save 2ms replace div with mul.

static float2 scale;

static float2 center, dimensions;

static float range, inv_max_dist, shade, slope;

static float2 overDimensions;

void init_vignette(uint32_t dim_x, uint32_t dim_y, float center_x, float center_y,

        float desired_scale, float desired_shade, float desired_slope) {

    range = 1.3 - 0.7*sqrt(desired_scale);

    shade = desired_shade;

    slope = desired_slope;

    overDimensions = ((float2)1.f) / dimensions;

}

void root(const uchar4 *in, uchar4 *out, uint32_t x, uint32_t y) {

    // Convert x and y to floating point coordinates with center as origin

    const float4 fin = rsUnpackColor8888(*in);

    float2 coord;

    coord.x = (float)x / dimensions.x;

    coord.y = (float)y / dimensions.y;

    const float4 fin = convert_float4(*in);

    float2 coord = {(float)x, (float)y};

    coord *= overDimensions;

    coord -= center;

    const float dist = length(scale * coord);

    const float lumen = shade / (1.0 + exp((dist * inv_max_dist - range) * slope)) + (1.0 - shade);

    float4 fout;

    fout.rgb = fin.rgb * lumen;

    fout.w = fin.w;

    *out = rsPackColorTo8888(fout);

    *out = convert_uchar4(fout);

}