Previous tutorial was all about setting up the GLSurfaceView. Be sure to read it beacuse it's a really importent one to be able to continue.
Building a polygon
In this tutorial we will render our first polygon.
3D models are built up with smaller elements (vertices, edges, faces, and polygons) which can be manipulated individually.
Vertex
A vertex (vertices in plural) is the smallest building block of 3D model. A vertex is a point where two or more edges meet. In a 3D model a vertex can be shared between all connected edges, paces and polygons. A vertex can also be a represent for the position of a camera or a light source. You can see a vertex in the image below marked in yellow.
To define the vertices on android we define them as a float array that we put into a byte buffer to gain better performance. Look at the image to the right and the code below to match the vertices marked on the image to the code. |
private float vertices[] = {
-1.0f, 1.0f, 0.0f, // 0, Top Left
-1.0f, -1.0f, 0.0f, // 1, Bottom Left
1.0f, -1.0f, 0.0f, // 2, Bottom Right
1.0f, 1.0f, 0.0f, // 3, Top Right
};
// a float is 4 bytes, therefore we multiply the number if vertices with 4.
ByteBuffer vbb = ByteBuffer.allocateDirect(vertices.length * 4);
vbb.order(ByteOrder.nativeOrder());
FloatBuffer vertexBuffer = vbb.asFloatBuffer();
vertexBuffer.put(vertices);
vertexBuffer.position(0);
Don't forget that a float is 4 bytes and to multiply it with the number of vertices to get the right size on the allocated buffer.
OpenGL ES have a pipeline with functions to apply when you tell it to render. Most of these functions are not enabled by default so you have to remember to turn the ones you like to use on. You might also need to tell these functions what to work with. So in the case of our vertices we need to tell OpenGL ES that it’s okay to work with the vertex buffer we created we also need to tell where it is.
// Enabled the vertex buffer for writing and to be used during rendering. gl.glEnableClientState(GL10.GL_VERTEX_ARRAY);// OpenGL docs. // Specifies the location and data format of an array of vertex // coordinates to use when rendering. gl.glVertexPointer(3, GL10.GL_FLOAT, 0, vertexBuffer); // OpenGL docs.
When you are done with the buffer don't forget to disable it.
// Disable the vertices buffer. gl.glDisableClientState(GL10.GL_VERTEX_ARRAY);// OpenGL docs.
Edge
Edge is a line between two vertices. They are border lines of faces and polygons. In a 3D model an edge can be shared between two adjacent faces or polygons. Transforming an edge affects all connected vertices, faces and polygons. In OpenGL ES you don't define the edges, you rather define the face by giving them the vertices that would build up the three edges. If you would like modify an edge you change the two vertices that makes the edge. You can see an edge in the image below marked in yellow.
Face
Face is a triangle. Face is a surface between three corner vertices and three surrounding edges. Transforming a face affects all connected vertices, edges and polygons.
The order does matter.
When winding up the faces it's important to do it in the right direction because the direction defines what side will be the front face and what side will be the back face. Why this is important is because to gain performance we don't want to draw both sides so we turn off the back face. So it's a good idea to use the same winding all over your project. It is possible to change what direction that defines the front face with glFrontFace.
gl.glFrontFace(GL10.GL_CCW); // OpenGL docs
To make OpenGL skip the faces that are turned into the screen you can use something called back-face culling. What is does is determines whether a polygon of a graphical object is visible by checking if the face is wind up in the right order.
gl.glEnable(GL10.GL_CULL_FACE); // OpenGL docs
It's ofcource possible to change what face side should be drawn or not.
gl.glCullFace(GL10.GL_BACK); // OpenGL docs
Polygon
Time to wind the faces, remember we have decided to go with the default winding meaning counter-clockwise. Look at the image to the right and the code below to see how to wind up this square. |
private short[] indices = { 0, 1, 2, 0, 2, 3 };
To gain some performance we also put this ones in a byte buffer.
// short is 2 bytes, therefore we multiply the number if vertices with 2. ByteBuffer ibb = ByteBuffer.allocateDirect(indices.length * 2); ibb.order(ByteOrder.nativeOrder()); ShortBuffer indexBuffer = ibb.asShortBuffer(); indexBuffer.put(indices); indexBuffer.position(0);
Don't forget that a short is 2 bytes and to multiply it with the number of indices to get the right size on the allocated buffer.
Render
Time to get something on the screen, there is two functions used to draw and we have to decide which one to use.
