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pebble-faces

This Pebble application downloads PNG images from the Internet, de-compress the PNG into a bitmap and then display it on the screen of Pebble. It will display pictures of the Pebble developer evangelism team!

It is designed to be easily reusable in your own application.

The main parts are:

  • The resource downloader (netimage.h / netdownload.c / pebble-app.js)

    This library provides a function netdownload_request(char *url) to download any URL from the Internet. The resource as to be small enough to fit in Pebble memory.

    You will first need to initialize the library with netdownload_initialize(callback). The callback will be called with an array of bytes when the file has been downloaded.

  • The PNG image loader (png.h, png.c, upng.h, upng.c)

    This library is based on the fantastic uPNG. The png.h and png.c provide a simple wrapper to make it easier to use on Pebble.

    To convert an array of bytes (for example one provided to the NetDownloadCallback) you can call gbitmap_create_with_png_data().

An earlier version of this example downloaded and loaded PBI images directly from the Internet. This new version is superior because PNG is a much easier format to generate on your servers and the images will be smaller on the network.

A downside of this version is the memory requirement: the uPNG library takes some code space in your app and you will need enough memory to have the PNG and the Bitmap loaded during decompression.

Preparing images

To reduce size as much as possible you should prepare your PNGs to match the size of the screen (or smaller) and to use only two colors. You should also remove any extra information from the PNG.

Using ImageMagick for example:

convert myimage.png \
  -adaptive-resize '144x168>' \
  -fill '#FFFFFF00' -opaque none \
  -type Grayscale -colorspace Gray \
  -colors 2 -depth 1 \
  -define png:compression-level=9 -define png:compression-strategy=0 \
  -define png:exclude-chunk=all \
  myimage.pbl.png

Notes:

  • -fill #FFFFFF00 -opaque none makes the transparency white
  • -adaptive-resize with > at end means resize only if larger, and maintains aspect ration
  • we exclude png chunks to reduce size (like when image was made, author)

If you want to use dithering to simulate Grey, you can use this command:

convert myimage.png \
  -adaptive-resize '144x168>' \
  -fill '#FFFFFF00' -opaque none \
  -type Grayscale -colorspace Gray \
  -black-threshold 30% -white-threshold 70% \
  -ordered-dither 2x1 \
  -colors 2 -depth 1 \
  -define png:compression-level=9 -define png:compression-strategy=0 \
  -define png:exclude-chunk=all \
  myimage.pbl.png

SDK 3.x

To convert your image to the palletized format (64 colors) used by Pebble Time, use:

convert myimage.png \
  -adaptive-resize '144x168>' \
  -fill '#FFFFFF00' -opaque none \
  -dither FloydSteinberg \
  -remap pebble_colors_64.gif \
  -define png:compression-level=9 -define png:compression-strategy=0 \
  -define png:exclude-chunk=all \
  myimage.pbl.png

Important: You need ImageMagick to do the conversion above. Mac OS X ships with GraphicsMagick which does not support the PNG options to compress and remove un-needed informations.

Resources

For more information about PNG on Pebbles, how to optimize memory usage and tips on image processing, please refer to the Advanced techniques videos from the Pebble Developer Retreat 2014.