Android - ColorMatrix 处理图像对比度

ColorMatrix 没有直接处理对比度的方法。
内部有一个 4x5的矩阵数组：

 * <pre>
 *   R&rsquo; = a*R + b*G + c*B + d*A + e;
 *   G&rsquo; = f*R + g*G + h*B + i*A + j;
 *   B&rsquo; = k*R + l*G + m*B + n*A + o;
 *   A&rsquo; = P*R + q*G + r*B + s*A + t;</pre>

找到 Android-GPUImage 实现

public static final String CONTRAST_FRAGMENT_SHADER = "" +
            "varying highp vec2 textureCoordinate;\n" +
            " \n" +
            " uniform sampler2D inputimageTexture;\n" +
            " uniform lowp float contrast;\n" +
            " \n" +
            " void main()\n" +
            " {\n" +
            "     lowp vec4 textureColor = texture2D(inputimageTexture, textureCoordinate);\n" +
            "     \n" +
            "     gl_FragColor = vec4(((textureColor.rgb - vec3(0.5)) * contrast + vec3(0.5)), textureColor.w);\n" +
            " }";

仿照 gl_FragColor 来处理：

//对比度范围 [0,2f]
val matrix = ColorMatrix() //4x5矩阵，rgba+偏移
matrix.setScale(lum, lum, lum, 1f)
val ary = matrix.array
ary[0] = (ary[0] - 0.5f) * contrast + 0.5f
ary[1] = (ary[1] - 0.5f) * contrast + 0.5f
ary[2] = (ary[2] - 0.5f) * contrast + 0.5f

ary[5] = (ary[5] - 0.5f) * contrast + 0.5f
ary[6] = (ary[6] - 0.5f) * contrast + 0.5f
ary[7] = (ary[7] - 0.5f) * contrast + 0.5f

ary[10] = (ary[10] - 0.5f) * contrast + 0.5f
ary[11] = (ary[11] - 0.5f) * contrast + 0.5f
ary[12] = (ary[12] - 0.5f) * contrast + 0.5f
paint.colorFilter = ColorMatrixColorFilter(matrix)