The two functions are:
public abstract void glDrawArrays(int mode, int first, int count) // OpenGL docs
glDrawArrays draws the vertices in that order they are specified in the construction of our verticesBuffer.
public abstract void glDrawElements(int mode, int count, int type, // OpenGL docs Buffer indices)
glDrawElements need a little bit more to be able to draw. It needs to know the order which to draw the vertices, it needs the indicesBuffer.
Since we already created the indicesBuffer I'm guessing that you figured out that's the way we are going.
What is common for this functions is that they both need to know what it is they should draw, what primitives to render. Since there is some various ways to render this indices and some of them are good to know about for debugging reasons. I'll go through them all.
What primitives to render
GL_POINTS
Draws individual points on the screen.
GL_LINE_STRIP
Series of connected line segments.
GL_LINE_LOOP
Same as above, with a segment added between last and first vertices.
GL_LINES
Pairs of vertices interpreted as individual line segments.
GL_TRIANGLES
Triples of vertices interpreted as triangles.
GL_TRIANGLE_STRIP
Draws a series of triangles (three-sided polygons) using vertices v0, v1, v2, then v2, v1, v3 (note the order), then v2, v3, v4, and so on. The ordering is to ensure that the triangles are all drawn with the same orientation so that the strip can correctly form part of a surface.
GL_TRIANGLE_FAN
Same as GL_TRIANGLE_STRIP, except that the vertices are drawn v0, v1, v2, then v0, v2, v3, then v0, v3, v4, and so on.
I think the GL_TRIANGLES is the easiest to use so we go with that one for now.
Putting it all togetter
So let's putting our square together in a class.
package se.jayway.opengl.tutorial;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.nio.FloatBuffer;
import java.nio.ShortBuffer;
import javax.microedition.khronos.opengles.GL10;
public class Square {
// Our vertices.
private float vertices[] = {
-1.0f, 1.0f, 0.0f, // 0, Top Left
-1.0f, -1.0f, 0.0f, // 1, Bottom Left
1.0f, -1.0f, 0.0f, // 2, Bottom Right
1.0f, 1.0f, 0.0f, // 3, Top Right
};
// The order we like to connect them.
private short[] indices = { 0, 1, 2, 0, 2, 3 };
// Our vertex buffer.
private FloatBuffer vertexBuffer;
// Our index buffer.
private ShortBuffer indexBuffer;
public Square() {
// a float is 4 bytes, therefore we multiply the number if
// vertices with 4.
ByteBuffer vbb = ByteBuffer.allocateDirect(vertices.length * 4);
vbb.order(ByteOrder.nativeOrder());
vertexBuffer = vbb.asFloatBuffer();
vertexBuffer.put(vertices);
vertexBuffer.position(0);
// short is 2 bytes, therefore we multiply the number if
// vertices with 2.
ByteBuffer ibb = ByteBuffer.allocateDirect(indices.length * 2);
ibb.order(ByteOrder.nativeOrder());
indexBuffer = ibb.asShortBuffer();
indexBuffer.put(indices);
indexBuffer.position(0);
}
/**
* This function draws our square on screen.
* @param gl
*/
public void draw(GL10 gl) {
// Counter-clockwise winding.
gl.glFrontFace(GL10.GL_CCW); // OpenGL docs
// Enable face culling.
gl.glEnable(GL10.GL_CULL_FACE); // OpenGL docs
// What faces to remove with the face culling.
gl.glCullFace(GL10.GL_BACK); // OpenGL docs
// Enabled the vertices buffer for writing and to be used during
// rendering.
gl.glEnableClientState(GL10.GL_VERTEX_ARRAY);// OpenGL docs.
// Specifies the location and data format of an array of vertex
// coordinates to use when rendering.
gl.glVertexPointer(3, GL10.GL_FLOAT, 0, // OpenGL docs
vertexBuffer);
gl.glDrawElements(GL10.GL_TRIANGLES, indices.length,// OpenGL docs
GL10.GL_UNSIGNED_SHORT, indexBuffer);
// Disable the vertices buffer.
gl.glDisableClientState(GL10.GL_VERTEX_ARRAY); // OpenGL docs
// Disable face culling.
gl.glDisable(GL10.GL_CULL_FACE); // OpenGL docs
}
}
We have to initialize our square in the OpenGLRenderer class.
// Initialize our square. Square square = new Square();
And in the draw function call on the square to draw.
public void onDrawFrame(GL10 gl) {
// Clears the screen and depth buffer.
gl.glClear(GL10.GL_COLOR_BUFFER_BIT | // OpenGL docs.
