Color gradient calculation and BMP creation in plain C

gistfile1.c

/*
* Project: Draw Gradient to BMP
* Author: Alexander Aigner
* Creation Date: -
*
* Description: Gradient sample.
*
* Creates a Color gradient, saves the result to a BMP file and opens
* the file in the default image viewer.
*
*/


#include <stdio.h>
#include <math.h>
#include <stdlib.h>

// RGB Parameter Struct
typedef struct {
  double r;
  double g;
  double b;
} COLOR_RGB_PARAMS;

// RGB Holder
typedef struct {
  unsigned char r;
  unsigned char g;
  unsigned char b;
} COLOR_RGB;


// Fills an array with RGB values for a gradient
int make_color_gradient(COLOR_RGB buffer[], COLOR_RGB_PARAMS *frequency, COLOR_RGB_PARAMS *phase, int center, int width, int len) {
  int i = 0;

  if (!(center && width && len))
    return 0;

  for (i = 0; i < len; ++i) {

    buffer[i].r = (unsigned char)(sin(frequency->r*i + phase->r) * width + center);
    buffer[i].g = (unsigned char)(sin(frequency->g*i + phase->g) * width + center);
    buffer[i].b = (unsigned char)(sin(frequency->b*i + phase->b) * width + center);
  }

  return 1;
}

// Safes Gradient to a bmp file
int gradient_to_file(unsigned char * filename, COLOR_RGB buffer[], int len, int width, int height) {

  FILE *f = NULL;  // File handle
  unsigned char *img = NULL; // Image data holder
  int datasize = sizeof(COLOR_RGB)*width*height; // Data size
  int filesize = 54 + datasize; // Image file size
  int color = 0; // Color Index
  double blocksize =  (double)len/width;
  // Image data buffer
  img = (unsigned char *)malloc(datasize);
  memset(img, 0, datasize);
  // Current Pixel
  COLOR_RGB *cpix;
  // Return variable
  int rtVar = 0;

  // Writting Data
  int x, y;
  for (y = 0; y < height; ++y) {

    for (x = 0; x < width; ++x) {

      color = (int)(double)x*blocksize;
      cpix = img + (y*width + x)*sizeof(COLOR_RGB);
      cpix->r = buffer[color].r;
      cpix->b = buffer[color].b;
      cpix->g = buffer[color].g;
    }
  }

  // Creating Header
  unsigned char bmpfileheader[14] = {'B', 'M', 0, 0, 0, 0, 0, 0, 0, 0, 54, 0, 0, 0};
  unsigned char bmpinfoheader[40] = {40, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 24, 0};
  unsigned char bmppad[3] = {0, 0, 0};

  bmpfileheader[2] = (unsigned char)(filesize);
  bmpfileheader[3] = (unsigned char)(filesize >> 8);
  bmpfileheader[4] = (unsigned char)(filesize >> 16);
  bmpfileheader[5] = (unsigned char)(filesize >> 24);
  bmpinfoheader[4] = (unsigned char)(width);
  bmpinfoheader[5] = (unsigned char)(width >> 8);
  bmpinfoheader[6] = (unsigned char)(width >> 16);
  bmpinfoheader[7] = (unsigned char)(width >> 24);
  bmpinfoheader[8] = (unsigned char)(height);
  bmpinfoheader[9] = (unsigned char)(height>> 8);
  bmpinfoheader[10] = (unsigned char)(height >> 16);
  bmpinfoheader[11] = (unsigned char)(height >> 24);

  // Writing Image
  if (!fopen_s(&f, filename, "wb")) {

    // Header ....
    fwrite(bmpfileheader, 1, 14, f);
    fwrite(bmpinfoheader, 1, 40, f);

    // ... and data with padding
    for (int i = 0; i < height; i++) {

      fwrite(img + (width*(height - i - 1) * 3), 3, width, f);
      fwrite(bmppad, 1, (4 - (width * 3) % 4) % 4, f);
    }
    rtVar = 1; // Success!
    fclose(f);
  }

  // frees image data buffer
  free(img);

  return rtVar;
}

int main(int argc, char *argv[]) {
  
  // define image file name:
  unsigned char *filename = "test.bmp";

  // image size:
  int img_width = 500;
  int img_height = 50;


  // Gradient - Parameters:
  COLOR_RGB_PARAMS freq, phase;
  int center, width, len;

  // Play arround to create different color gradients
  center = 128;
  width = 127;
  len = 50;

  freq.r = 0.1;
  freq.g = 0.1;
  freq.b = 0.1;

  phase.r = 0;
  phase.g = 2;
  phase.b = 4;

  // Creating gradient buffer
  COLOR_RGB *result = (COLOR_RGB *)calloc(len, sizeof(COLOR_RGB));

  // Calculating data
  if (make_color_gradient(result, &freq, &phase, center, width, len)) {

    // Saving gradient to file
    if (gradient_to_file(filename, result, len, img_width, img_height)) {

      printf("Success, open BMP in default viewer ...\n");
      // Opens file in default viewer
      system(filename);
    }

    
  }

  return 0;
}