Android: optimize app loading time

No one likes to wait for an app to launch for more than several seconds. It is critical to optimize the app loading time.

My app loads a large 3D model at launching. It is a custom binary format consisting of vertex array and index array. The vertex format is interleaved since it is from an iOS app. Each vertex is 8 floats: 3 for position, 3 for normal, and 2 for texture coordinates.

It is not suitable for Android if you use Java instead of NDK. So I write a small Java program to convert the binary file so that there is a position array, a normal array and a texture coordinates array. When I load the mesh, I can read all positions to a byte array, wrap it as a byte buffer, use it as a float buffer, then copy it to a direct float buffer, which could be used by OpenGL ES. Same for normals and tex coords. This optimization speeds up the file I/O by 50%. Although it is still kind of slow (4 secs). Most of time is spent copying from the byte array wrapped as float buffer to the direct float buffer.

Since 4 seconds is still slow for launching an app, I ran the mesh loading code in another thread, and add a flag for mesh ready. If mesh is not ready, the render code will skip rendering the mesh. With this change, the app is very fast to load.