GL10.GL_DEPTH_BUFFER_BIT);
// Draw our square.
square.draw(gl); // ( NEW )
}
If you run the application now the screen is still black. Why? Because OpenGL ES render from where the current position is, that by default is at point: 0, 0, 0 the same position that the view port is located. And OpenGL ES don’t render the things that are too close to the view port. The solution to this is to move the draw position a few steps into the screen before rendering the square:
// Translates 4 units into the screen. gl.glTranslatef(0, 0, -4); // OpenGL docs
I will talk about the different transformations in the next tutorial.
Run the application again and you will see that the square is drawn but quickly moves further and further into the screen. OpenGL ES doesn’t reset the drawing point between the frames that you will have to do yourself:
// Replace the current matrix with the identity matrix gl.glLoadIdentity(); // OpenGL docs
Now if you run the application you will see the square on a fixed position.
References
The info used in this tutorial is collected from:
Android Developers
OpenGL ES 1.1 Reference Pages
You can download the source for this tutorial here: Tutorial_Part_II
You can also checkout the code from: code.google.com
Previous tutorial: OpenGL ES Tutorial for Android – Part I – Setting up the view
Next tutorial: OpenGL ES Tutorial for Android – Part III – Transformations
Per-Erik Bergman
Consultant at Jayway
62 comments ↓
This is the first OpenGL tutorial i got the first time around. I must say the 3D tube you included to illustrate Vertex, Edge, Face and Polygon was well chosen. Please keep it up. You have been bookmarked…thanksalot,–
Very informative. Best and most up to date android opengl tutorial I’ve seen. Can’t wait for part 3. Keep up the excellent work for us nubs.
Best Android + GL tutorial ever! You helped me alot with getting my game to work with opengl! Thanks a lot man!
Best OpenGL tutorial i have ever gone through. Waiting eagerly for the next one. Please let me know when will the next one get published.
I’m working on the next tutorial. Hopefully, it will be published next week.
Wonderful tutorial; exactly what I was looking for!
:thumb up:
cheers!
Well explained GL ES ,excellent!
Thanks !
River
Eagerly waiting for your next tutorial.Next week is about to get over, please let me know when can I expect the next tutorial.
Can’t wait for the next tutorial… this is by FAR the best explanation I have found on the subject. Well done for being so descriptive, and not assuming we’ve all been programming graphics since the womb.
Tusen takk! Your tutorial is great and is just what I needed to get me up and running with Android and OpenGL ES. Can you recommend any books as well until the next installment! Hadet!
Nice tutorial, thanks. I notice it crashes when running on my G1 though.
The problem is in Cube.draw(GL10). glDrawElements is called with vertices.length as the count, but I believe it should be indices.length. This fixes the crash.
Regards,
Hal
As Hal, I too had crashes on a donut avd (1.6), the fix suggested by Hall has solved that problem.
Still an excellent tutorial. Thanks.
Thanks for the input. And of course it should be indics.length, my bad. I have updated the source and the tutorials affected.
Great tutorial! One thing I’m wondering is if it’s possible to use int arrays instead of float arrays for defining vertices. My understanding is that floating point performance is pretty bad on android devices because there is no dedicated floating point processing unit. I ask now because I ran across this post at andev.org: http://www.anddev.org/float_or_int_as_vertex_buffer-t9940.html
Your input would be greatly appreciated!
I’m trying to use DrawArrays with GL_POINTS, but my point(s) are only ever rendered in the middle of the screen, if at all (and the strangest code changest can prevent them from being appearing, like adding an extra LoadIdentity). I’ve put in a color-changing if-statement to confirm that my buffer contains the coordinates that I want it to, which it definitely does. Even changing the parameters of glOrthof (so that the origin should be to the left or right of the viewport) doesn’t prevent my point from just appearing in the middle of the viewport, regardless of the point’s coordinates. The only thing that ever moves the point on the screen at all is if I change the size or location of the actual viewport (not the ‘camera’). Can anyone explain this? Could it just be a problem with the AVD simulation?
thank you, it’s very good lecture for me.
but, i can draw square using just your code.
so i add other code.
onDrawFrame method is added gl.glEnableClientState(GL10.GL_VERTEX_ARRAY);
gl.glEnableClientState(GL10.GL_COLOR_ARRAY);
and
also i modify indices[] from short to byte in Square.java
like do this, i can draw it.
can other people draw Square??
i wander.
oh, i’m mistake. it executes very well when i add code of previous lecture
I’m taking my firsts steps into OpenGL and I have to say this tutorial is AMAZING!
