Nov 3, 2014

Trying out picamera's overlay function: Raspberry Pi drive recorder with GPS logger

I introduced a new version of picamera and its new feature on last article. This new feature lets us add overlays to the preview. You'll never have to continuously capture image from camera and add some image or text to it before sending to display device as preview; just render preview with show_preview() and add as many layer as you wish to show some statistics, recording status or anything you want.


Below are example of both implementations.

1. Capture image, add text to it, and send it to display device continuously

It was the only way to add modification to preview until picamera ver. 1.8 came out. Because this implementation shows preview by continuously updating image, frame rate can be lower as the image modification process gets slower; Slower the loop process, fewer the image capture, poorer the preview quority.

2. Show preview on the bottom layer and add overlay to show text

This way, preview can be shown with show_preview() just like plain procedure without any modification. Text or additional image can be added as an overlay on top, and this layer may be replaced with your desired frequency. As you probably figured out, the additional layer doesn't affect preview quality. The problem is that this preview output directly goes to default display device, /dev/fb0, while PiTFT is assigned to /dev/fb1. Therefore we need to copy /dev/fb0 output to /dev/fb1 with rpi-fbcp. Fbcp takes a snapshot of /dev/fb0, copies it to /dev/fb1 and waits 25ms before repeating. Snapshotting takes ~10ms and with a 25ms delay it gives roughly 1000/(10+25) = 28fps.

Comparison

Despite the latency rpi-fbcp gives, the solution with overlay works better for me. I also like this in a point that we can separate preview and overlay implementation.
Now I'll take a break and wait till PiTFT HAT comes out.

EDIT 2015-01-25: Now PiTFT HAT is out in the market.
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