Asked Questions (FAQ)

6.1. Can I put the preview in a window?

No. The camera module’s preview system is quite crude: it simply tells the GPU to overlay the preview on the Pi’s video output. The preview has no knowledge (or interaction with) the X-Windows environment (incidentally, this is why the preview works quite happily from the command line, even without anyone logged in).

That said, the preview area can be resized and repositioned via the window attribute of the preview object. If your program can respond to window repositioning and sizing events you can “cheat” and position the preview within the borders of the target window. However, there’s currently no way to allow anything to appear on top of the preview so this is an imperfect solution at best.

6.2. Help! I started a preview and can’t see my console!

As mentioned above, the preview is simply an overlay over the Pi’s video output. If you start a preview you may therefore discover you can’t see your console anymore and there’s no obvious way of getting it back. If you’re confident in your typing skills you can try calling stop_preview() by typing “blindly” into your hidden console. However, the simplest way of getting your display back is usually to hit Ctrl+D to terminate the Python process (which should also shut down the camera).

When starting a preview, you may want to set the alpha parameter of the start_preview() method to something like 128. This should ensure that when the preview is displayed, it is partially transparent so you can still see your console.

6.3. The preview doesn’t work on my PiTFT screen

The camera’s preview system directly overlays the Pi’s output on the HDMI or composite video ports. At this time, it will not operate with GPIO-driven displays like the PiTFT. Some projects, like the Adafruit Touchscreen Camera project, have approximated a preview by rapidly capturing unencoded images and displaying them on the PiTFT instead.

6.4. How much power does the camera require?

The camera requires 250mA when running. Note that simply creating a PiCamera object means the camera is running (due to the hidden preview that is started to allow the auto-exposure algorithm to run). If you are running your Pi from batteries, you should close() (or destroy) the instance when the camera is not required in order to conserve power. For example, the following code captures 60 images over an hour, but leaves the camera running all the time:

import picamera
import time

with picamera.PiCamera() as camera:
    camera.resolution = (1280, 720)
    time.sleep(1) # Camera warm-up time
    for i, filename in enumerate(camera.capture_continuous('image{counter:02d}.jpg')):
        print('Captured %s' % filename)
        # Capture one image a minute
        time.sleep(60)
        if i == 59:
            break

By contrast, this code closes the camera between shots (but can’t use the convenient capture_continuous() method as a result):

import picamera
import time

for i in range(60):
    with picamera.PiCamera() as camera:
        camera.resolution = (1280, 720)
        time.sleep(1) # Camera warm-up time
        filename = 'image%02d.jpg' % i
        camera.capture(filename)
        print('Captured %s' % filename)
    # Capture one image a minute
    time.sleep(59)

Note

Please note the timings in the scripts above are approximate. A more precise example of timing is given in Capturing timelapse sequences.

If you are experiencing lockups or reboots when the camera is active, your power supply may be insufficient. A practical minimum is 1A for running a Pi with an active camera module; more may be required if additional peripherals are attached.

6.5. How can I take two consecutive pictures with equivalent settings?

See the Capturing consistent images recipe.

6.6. Can I use picamera with a USB webcam?

No. The picamera library relies on libmmal which is specific to the Pi’s camera module.

6.7. How can I tell what version of picamera I have installed?

The picamera library relies on the setuptools package for installation services. You can use the setuptools pkg_resources API to query which version of picamera is available in your Python environment like so:

>>> from pkg_resources import require
>>> require('picamera')
[picamera 1.2 (/usr/local/lib/python2.7/dist-packages)]
>>> require('picamera')[0].version
'1.2'

If you have multiple versions installed (e.g. from pip and apt-get) they will not show up in the list returned by the require method. However, the first entry in the list will be the version that import picamera will import.

If you receive the error “No module named pkg_resources”, you need to install the pip utility. This can be done with the following command in Raspbian:

$ sudo apt-get install python-pip

6.8. How come I can’t upgrade to the latest version?

If you are using Raspbian, firstly check that you haven’t got both a PyPI (pip) and an apt (apt-get) installation of picamera installed simultaneously. If you have, one will be taking precedence and it may not be the most up to date version.

Secondly, please understand that while the PyPI release process is entirely automated (so as soon as a new picamera release is announced, it will be available on PyPI), the release process for Raspbian packages is semi-manual. There is typically a delay of a few days after a release before updated picamera packages become accessible in the Raspbian repository.

Users desperate to try the latest version may choose to uninstall their apt based copy (uninstall instructions are provided in the installation instructions, and install using pip instead. However, be aware that keeping a PyPI based installation up to date is a more manual process (sticking with apt ensures everything gets upgraded with a simple sudo apt-get upgrade command).

6.9. Why is there so much latency when streaming video?

The first thing to understand is that streaming latency is nothing to do with the encoding or sending end of things (i.e. the Pi), but mostly to do with the playing or receiving end. If the Pi weren’t capable of encoding a frame before the next frame arrived, it wouldn’t be capable of recording video at all (because its internal buffers would rapidly become filled with unencoded frames).

So, why do players typically introduce several seconds worth of latency? The primary reason is that most players (e.g. VLC) are optimized for playing streams over a network. Such players allocate a large (multi-second) buffer and only start playing once this is filled to guard against possible future packet loss.

A secondary reason that all such players allocate at least a couple of frames worth of buffering is that the MPEG standard includes certain frame types that require it:

  • I-frames (intra-frames, also known as “key frames”). These frames contain a complete picture and thus are the largest sort of frames. They occur at the start of playback and at periodic points during the stream.
  • P-frames (predicted frames). These frames describe the changes from the prior frame to the current frame, therefore one must have successfully decoded the prior frame in order to decode a P-frame.
  • B-frames (bi-directional predicted frames). These frames describe the changes from the next frame to the current frame, therefore one must have successfully decoded the next frame in order to decode the current B-frame.

B-frames aren’t produced by the Pi’s camera (or, as I understand it, by most real-time recording cameras) as it would require buffering yet-to-be-recorded frames before encoding the current one. However, most recorded media (DVDs, BDs, and hence network video streams) do use them, so players must support them. It is simplest to write such a player by assuming that any source may contain B-frames, and buffering at least 2 frames worth of data at all times to make decoding them simpler.

As for the network in between, a slow wifi network may introduce a frame’s worth of latency, but not much more than that. Check the ping time across your network; it’s likely to be less than 30ms in which case your network cannot account for more than a frame’s worth of latency.

TL;DR: the reason you’ve got lots of latency when streaming video is nothing to do with the Pi. You need to persuade your video player to reduce or forgo its buffering.