CODEWITHSUNDEEP
androidopengl-es
user29671
user29671

Reputation: 100

Frustumf() function in android

I am using a sample code and not able to understand a function Frustumf(). I want to change far argument value in it. my mean I want to see my object smaller then current view.

This is part of Myrenderer class in android.

public void onSurfaceChanged(GL10 gl, int width, int height) {
    gl.glViewport(0, 0, width, height);
    float aspect = (float)width / height;
    gl.glMatrixMode(GL10.GL_PROJECTION);
    gl.glLoadIdentity();
    gl.glFrustumf(-aspect, aspect, -1.0f, 1.0f, 1.0f, 50.0f);
}

Upvotes: 0

Views: 83

Answers (2)

Reto Koradi
Reto Koradi

Reputation: 54592

The far argument has no effect on the size of the object. It only controls the far clipping plane.

As the fancy diagram in @datenwolf's answer shows, the left, right, bottom and top values are measured at the near plane. So it's the relationship of those values with the near value that control the field of view, which in turn controls the size of your object.

More precisely, if bottom and top have the same magnitude, the field of view in the vertical direction is calculated as:

fovAngle = 2 * atan(top / near)

This means that making the near value smaller or making the top value larger increases the field of view angle, and making the near value larger or the top value smaller decreases the field of view.

To make your object smaller using the projection matrix, without changing the clipping planes, you can multiply the first 4 arguments by a factor. E.g.:

gl.glFrustumf(-2.0f * aspect, 2.0f * aspect, -2.0f, 2.0f, 1.0f, 50.0f);

Since you're increasing the field of view angle, this will also result in a stronger perspective effect. Which may or may not be desirable.

The more standard way of controlling the size of your objects is by modifying the view matrix, while leaving the projection unchanged. For example, to scale by a factor of 2:

gl.glMatrixMode(GL10.GL_MODELVIEW);
gl.glScalef(2.0f, 2.0f, 2.0f);

Upvotes: 1

datenwolf
datenwolf

Reputation: 162164

Here's a picture produced by the program https://github.com/datenwolf/codesamples/tree/master/samples/OpenGL/frustum

The grid is in the XY plane with Z=0, the labeled measure arrows depict the parameters for the frustum.

Frustum and a Scene

Upvotes: 1

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