CODEWITHSUNDEEP
androidopengl-es
WhiteShark88
WhiteShark88

Reputation: 23

android OpenGL ES coordinates mapping

I did a lot of search and nothing solved my problem. I'm both new to android and to 3d programming. I'm working on an Android project where I need to draw a 3d object on the android device using opengl es. For each pixel I have Distance value between 200 and 9000, which needs to be mapped as a Z coordinate. The object is 320x240.

The questions are:

Upvotes: 1

Views: 3277

Answers (3)

Kevin Roy
Kevin Roy

Reputation: 111

Adding to what James had mentioned about normalizing to [-1,1]. A little bit of code :

FIll in data in a flat array as x,y,z assuming you are using a vertex shader similar to :

 "attribute vec3 coord3d;" +
    "uniform mat4 transform;" +
    "void main(void) {" +
    "  gl_Position = transform * vec4(coord3d.xyz, 1.0f);" + // size of 3 with a=1.0f for all points
    "  gl_PointSize = 10.0;"+
    "}"

Get the attribute :

attribute_coord3d = glGetAttribLocation(program, "coord3d");

Create VBO:

glGenBuffers(1, vbo,0);

Bind

glBindBuffer(GL_ARRAY_BUFFER, vbo[0]);

Put data in:

glBufferData(GL_ARRAY_BUFFER, size:SIZE_OF_ARRAY, makeFloatBuffer(FlatArray), GL_STATIC_DRAW);

where makeFloatBuffer is a function that creates a buffer:

  private FloatBuffer makeFloatBuffer(float[] arr) {
                ByteBuffer bb = ByteBuffer.allocateDirect(arr.length*4);
                bb.order(ByteOrder.nativeOrder());
                FloatBuffer fb = bb.asFloatBuffer();
                fb.put(arr);
                fb.position(0);
                return fb;
            }

Bind and Point to buffer:

    glBindBuffer(GL_ARRAY_BUFFER, vbo[0]);
    glEnableVertexAttribArray(attribute_coord3d);
    glVertexAttribPointer(attribute_coord3d, 
    size:3,GL_FLOAT,false,stride:vertexStride, 0);

where vertexStride = num_components*Float.BYTES; in our case num_components = 3 // x,y,z.

Draw:

glDrawArrays(GL_POINTS, 0, NUM_OF_POINTS);

Disable VBO:

glDisableVertexAttribArray(attribute_coord2d);

Upvotes: 0

James Coote
James Coote

Reputation: 1973

OpenGL doesn't really work well with large numbers (like anything over 10.0f, just the way it is designed). It would be better to convert your coordinates to be between -1 and 1 (i.e. normalize) than to try and make openGL use coordinates of 50f or 290f.

The reason the coordinates are normalized to between -1 and 1 is because model coordinates are only supposed to be relative to each other and not indicative of their actual dimensions in a specific game/app. The model could be used in many different games/apps with different coordinate systems, so you want all the model coordinates to be in some normalized standard form, so the programmer can then interpret in their own way.

To normalize, you loop through all your coordinates and find the value furthest from 0 i.e. 

float maxValueX = 0;
float maxValueY = 0;
float maxValueZ = 0;

// find the max value of x, y and z
for(int i=0;i<coordinates.length'i++){
    maxValueX = Math.max(Math.abs(coordinates[i].getX()), maxValueX);
    maxValueY = Math.max(Math.abs(coordinates[i].getY()), maxValueY);
    maxValueZ = Math.max(Math.abs(coordinates[i].getZ()), maxValueZ);
}

// convert all the coordinates to be between -1 and 1
for(int i=0;i<coordinates.length'i++){
    Vector3f coordinate = coordinates[i];
    coordinate.setX(coordinate.getX() / maxValueX);
    coordinate.setY(coordinate.getY() / maxValueY);
    coordinate.setZ(coordinate.getZ() / maxValueZ);
}

You only need to do this once. Assuming you are storing your data in a file, you can write a little utility program that does the above to the file and save it, rather than doing it every time you load the data into your app

Upvotes: 1

John Ericksen
John Ericksen

Reputation: 11113

Checkout the GLSurfaceView Activity in the APIDemos that ship with the Android SDK. That will give you a basic primer on how Android handles rendering through OpenGL ES. This is located in android-sdk/samples/android-10/ApiDemos. Make sure you have downloaded the 'Samples for SDK' under the given API level.

Here's a couple of resources to get you started as well:

Android Dev Blog on GLSurfaceView

Instructions on OpenGLES

Android Development Documentation on OpenGL

Hope that helps.

Upvotes: 0

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