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POIs Google's Android OpenGL tutorial teaching incorrect linear algebra?
 

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  1. POIs Google's Android OpenGL tutorial teaching incorrect linear algebra?
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    <p>After helping another user with a question regarding the <a href="http://developer.android.com/training/graphics/opengl/touch.html" rel="noreferrer">Responding to Touch Events</a> Android tutorial, I downloaded the source code, and was quite baffled by what I saw. The tutorial seems to not be able to decide whether it wants to use row vectors or column vectors, and it looks all mixed up to me.</p> <p>On the Android Matrix page, they claim that their convention is column-vector/column-major, which is typical of OpenGL.</p> <p>Am I right, or is there something I am missing? Here are the relevant bits of it:</p> <p>Start out by creating a MVPMatrix by multiplying mProjMatrix * mVMatrix. So far so good.</p> <pre><code> // 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) </code></pre> <hr> <p>Next they are appending a rotation to the left hand side of the MVPMatrix? This seems a little weird.</p> <pre><code> // Create a rotation for the triangle Matrix.setRotateM(mRotationMatrix, 0, mAngle, 0, 0, -1.0f); // Combine the rotation matrix with the projection and camera view Matrix.multiplyMM(mMVPMatrix, 0, mRotationMatrix, 0, mMVPMatrix, 0) </code></pre> <hr> <p>Uploading in non-transposed order.</p> <pre><code> GLES20.glUniformMatrix4fv(mMVPMatrixHandle, 1, false, mvpMatrix, 0); </code></pre> <hr> <p>Finally in their shader, a vector*matrix multiplication?</p> <pre><code> // the matrix must be included as a modifier of gl_Position " gl_Position = vPosition * uMVPMatrix;" </code></pre> <hr> <p>Adding this all together, we get:</p> <pre><code>gl_Position = vPosition * mRotation * mProjection * mView; </code></pre> <p>Which is not correct by any stretch of my imagination. Is there any explanation that I'm not seeing as to what's going on here?</p>
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    1. COTwo possibilities for me. Either the example is wrong or they implemented the matrix operations differently. [see](http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android/2.2_r1.1/android/opengl/Matrix.java)
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    2. COCould you clarify the problem, please?
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