[SO-a/53772151/5690248] `GPUImageFilter` with subtract blending function implemented

ColorMatrixSubtractFilter.java

package the.dreams.wind.blendingfilter;

import android.opengl.GLES20;

import jp.co.cyberagent.android.gpuimage.filter.GPUImageFilter;

class ColorMatrixSubtractFilter extends GPUImageFilter {
    private static final String FRAGMENT_SHADER = "precision mediump float;" +
            "struct ColorFilter {" +
            "  mat4 factor;" +
            "  vec4 shift;" +
            "};" +
            "uniform sampler2D inputImageTexture;" +
            "uniform ColorFilter uColorFilter;" +
            "varying vec2 textureCoordinate;" +
            "void main() {" +
            "  vec4 originalColor = texture2D(inputImageTexture, textureCoordinate);" +
            "  originalColor.rgb *= originalColor.a;" +
            "  vec4 filteredColor = (originalColor * uColorFilter.factor) + uColorFilter.shift;" +
            "  filteredColor.rgb *= filteredColor.a;" +
            "  gl_FragColor = vec4(originalColor.rgb - filteredColor.rgb, originalColor.a);" +
            "}";
    private final float[] mColorFilter;

    // ========================================== //
    // Lifecycle
    // ========================================== //

    ColorMatrixSubtractFilter(float[] mColorFilter) {
        this(NO_FILTER_VERTEX_SHADER, mColorFilter);
    }

    @SuppressWarnings("WeakerAccess")
    ColorMatrixSubtractFilter(String vertexShader, float[] colorFilter) {
        super(vertexShader, FRAGMENT_SHADER);
        mColorFilter = colorFilter;
    }

    // ========================================== //
    // GPUImageFilter
    // ========================================== //


    @Override
    public void onInitialized() {
        super.onInitialized();
        attachColorFilter(getProgram());
    }

    // ========================================== //
    // Private
    // ========================================== //

    private void attachColorFilter(int program) {
        final float[] colorFilterFactor = new float[4 * 4];
        final float[] colorFilterShift = new float[4];
        for (int i = 0; i < mColorFilter.length; i++) {
            final float value = mColorFilter[i];
            final int calculateIndex = i + 1;
            if (calculateIndex % 5 == 0) {
                colorFilterShift[calculateIndex / 5 - 1] = value / 255;
            } else {
                colorFilterFactor[i - calculateIndex / 5] = value;
            }
        }
        final int colorFactorLocation = GLES20.glGetUniformLocation(program,
                "uColorFilter.factor");
        setUniformMatrix4f(colorFactorLocation, colorFilterFactor);
        final int colorShiftLocation = GLES20.glGetUniformLocation(program,
                "uColorFilter.shift");
        setFloatVec4(colorShiftLocation, colorFilterShift);
    }
}