1) OpenGL provides low-level control over graphics rendering which allows for customization and optimization compared to higher-level APIs. 2) The OpenGL rendering process in Android involves setting up an OpenGL context, initial setup, loading resources, compiling/linking shaders, uploading resources to the GPU, clearing the screen, and drawing. 3) Shaders written in the OpenGL Shading Language (GLSL) control how vertices and fragments are processed in the rendering pipeline.

1. Android & OpenGL ES 2+
from an app developers point of view
Droidcon Berlin, 2014

2. Andreas Nilsson
Software specialist @Jayway
!
Broad interest in graphics, both 2D and 3D
!
www.jayway.com/blog

3. Inspiration
AndreasEarth
BACKUP PLAN(ET)

4. Terminology
• Matrix - A NxN array of values
• Vertex - A point in space which act as the basic
building block for shapes e.g. line, triangle, etc.
• Mesh - A structure compromised of triangles
• Shader - Small program running on the GPU in
parallell.

5. Background - OpenGL
• An abstract graphics API - That is it
• Written in C but not limited to it
• Bindings for most programming languages
• Low level ⬌ Close to hardware

6. Better control of when things are rendered - Possible to
make specific optimizations
Why OpenGL?
Control

7. Control
Rendering of ambiguous 3d meshes
Models
Why OpenGL?

8. Control ModelsEffects
Why OpenGL?
Custom effects, that can only be achieved with a
shader

9. Why not RenderScript ?
It is platform specific which limits its usage, etc.
My preference: OpenGL + OpenCL

10. Android Wrapping of OpenGL
Model
float[…]
FloatBuffer
Drawing
OpenGL
GLES20
Abstract
Android
Native
Context
GLSurfaceView
EGL

11. EGL
• Platform independent abstract API for setting up an
OpenGL context/surface
• Query graphics card for context capabilities
• GLSurfaceView wraps EGL

12. OpenGL ES Versions
OpenGL ES 1.x - Fixed pipeline
OpenGL ES 3.x- Extensions and Optimizations
OpenGL ES 2.x - Shader support
OpenGL
OpenGL ES
⇎
⇕

13. OpenGL in Android
GLSurfaceView
GLSurfaceView.Renderer
- A view, set-up for rendering with OpenGL
- Runs in its own thread
- OpenGL context can be lost onPause()
- Where the actual rendering with OpenGL happens

14. GLSurfaceView.Renderer - Quick Look
• onSurfaceCreated() - The OpenGL context is now created
and we can start loading our resources
• onSurfaceChanged() - Called when the size has changed e.g.
device rotation
• onDrawFrame() - Where the fun happens!
• Never mind the parameter GL10, for GLES20+

15. 1. Create an OpenGL Context
2. Initial OpenGL Setup
-clear color, cull mode, etc
3. Load Resources
-textures, models, shaders, etc
4. Compile and link shaders
5. Upload Resources to GPU
6. Clear Screen
7. Draw
Draw Loop
Rendering: Step-By-Step

16. GLSL ES
The OpenGL Shading Language

17. GLSL - OpenGL Shading Language
• A C-style language for writing OpenGL shaders
• OpenGL ES 2.0 has two types of shaders
- Vertex shader
- Fragment shader
• Vertex shader - A program that runs per vertex
• Fragment shader - A program that runs per fragment

18. Rendering Pipeline
5.929688
4.125000
0.000000
5.832031
4.494141
0.000000
5.945313
0.000000
6.429688
4.125000
0.000000
5.387188
4.125000
2.747500
5.297100
4.494141
…
x.xxxxxx
][
Vertex Shader Fragment Shader
Simplified!

19. GLSL Qualifiers
• Uniform - A value that is the same for all vertices and/or pixels
• Attribute - A value defined per vertex, only available in a vertex
shader
• Varying - Data shared between shaders down the pipeline and
since they don’t map 1:1 they are also interpolated

20. Simple Vertex Shader
attribute vec4 a_position;
attribute vec3 a_color;
uniform mat4 mvp;
varying vec3 color;
void main()
{
color = a_color
gl_Position = mvp * a_position;
}
Calculations are done per vertex

21. Simple Fragment Shader
precision highp float; // ES Specific
varying vec3 color;
void main()
{
float alpha = 1.0;
gl_FragColor = vec4(color, alpha);
}
Calculations are done per fragment

22. We have a battery!
Running @60 FPS for no reason drains the battery
Solution: RENDERMODE_WHEN_DIRTY
General Tips & Tricks
• Limited memory bandwidth - Offload memory bus with computations in shader
• If-statements - In shader is bad for parallelism, both branches are usually
evaluated
• Method calls - In draw-loop may impact performance. Use direct access to data
instead. Bad for encapsulation, good for performance
• Memory allocation - In draw loop should be avoided. Allocate everything on
beforehand or on-demand
• Power of 2 textures - Good practice to use. Non-power-of-two textures has some
limitations in OpenGL ES 2.0

23. Sample Project Demo
github.com/andreasnilsson/OGLHelloWorld
Example Project:

24. Questions?
plus.google.com/+AndreasNilsson1985
se.linkedin.com/pub/andreas-nilsson/14/685/966
Droidcon attendees notice
I was unable to host the requested shaders for the earth demo. I might post a
blogpost about it in the near future so follow me on google plus or the Jayway
blog. You are feel to ask me questions about it though.
Finally, Thanks for your questions and interest in my presentation.
Feel free to share any feedback to help me improve my presentations.