blur_float.cpp

// Copyright (C) 2017 Basile Fraboni
// Copyright (C) 2014 Ivan Kutskir
// All Rights Reserved
// You may use, distribute and modify this code under the
// terms of the MIT license. For further details please refer 
// to : https://mit-license.org/
//

//!
//! \file blur.cpp
//! \author Basile Fraboni
//! \date 2017
//!
//! \brief The software is a C++ implementation of a fast  
//! Gaussian blur algorithm by Ivan Kutskir. For further details 
//! please refer to : 
//! http://blog.ivank.net/fastest-gaussian-blur.html
//!
//! Floating point version
//!

#define STB_IMAGE_IMPLEMENTATION
#include "stb_image.h"
#define STB_IMAGE_WRITE_IMPLEMENTATION
#include "stb_image_write.h"

#include <iostream>
#include <cmath>
#include <chrono>

//!
//! \fn void std_to_box(int boxes[], float sigma, int n)  
//!
//! \brief this function converts the standard deviation of 
//! Gaussian blur into dimensions of boxes for box blur. For 
//! further details please refer to :
//! https://www.peterkovesi.com/matlabfns/#integral
//! https://www.peterkovesi.com/papers/FastGaussianSmoothing.pdf
//!
//! \param[out] boxes   boxes dimensions
//! \param[in] sigma    Gaussian standard deviation
//! \param[in] n        number of boxes
//!
void std_to_box(int boxes[], float sigma, int n)  
{
    // ideal filter width
    float wi = std::sqrt((12*sigma*sigma/n)+1); 
    int wl = std::floor(wi);  
    if(wl%2==0) wl--;
    int wu = wl+2;
                
    float mi = (12*sigma*sigma - n*wl*wl - 4*n*wl - 3*n)/(-4*wl - 4);
    int m = std::round(mi);
                
    for(int i=0; i<n; i++) 
        boxes[i] = ((i < m ? wl : wu) - 1) / 2;
}

//!
//! \fn void horizontal_blur(float * in, float * out, int w, int h, int r)    
//!
//! \brief this function performs the horizontal blur pass for box blur. 
//!
//! \param[in,out] in       source channel
//! \param[in,out] out      target channel
//! \param[in] w            image width
//! \param[in] h            image height
//! \param[in] r            box dimension
//!
void horizontal_blur(float * in, float * out, int w, int h, int r) 
{
    float iarr = 1.f / (r+r+1);
    #pragma omp parallel for
    for(int i=0; i<h; i++) 
    {
        int ti = i*w, li = ti, ri = ti+r;
        float fv = in[ti], lv = in[ti+w-1], val = (r+1)*fv;

        for(int j=0; j<r; j++) val += in[ti+j];
        for(int j=0  ; j<=r ; j++) { val += in[ri++] - fv      ; out[ti++] = val*iarr; }
        for(int j=r+1; j<w-r; j++) { val += in[ri++] - in[li++]; out[ti++] = val*iarr; }
        for(int j=w-r; j<w  ; j++) { val += lv       - in[li++]; out[ti++] = val*iarr; }
    }
}

//!
//! \fn void total_blur(float * in, float * out, int w, int h, int r)   
//!
//! \brief this function performs the total blur pass for box blur. 
//!
//! \param[in,out] in       source channel
//! \param[in,out] out      target channel
//! \param[in] w            image width
//! \param[in] h            image height
//! \param[in] r            box dimension
//!
void total_blur(float * in, float * out, int w, int h, int r) 
{
    float iarr = 1.f / (r+r+1);
    #pragma omp parallel for
    for(int i=0; i<w; i++) 
    {
        int ti = i, li = ti, ri = ti+r*w;
        float fv = in[ti], lv = in[ti+w*(h-1)], val = (r+1)*fv;
        for(int j=0; j<r; j++) val += in[ti+j*w];
        for(int j=0  ; j<=r ; j++) { val += in[ri] - fv    ; out[ti] = val*iarr; ri+=w; ti+=w; }
        for(int j=r+1; j<h-r; j++) { val += in[ri] - in[li]; out[ti] = val*iarr; li+=w; ri+=w; ti+=w; }
        for(int j=h-r; j<h  ; j++) { val += lv     - in[li]; out[ti] = val*iarr; li+=w; ti+=w; }
    }
}

