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POOpenGL ES Android Matrix Transformations
 

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    <p>I have a renderer implementing GLSurfaceView.Renderer interface; a subclass of GLSurfaceView and some classes representing my objects I want to draw. I have the code from <a href="http://developer.android.com/training/graphics/opengl/motion.html" rel="noreferrer">http://developer.android.com/training/graphics/opengl/motion.html</a> I want to expand this and add some moving along axes and can't manage it. The object is only rotated. and here is my code:</p> <pre><code>public class NotMyCoolRenderer implements GLSurfaceView.Renderer { public GLShip mTriangle; private GLBackgroundStar mSquare; private final float[] mMVPMatrix = new float[16]; private final float[] mProjMatrix = new float[16]; private final float[] mVMatrix = new float[16]; private final float[] mModelMatrix = new float[16]; private final float[] tempMatrix = new float[16]; public void onDrawFrame(GL10 unused) { // Draw background color GLES20.glClear(GLES20.GL_COLOR_BUFFER_BIT); // Set the camera position (View matrix) Matrix.setLookAtM(mVMatrix, 0, 0, 0, -3, 0f, 0f, 0f, 0f, 1.0f, 0.0f); // Calculate the projection and view transformation Matrix.multiplyMM(mMVPMatrix, 0, mProjMatrix, 0, mVMatrix, 0); // Draw square mSquare.draw(mMVPMatrix); // Now moving on to triangle aka ship Matrix.setIdentityM(mModelMatrix, 0); Matrix.translateM(mModelMatrix, 0, 0.1f, 0f, 0); Matrix.rotateM(mModelMatrix, 0, mTriangle.mAngle, 0, 0, -1.0f); Matrix.multiplyMM(tempMatrix, 0, mVMatrix, 0, mProjMatrix, 0); Matrix.multiplyMM(mMVPMatrix, 0, mModelMatrix , 0, tempMatrix , 0); // Draw triangle mTriangle.draw(mMVPMatrix); } public void onSurfaceChanged(GL10 unused, int width, int height) { // Adjust the viewport based on geometry changes, // such as screen rotation GLES20.glViewport(0, 0, width, height); float ratio = (float) width / height; // this projection matrix is applied to object coordinates // in the onDrawFrame() method Matrix.frustumM(mProjMatrix, 0, -ratio, ratio, -1, 1, 3, 7); } public class GLShip { public volatile float mAngle; private final String vertexShaderCode = // This matrix member variable provides a hook to manipulate // the coordinates of the objects that use this vertex shader "uniform mat4 uMVPMatrix;" + "attribute vec4 vPosition;" + "void main() {" + // the matrix must be included as a modifier of gl_Position " gl_Position = uMVPMatrix * vPosition;" + "}"; public void draw(float[] mvpMatrix) { // Add program to OpenGL environment GLES20.glUseProgram(mProgram); // get handle to vertex shader's vPosition member mPositionHandle = GLES20.glGetAttribLocation(mProgram, "vPosition"); // Enable a handle to the triangle vertices GLES20.glEnableVertexAttribArray(mPositionHandle); // Prepare the triangle coordinate data GLES20.glVertexAttribPointer(mPositionHandle, COORDS_PER_VERTEX, GLES20.GL_FLOAT, false, vertexStride, vertexBuffer); // get handle to fragment shader's vColor member mColorHandle = GLES20.glGetUniformLocation(mProgram, "vColor"); // Set color for drawing the triangle GLES20.glUniform4fv(mColorHandle, 1, color, 0); // get handle to shape's transformation matrix mMVPMatrixHandle = GLES20.glGetUniformLocation(mProgram, "uMVPMatrix"); NotMyCoolRenderer.checkGlError("glGetUniformLocation"); // Apply the projection and view transformation GLES20.glUniformMatrix4fv(mMVPMatrixHandle, 1, false, mvpMatrix, 0); NotMyCoolRenderer.checkGlError("glUniformMatrix4fv"); // Draw the triangle GLES20.glDrawArrays(GLES20.GL_TRIANGLES, 0, vertexCount); // Disable vertex array GLES20.glDisableVertexAttribArray(mPositionHandle); } } </code></pre> <p>My expectations are that on each redrawing the object would be rotated by mAngle and traslated along Y-Axis by 1f. I can see only rotating (a bit projected too) though. I actually got some questions regarding that: how do I apply my translation matrix and what is the best practice of dividing the opengl functionality? Should not the modelMatrix be stored in the object itself rather than in renderer? Should the matrix operations be performed in the renderer-class? I have grouped them together since I guess that they are all related.</p>
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    1. COHmm. still nothing to explain how mProjMatrix is initialized.
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    2. COHere's a final hint: try to replicate step by step the lessons from http://learningwebgl.com/blog/?page_id=1217 . It covers only javascript, but the differences are next to minimal. The API is the same and the matrix math is the same. Good luck!
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    3. CO@AkiSuihkonen In another [thread](http://stackoverflow.com/questions/11925647/is-googles-android-opengl-tutorial-teaching-incorrect-linear-algebra) people suggested (for the same code of the same tutorial) using `gl_Position = uMVPMatrix * vPosition;` instead of `gl_Position = vPosition * uMVPMatrix;`. If i follow the suggestion, i cant see my triangle at all
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