Negative image filter to Black & White mode

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I am using https://github.com/natario1/CameraView this library for capturing a negative image to positive and it is using the openGl shaders. I need a filter in which I can capture a negative image to positive in Black & White mode not in the normal color mode (which is currently available in the library). I tried to mix the two filters i.e first capture the negative image to positive in color mode and then apply the Black & White mode filter but as I am new to openGl, I was unable to do this. Please help me in this regard. It would be highly appreciated. The shaders which I am using are as follows :

This shader is used to convert the negative to positive in color mode.

private final static String FRAGMENT_SHADER = "#extension GL_OES_EGL_image_external : require\n"
            + "precision mediump float;\n"
            + "varying vec2 "+DEFAULT_FRAGMENT_TEXTURE_COORDINATE_NAME+";\n"
            + "uniform samplerExternalOES sTexture;\n"
            + "void main() {\n"
            + "  vec4 color = texture2D(sTexture, "+DEFAULT_FRAGMENT_TEXTURE_COORDINATE_NAME+");\n"
            + "  float colorR = (1.0 - color.r) / 1.0;\n"
            + "  float colorG = (1.0 - color.g) / 1.0;\n"
            + "  float colorB = (1.0 - color.b) / 1.0;\n"
            + "  gl_FragColor = vec4(colorR, colorG, colorB, color.a);\n"
            + "}\n";

This shader is used to change the normal positive image in Black & White mode.

private final static String FRAGMENT_SHADER = "#extension GL_OES_EGL_image_external : require\n"
            + "precision mediump float;\n"
            + "varying vec2 "+DEFAULT_FRAGMENT_TEXTURE_COORDINATE_NAME+";\n"
            + "uniform samplerExternalOES sTexture;\n" + "void main() {\n"
            + "  vec4 color = texture2D(sTexture, "+DEFAULT_FRAGMENT_TEXTURE_COORDINATE_NAME+");\n"
            + "  float colorR = (color.r + color.g + color.b) / 3.0;\n"
            + "  float colorG = (color.r + color.g + color.b) / 3.0;\n"
            + "  float colorB = (color.r + color.g + color.b) / 3.0;\n"
            + "  gl_FragColor = vec4(colorR, colorG, colorB, color.a);\n"
            + "}\n";

Please help in making a filter which can direct capture the negative image to positive in Black & White mode.

Thanks.
android-camera2 opengl-es glsl shader android
1 Answers

You can do that with a one-liner in a single shader:

gl_FragColor = vec4(vec3(dot(1.0 - color.rgb, vec3(1.0/3.0))), color.a);

Explanation:

the inverse color is:

vec3 inverseColor = 1.0 - color.rgb;

For the gray scale there are 2 opportunities. Either straight forward

float gray = (inverseColor.r + inverseColor.g + inverseColor.b) / 3.0;

Or by using the dot product:

float gray = dot(1.0 - inverseColor.rgb, vec3(1.0/3.0));

Finally construct a vec3 from gray:

gl_FragColor = vec4(vec3(gray), color.a);
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