OpenGL ES 2.0 Camera Issues

问题:

I'm working with Android and OpenGL ES 2.0 and I am having an issue that I can't really formulate into a solid question. In the image, http://i.imgur.com/XuCHF.png, I basically have a shape to represent a ship in the middle and when it's moved to the side it gets stretched toward the vanishing point. What I am wanting to accomplish is to have the ship maintain most of its shape when it is moved. I believe it may be due to my matrices but every resource I've looked seems to use the same method.

//Setting up the projection matrix
final float ratio = (float) width / height;
final float left = -ratio;
final float right = ratio;
final float bottom = -1.0f;
final float top = 1.0f;
final float near = 1.0f;
final float far = 1000.0f;
Matrix.frustumM(projection_matrix, 0, left, right, bottom, top, near, far);

//Setting the view matrix
Matrix.setLookAtM(view_matrix, 0, 0.0f, 1.0f, 0.0f, 0.0f, 1.0f, -1.0f, 0.0f, 1.0f, 0.0f);

//Setting the model matrix
Matrix.setIdentityM(model_matrix, 0);
Matrix.translateM(model_matrix, 0, 2f, 0f, 0f);

//Setting the model-view-projection matrix
Matrix.multiplyMM(mvp_matrix, 0, view_matrix, 0, model_matrix, 0);
Matrix.multiplyMM(mvp_matrix, 0, GL.projection_matrix, 0, mvp_matrix, 0);
GLES20.glUniformMatrix4fv(mvp_matrix_location, 1, false, mvp_matrix, 0);

The shaders are very basic as well:

private final static String vertex_shader = 
      "uniform mat4 u_mvp_matrix;"
    + "attribute vec4 a_position;"
    + "void main()"
    + "{"
    + "  gl_Position = u_mvp_matrix * a_position;"
    + "}";

private final static String fragment_shader = 
       "precision mediump float;"
     + "void main()"
     + "{"
     + "  gl_FragColor = vec4(0.0, 0.0, 1.0, 1.0);"
     + "}";

Any thoughts/insight is greatly appreciated.

Thanks.

回答1:

That is normal - that is how perspective projection should look like. Although in your case it looks really stretched one - with wide field of view.

Try using instead of frustumM method perspectiveM(projection_matrix, 0, 45.0f, ratio, near, far). Or if you must use frustumM, calculate left/right/bottom/top like this:

float fov = 60; // degrees, try also 45, or different number if you like
top = tan(fov * PI / 360.0f) * near;
bottom = -top;
left = ratio * bottom;
right = ratio * top;

回答2:

If you don't want any perspective effect at all, then use Matrix.orthoM instead of Matrix.frustumM.

To just make the perspective effect less extreme, you need to reduce the field of view -- That is, increase near or bring top and bottom closer to zero. (You probably want right = top * ratio, and similarly with left if you're going to fiddle with top and bottom values.)