1. OpenGL ES
on Android

2. OpenGL ES
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subset of OpenGL for Embedded Systems
royalty free
adopted by every major handset OS
version 1.x fixed function
version 2.x fully programmable
version 3.x builds on version 2
http://www.khronos.org/opengles/
http://en.wikipedia.org/wiki/OpenGL_ES

3. Android GLSurfaceView
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built in to Android
manages OpenGL ES surfaces
draws into the Android view system
provides dedicated render thread
handles EGL interface with window system
supports continuous or on-demand
rendering
● makes getting started relatively easy

4. Android Renderer
● a custom implementation
○ GLSurfaceView.Renderer
● 3 interface methods called by system
○ onSurfaceCreated
○ onSurfaceChanged
○ onDrawFrame
● define model data in renderer
○ pass to OpenGL ES in onDrawFrame
● define matrices in renderer

5. Model Data
● OpenGL ES version 1
○ set points and colors with direct method calls
glBegin(GL_TRIANGLES);
glVertex3f(-0.5f, -0.25f, 0.0f);
glColor3f(1.0f, 0.0f, 0.0f);
...
glEnd();
● OpenGL ES version 2
○ define arrays and buffers to draw with
final float[] triangle1VerticesData = {
-0.5f, -0.25f, 0.0f, // X, Y, Z,
1.0f, 0.0f, 0.0f, 1.0f, // R, G, B, A
0.5f, -0.25f, 0.0f,
0.0f, 0.0f, 1.0f, 1.0f,
0.0f, 0.559016994f, 0.0f,
0.0f, 1.0f, 0.0f, 1.0f
};

6. Buffers
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Android is written in Java
OpenGL ES is written in C
model data is passed via buffers
model data is cached somewhere
○ client memory: FloatBuffer
○ graphics memory: VBO, IBO

7. Matrices
● model matrix
○ places a drawing in the world
● view matrix
○ positions drawings relative to our eye
● projection matrix
○ projects the view onto the screen
○ enables 3 dimensional perspective

8. Shaders
● GLSL ES: http://www.khronos.org/opengles/sdk/docs/manglsl/
● model data passes through shaders
● vertex shaders
○ perform operations on each vertex
○ pass results to fragment shaders
● fragment shaders
○ use results of vertex shaders
○ applies additional operations per pixel
○ draws to screen by sending data to OpenGL ES

9. Shader Program
● links vertex output to fragment input
● provides mechanism to pass model data
○ App puts model data in buffers
○ OpenGL ES passes data from buffers to shaders
● used by OpenGL ES to execute drawing
● App can have multiple shader programs

10. Light
● version 2 requires us to write our own lighting
○ per-vertex
○ per-fragment
● ray tracing
○ very accurate and realistic but very expensive
● rasterization
○ very good approximation
○ fast enough for real time graphics on mobile devices

11. Light Types
● Ambient
○ pervades entire scene, no single source
○ indirect diffuse lighting
● Diffuse
○ light from a source bounces directly off an object
○ illumination of object depends on angle to source
● Specular
○ moves as we move relative to an object
○ perceived as "shininess"

12. Light Sources
● Directional
○ consistent strength from undefined location
○ consistent direction no matter where you are
● Point
○ strength fades from a center point
○ travels in all directions
● Spot
○ strength fades from a point
○ travels with attenuation and direction

13. Textures
● Coordination
○ OpenGL y-axis is flipped relative to images
■ more expensive: flip bitmap when loaded
■ less expensive: flip texture coordinates
○ texture coordinates are called texels
○ (s,t) instead of (x,y)
● Load image bitmap into application
● Supply shaders with texture data
● Shaders draw the texture

14. Texture Filtering
● minification & magnification
● mipmapping
● nearest neighbor
○ blocky when magnified
● bilinear
○ smoother when magnified
● trilinear
○ smoother when multiple levels are side by side

15. Blending
● effect of combining colors
● OpenGL provides several modes
○ additive
○ multiplicative
○ interpolative
● occurs in fragment shader
○ after calculating final color
○ normally overwrites previous value
○ blending… blends with previous value

16. Vertex Buffer Objects
● instead of transferring data every frame
○ from client memory to OpenGL
● transfer once and draw from graphics cache
● generate an OpenGL buffer
○ give it the data in the client buffer
○ free client memory

17. Index Buffer Objects
● VBO's can contain a lot of redundant data
○ specifically, redundant vertex data
● instead, define each vertex once only in IBO
● reference each vertex by index in IBO