Headless Kivy

Provides utilities to render Kivy applications headlessly. It calls a callback whenever something has changed in the framebuffer in a locality.

It can be used to render the Kivy application on a custom display like an SPI display, it provides tools for local updates, limiting the bandwidth and limiting the fps based on the spec of the display.

It can also be used in test environments with it tools for snapshot testing.

You can control the orientation of the display and flipping the display horizontally and vertically.

The renderer is optimized to not schedule a render when nothing has changed since the last rendered frame, by default it divides the screen into tiles and checks each tile for changes separately.

It can be configured to use double buffering, so that the next frame is generated while the last frame is being transmitted to the display.

You can have multiple instances of the headless renderer in the same application, each works as a portal to your display (or multiple different displays).

📦 Installation

pip install headless-kivy

To use its test tools, you can install it with the following command:

pip install headless-kivy[test]

🛠 Usage

1. Call setup_headless() before inheriting the HeadlessWidget class for the root widget of your application, and provide the optional parameters as needed. For example (these are all default values, you only need to provide the ones you want to change):

   setup_headless(
       width=240,
       height=240,
       bandwidth_limit=1000000, # number of pixels per second
       bandwidth_limit_window=.1, # allow bandwidth_limit x bandwidth_limit_window pixels to be transmitted in bandwidth_limit_window seconds
       bandwidth_limit_overhead=1000, # each draw command, regardless of the size, has equivalent of this many pixels of cost in bandwidth
       is_debug_mode=False,
       rotation=1, # gets multiplied by 90 degrees
       flip_horizontal=True,
       double_buffering=True, # let headless kivy generate the next frame while the previous callback is still running
       window_mode='hidden', # 'hidden' (default) renders off-screen; 'auto' lets SDL grab a physical display
   )
   

2. Inherit the HeadlessWidget class for the root widget of your Kivy application. For example:

   class FboFloatLayout(FloatLayout, HeadlessWidget):
       pass
   

3. Run the Kivy app as you normally would.

Checkout Ubo App to see a sample implementation.

⚙️ Parameters

These parameters can be set to control the behavior of headless kivy:

callback

A callback function that will be called when the screen data changes. It should have this signature:

def render(
    *,
    rectangle: tuple[int, int, int, int],
    data: NDArray[np.uint8],
    data_hash: int,
    last_render_thread: Thread,
) -> None: ...

rectangle is a tuple with the coordinates and size of the changed area in the (x, y, width, height) format.

data is a numpy array with the screen RGB data in the uint8 format. So its dimensions are (width, height, 3).

data_hash is probably not very useful for most cases, it is mostly for logging and debugging purposes.

It always runs in a new thread, the previous thread is provided so that it can call its join if desired.

bandwidth_limit

Maximum bandwidth usage in pixels per second, no limit if set to 0.

bandwidth_limit_window

Length of the time window in seconds to check the bandwidth limit.

bandwidth_limit_overhead

The overhead of each draw command in pixels, regardless of its size.

width

The width of the display in pixels.

height

The height of the display in pixels.

is_debug_mode

If set to True, the application will print debug information, including FPS.

double_buffering

Is set to True, it will let Kivy generate the next frame while sending the last frame to the display.

rotation

The rotation of the display. It will be multiplied by 90 degrees.

flip_horizontal

If set to True, it will flip the display horizontally.

flip_vertical

If set to True, it will flip the display vertically.

window_mode

Controls whether Kivy is allowed to grab a physical display. Two values:

• 'hidden' (default): headless-kivy sets SDL_VIDEODRIVER=offscreen before Kivy initializes, so SDL renders to an off-screen OpenGL context. No HDMI / DSI / KMSDRM connector is touched, and the FBO callback still receives frames. This is the right setting when the only "real" display you care about is driven from the callback (e.g. an SPI panel on a Raspberry Pi) — an attached HDMI monitor will keep showing whatever it was showing before (typically the Linux console) instead of being hijacked by Kivy's window.
• 'auto': headless-kivy does not touch SDL_VIDEODRIVER; SDL picks a display normally. On a Pi kiosk this means the KMSDRM video driver will grab the framebuffer and Kivy's window will cover the connected display.

You can also set this via environment variable:

export HEADLESS_KIVY_WINDOW_MODE=auto

If SDL_VIDEODRIVER is already set in the environment when you call setup_headless_kivy, headless-kivy will respect it and leave it alone (an INFO log line is emitted in that case).

🤝 Contributing

You need to have uv installed on your machine.

To install the required dependencies, run the following command in the root directory of the project:

uv sync

⚠️ Important Note

This project has only been tested with the ST7789 SPI display module. Other display modules might not be compatible or may require changing the parameters or even modifications to the code.

🔒 License

This project is released under the Apache-2.0 License. See the LICENSE file for more details.