Wear OS improves battery life by tracking memory use. Watch faces using the Watch Face Format have memory limits, as per the Wear OS app quality guidelines:
- Ambient mode: 10 MB maximum memory use.
- Interactive mode: 100 MB maximum memory use.
Memory usage calculation
To compute memory usage for an image or bitmap font in a watch face using the Watch Face Format, the system does the following:
- Decompress the image or font.
- Check if the following optimizations apply:
- Resizing to better fit the screen
- Cropping transparent pixels
- Downsampling to RGB565, without loss of fidelity
Based on the resulting bounding box, the size is computed as follows:
- For images and fonts using RGBA8888: 4 x width x height
- For images and fonts using RGB565: 2 x width x height
- For images and fonts using the ALPHA_8 bitmap configuration: width x height
Interactive mode
To compute memory usage for interactive mode, the system sums the following values:
- The unprocessed size of any vector fonts
- The estimated usage of the system's default font
- The total size of the images and bitmap fonts after cropping, resizing, and reformatting is applied
Configurations
For watch faces with configurations, the system attempts to calculate the total size of the watch face resources across different configurations. If the number of combinations is very large, the system may overestimate how many resources are used simultaneously.
Ambient mode and layers
The system assumes ambient mode uses up to three full-screen layers, two of which are static. The layers include:
- The watch face background. The system treats this as one image, regardless of how many images the background comprises.
- Moving parts like hands, digital displays, or dynamic elements.
- Remaining elements from the source XML file.
Large bitmap fonts often use the most memory in ambient mode.
Methods of reducing memory usage
Use the following optimizations to reduce memory usage.
Crop and resize bitmap fonts
Crop your images and BitmapFont objects to match the display size.
Wear OS draws watch faces with all images decompressed. A mostly blank full-screen image might consume 3 KB on disk, but 750 KB or more on a 450-pixel x 450-pixel screen.
Use consistent bitmap font heights
When using a BitmapFont, ensure all images for a character have the same
height. Likewise, ensure all images for words have the same height.
Use consistent frame sizes in animations
Instead of moving an image across a watch face, update the elements in the image and keep the bounding box position fixed. For example, to animate a circle on your watch face, change its color instead of rolling it.
This technique shrinks the size of the animation's calculated bounding box.
Deduplicate images
To display an image multiple times, include only one image resource and reference it multiple times.
Show progress using arcs
To simulate a progress bar finishing after 1 minute or 1 hour, don't use 60
images. Use an Arc object with an expression controlling its length, as
shown here:
<PartDraw angle="0" width="400" height="400" name="ProgressBar" pivotX="0.5" pivotY="0.5 x="40" y="40"> <Arc centerX="200" centerY="200" width="400" height="400" startAngle="0" endAngle="360"> <!-- Completes a "progress loop" every minute. --> <Transform target="endAngle" value="0 + (clamp([SECOND], 0, 60) - 0) * 6" /> <Stroke cap="ROUND" color="#123456" thickness="10" /> </Arc> </PartDraw>
To display a noncontinuous line, for example to achieve a retro digital watch
style look, use a dash property for a Stroke object or a semitransparent
mask image overlay.
Place watch hands and complications at the end of the source file
XML nodes are drawn in the order listed in the source XML. By putting watch hands and complications at the end, you enable the system to eliminate an entire layer from the ambient mode memory calculation.
Evaluate the memory usage of your watch face
To measure the memory usage of your watch face, use the memory footprint
evaluator tool, available in the watchface repository on GitHub.