I am trying to create an application that will display text using OpenGL. I get unexpected behavior when I try to use Matrix.translateM to move the specified object to a specific position. All other transformation matrices work as I would expect from them.
This is what I get when I call Matrix.translateM(model_matrix, 0, -1.0f, 0, 0) : 
This image is without translation: 
This is my render code.
@Override public void onSurfaceCreated(GL10 unused, EGLConfig config) { GLES20.glClearColor(0.5f, 0.5f, 0.5f, 1.0f); GLES20.glDisable(GLES20.GL_DEPTH_TEST); GLES20.glEnable(GLES20.GL_BLEND); GLES20.glBlendFunc(GLES20.GL_SRC_ALPHA, GLES20.GL_ONE_MINUS_SRC_ALPHA); // create programs simple_texture = new SimpleTexture(); // create shapes square = new Square(); // create debug text handlers text_fps = new Text(this.font, this.text_scale); text_fps.setSize(50); text_fps.setText("Test\nLonger line"); // set camera position final float eyeX = 0.0f; final float eyeY = 0.0f; final float eyeZ = -3.0f; final float lookX = 0.0f; final float lookY = 0.0f; final float lookZ = 0.0f; final float upX = 0.0f; final float upY = 1.0f; final float upZ = 0.0f; Matrix.setLookAtM(view_matrix, 0, eyeX, eyeY, eyeZ, lookX, lookY, lookZ, upX, upY, upZ); } @Override public void onSurfaceChanged(GL10 unused, int width, int height) { GLES20.glViewport(0, 0, width, height); // set projection matrix float ratio = (float) width / height; Matrix.orthoM(projection_matrix, 0, -ratio, ratio, -1, 1, 3, 7); setScreenRatio(ratio); setScreenHeight(height); } @Override public void onDrawFrame(GL10 unused) { GLES20.glClear(GLES20.GL_COLOR_BUFFER_BIT); // calculate projection and view transformation Matrix.multiplyMM(vp_matrix, 0, projection_matrix, 0, view_matrix, 0); // if we are connected to debugger draw statistics drawStatistics(vp_matrix); }
And this is my draw code for text.
// use predefined program GLES20.glUseProgram(program); // get attribute positions attr_matrix = GLES20.glGetUniformLocation(program, "u_Matrix"); attr_position = GLES20.glGetAttribLocation(program, "a_Position"); attr_texture_position = GLES20.glGetAttribLocation(program, "a_TexturePosition"); attr_texture = GLES20.glGetUniformLocation(program, "u_Texture"); // set program parameters GLES20.glEnableVertexAttribArray(attr_position); GLES20.glVertexAttribPointer(attr_position, 2, GLES20.GL_FLOAT, false, 2 * 4, square_buffer); // set texture parameters GLES20.glEnableVertexAttribArray(attr_texture_position); GLES20.glVertexAttribPointer(attr_texture_position, 2, GLES20.GL_FLOAT, false, 2 * 4, texture_buffer); // set matrix float[] result = new float[16]; float ratio = (float) texture_height / texture_width; float screen_scale = (float) texture_height / PageRenderer.getScreenHeight(); Matrix.setIdentityM(model_matrix, 0); Matrix.translateM(model_matrix, 0, -1, 0, 0); Matrix.scaleM(model_matrix, 0, screen_scale, screen_scale * ratio, screen_scale); Matrix.setIdentityM(result, 0); Matrix.multiplyMM(result, 0, matrix, 0, model_matrix, 0); GLES20.glUniformMatrix4fv(attr_matrix, 1, false, result, 0); // assign texture GLES20.glActiveTexture(GLES20.GL_TEXTURE0); GLES20.glBindTexture(GLES20.GL_TEXTURE_2D, index); GLES20.glUniform1i(attr_texture, 0); // perform drawing GLES20.glDrawArrays(GLES20.GL_TRIANGLE_STRIP, 0, 4); // disable vertex array GLES20.glDisableVertexAttribArray(attr_position); GLES20.glDisableVertexAttribArray(attr_texture_position);
Shaders:
int vertex_shader = PageRenderer.loadShader( GLES20.GL_VERTEX_SHADER, "uniform mat4 u_Matrix;" + "attribute vec4 a_Position;" + "attribute vec2 a_TexturePosition;" + "varying vec2 v_TexturePosition;" + "void main() {" + " gl_Position = a_Position * u_Matrix;" + " v_TexturePosition = a_TexturePosition;" + "}" ); int fragment_shader = PageRenderer.loadShader( GLES20.GL_FRAGMENT_SHADER, "precision mediump float;" + "varying vec2 v_TexturePosition;" + "uniform sampler2D u_Texture;" + "void main() {" + " gl_FragColor = texture2D(u_Texture, v_TexturePosition);" + "}" );
Scaling and rotation of the work, as expected, but I can not understand why the translation is wrong. Please note that until recently, I did not use OpenGL.
I expected for translateM move the text to the left rather than “rotate” it.