Many Thanks! Keep up the excellent work
Thanks a lot for your tutorial. I understood very well this sample program.can u explain little more clear about indices. Waiting eagerly for more tutorials from u..
hey i couldn’t get the indices part?? seeing the image for square and seeing the code! I couldn’t get how its done in short indices!!?? pls help me there!!I got the triangles part but not the square indices??
hey Guys Im gettin NullPointerExceptio in gl.glVertexPOinter()??
Dude, you’re just rock!
You rock, I meant
Great tutorials pls keep up the good work!
Great Tutorial. One question though.
The last step to get the square in a fixed position using
// Replace the current matrix with the identity matrix
gl.glLoadIdentity(); // OpenGL docs
Where exactly does this line go?
My square never appears when i put this line in and without this line my square goes “into” the screen.
Thank u..nice tutorial..the OpenGL docs reference hyperlink given for glVertexPointer is pointing to glDrawElemtents. hope u will correct it..
Excellent tutorial, thanks
Do you recommend same techniques for 2D rendering?
Great tutorial, thanks!
Excellent, very intuitive and clear.
[...] Next tutorial: OpenGL ES Tutorial for Android – Part II – Building a polygon [...]
ashik: We only work with triangles so the indices is the way we tell OpenGL how to build our triangles.
Adrian: glLoadIdentity() resets the current matrix it’s like moving the pen back to it’s original location.
RGB tool: It depends some hardware support direct texture rendering and that is faster but sure, this is a good way to start.
Excellent, Great tutorial!
thanks!
This is awesome, thanks!
Hi
I am trying to plot ecg on android, using opengl is there any way, if anybody have idea please help in this.
naveenpatel
Form India
I tried the code and debugged on my htc wildfire.. and still, just blank screen. No error and I put together everything from the two tutorials.
Weird
Excellent!
Great Tutorial!
cannot see square. am using eclipse to launch android emulator. what color is the square suppose to be? any suggestions?
Thank you for your great work.
Thank you for your great tutorial.
rEALLY AN AWSOME TUTORIAL 1ST STEPWISE TUTORIAL OF 3D
Hi,
A very good tutorial for beginner like me.
But I was wondering, instead of declaring the vertices and indices manually one by one for the cube, do we have other method?
Please..
Damn, this tutorial is dope!
Man this tutorial makes more sense than the stock android one.
So basically, we need to make a custom class for each particular shape we want to render?
I’m used to just rendering in straight OpenGL, and not OpenGL ES, so this is somewhat foreign to me.
I feel like there has to be some simple library which contains all of the rendering classes for each of the primitive shapes?
I confess, I’m a newb and just want to take as many shortcuts as possible to get up and running, but does such a library exist?
Very nice.Thank’s
Hi..the program exits unexpectedly. When i checked logcat in eclipse it mentioned a nullpointer exception at OpenGLrenderer.onDrawFrame.. any ideas or help would be really appreiciated!
Thanks a lot
I figured it out ..thanks .I , by accident, added a return type to the constructor
[...] Bergman has an excellent break down of what some of the basic terms that have confused me [...]
i am not getting squre when i run
Gracias por el tuturial, hasta donde voy me ha quedado bastante claro lo que has enseñado
Thanks this is very helpful!
Thanks for your sharing^^
I find this an excelent tutorial.
I have several objects, including a torus as in this example.
A torus has quite some symmetry. I can make one big buffer including al the vertices of the torus, and call the draw function once every frame, or i can make a small patch of the torus an call this patch 16 times per frame with different position/ orientation. What will give the best performance?
Thanks alot
Hello everyone, I am learning opengl es these days. and I got stuck by the difference on the surface and surfaceview.
so is there anybody who can tell me the difference between surface and surfaceview in android.
it is urgent.
Many thanks.
Simply a lifesaver – Many thanks
[...] http://blog.jayway.com/2009/12/04/opengl-es-tutorial-for-android-%E2%80%93-part-ii-building-a-polygo... [...]
Wonderful !! Thanks…
Fantastische! I made a rotating cube without lighting and texture. Wow!
When I ran the app, it always said The (app name) has stopped unexpectedly.
What might be the problem???
Alright. I figured it out~
Log.i(“Woww”, “Now this one i must say excellent !”);
Excellent tutorial, you explain these stuff really well. I don’t know much about openGL and it’s the first tutorial I feel I really understood
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