//!
//! \fn void box_blur(float * in, float * out, int w, int h, int r)    
//!
//! \brief this function performs a box blur pass. 
//!
//! \param[in,out] in       source channel
//! \param[in,out] out      target channel
//! \param[in] w            image width
//! \param[in] h            image height
//! \param[in] r            box dimension
//!
void box_blur(float *& in, float *& out, int w, int h, int r) 
{
    std::swap(in, out);
    horizontal_blur(out, in, w, h, r);
    total_blur(in, out, w, h, r);
    // Note to myself : 
    // here we could go anisotropic with different radiis rx,ry in HBlur and TBlur
}

//!
//! \fn void fast_gaussian_blur(float * in, float * out, int w, int h, float sigma)   
//!
//! \brief this function performs a fast Gaussian blur. Applying several
//! times box blur tends towards a true Gaussian blur. Three passes are sufficient
//! for good results. For further details please refer to :  
//! http://blog.ivank.net/fastest-gaussian-blur.html
//!
//! \param[in,out] in       source channel
//! \param[in,out] out      target channel
//! \param[in] w            image width
//! \param[in] h            image height
//! \param[in] r            box dimension
//!
void fast_gaussian_blur(float *& in, float *& out, int w, int h, float sigma) 
{
    // sigma conversion to box dimensions
    int boxes[3];
    std_to_box(boxes, sigma, 3);
    box_blur(in, out, w, h, boxes[0]);
    box_blur(out, in, w, h, boxes[1]);
    box_blur(in, out, w, h, boxes[2]);
}

//! \code{.cpp}
int main(int argc, char * argv[])
{   
    if( argc < 2 ) exit(1);
    const char * image_file = argv[1];
    const float sigma = argc > 2 ? std::atof(argv[2]) : 3.;
    const char * output_file = argc > 3 ? argv[3] : "blur.png";
    
    // Image loading
    int width, height, channels;
    unsigned char * image_data = stbi_load(argv[1], &width, &height, &channels, 0);
    std::cout << "Source image: " << width<<"x" << height << " ("<<channels<<")" << std::endl;
    if(channels < 3)
    {
        std::cout<< "Input images must be RGB images."<<std::endl;
        exit(1);
    }

    // copy data
    int size = width * height;
    
    // output channels r,g,b
    float * newb = new float[size];
    float * newg = new float[size];
    float * newr = new float[size];
    
    // input channels r,g,b
    float * oldb = new float[size];
    float * oldg = new float[size];
    float * oldr = new float[size];

    // channels copy r,g,b
    for(int i = 0; i < size; ++i)
    {
        oldb[i] = image_data[channels * i + 0] / 255.f;
        oldg[i] = image_data[channels * i + 1] / 255.f;
        oldr[i] = image_data[channels * i + 2] / 255.f;
    }

    // per channel filter
    auto start = std::chrono::system_clock::now();
    fast_gaussian_blur(oldb, newb, width, height, sigma);
    fast_gaussian_blur(oldg, newg, width, height, sigma);
    fast_gaussian_blur(oldr, newr, width, height, sigma);
    auto end = std::chrono::system_clock::now();

    // stats
    float elapsed = std::chrono::duration_cast<std::chrono::milliseconds>(end-start).count();
    std::cout << "time " << elapsed << "ms" << std::endl;

    // channels copy r,g,b
    for(int i = 0; i < size; ++i)
    {
        image_data[channels * i + 0] = (unsigned char) std::min(255.f, std::max(0.f, 255.f * newb[i]));
        image_data[channels * i + 1] = (unsigned char) std::min(255.f, std::max(0.f, 255.f * newg[i]));
        image_data[channels * i + 2] = (unsigned char) std::min(255.f, std::max(0.f, 255.f * newr[i]));
    }
    
    // save
    std::string file(output_file);
    std::string ext = file.substr(file.size()-3);
    if( ext == "bmp" )
        stbi_write_bmp(output_file, width, height, channels, image_data);
    else if( ext == "jpg" )
        stbi_write_jpg(output_file, width, height, channels, image_data, 90);
    else
    {
        if( ext != "png" )
        {
            std::cout << "format '" << ext << "' not supported writing default .png" << std::endl; 
            file = file.substr(0, file.size()-4) + std::string(".png");
        }
        stbi_write_png(file.c_str(), width, height, channels, image_data, channels*width);
    }
    stbi_image_free(image_data);

    // clean memory
    delete[] newr;
    delete[] newb;
    delete[] newg;
    delete[] oldr;
    delete[] oldb;
    delete[] oldg;

    image_data = stbi_load(argv[1], &width, &height, &channels, 0);

    return 0;
}
//! \endcode