QuadRender.kt

override fun onDrawFrame(gl: GL10?) {
   // clear the our "screen"
   GLES30.glClearColor(0.0f, 0.0f, 0.0f, 0.0f)
   GLES30.glClear(GLES30.GL_DEPTH_BUFFER_BIT or GLES30.GL_COLOR_BUFFER_BIT)

   // use program
   GLES30.glUseProgram(program)

   // set shader input (built-in attributes)
   setAttribute(inPositionHandle, VERTEX_SHADER_IN_POSITION, 3, TRIANGLE_VERTICES_DATA_POS_OFFSET) // 3 because 3 floats for position
   setAttribute(inTextureHandle, VERTEX_SHADER_IN_TEXTURE_COORD, 2, TRIANGLE_VERTICES_DATA_UV_OFFSET) // 2 because 2 floats for texture coordinates

   // update matrix
   Matrix.setIdentityM(matrixMVP, 0)
   GLES30.glUniformMatrix4fv(uMVPMatrixHandle, 1, false, matrixMVP, 0)
   GLES30.glUniformMatrix4fv(uSTMatrixHandle, 1, false, matrixSTM, 0)

   // (!) update uniforms for fragment shader
   val uMyUniformValue = floatArrayOf(1.0f, 0.75f, 0.95f, 1f) // some values that we're going to send to the fragment shader
   GLES30.glUniform4fv(uMyUniform, 1, uMyUniformValue, 0)

   // activate blending for textures (to support transparency)
   GLES30.glBlendFunc(GLES30.GL_SRC_ALPHA, GLES30.GL_ONE_MINUS_SRC_ALPHA)
   GLES30.glEnable(GLES20.GL_BLEND)

   // draw our quad
   GLES30.glDrawArrays(GLES30.GL_TRIANGLE_STRIP, 0, 4)
   checkGlError("glDrawArrays")

   GLES30.glFinish()
}