GLES shaders for look up table texture - false colorizing a greyscale image

lut-shaders.java

// Simple vertex shader
private static final String VERTEX_SHADER =
        "uniform mat4 uMVPMatrix;\n" +
        "uniform mat4 uTexMatrix;\n" +
        "attribute vec4 aPosition;\n" +
        "attribute vec4 aTextureCoord;\n" +
        "varying vec2 vTextureCoord;\n" +
        "void main() {\n" +
        "    gl_Position = uMVPMatrix * aPosition;\n" +
        "    vTextureCoord = (uTexMatrix * aTextureCoord).xy;\n" +
        "}\n";

// Fragment shader for use with external 2D textures + an lut texture
private static final String FRAGMENT_SHADER_EXT =
        "#extension GL_OES_EGL_image_external : require\n" +
        "precision mediump float;\n" +
        "varying vec2 vTextureCoord;\n" +
        "uniform samplerExternalOES sTexture;\n" +
        "uniform sampler2D lutTexture;\n" +
        "void main() {\n" +
        "   float lum = texture2D(sTexture, vTextureCoord).r;" +
        "   lum = clamp(lum, 0.0, 1.0);" +
        "   gl_FragColor = texture2D(lutTexture, vec2(lum, 0.5));\n" +
        "}\n";