[C++知识库]C实现纵横比保持的RGB图像缩放

双线性插值，实现图像scale down, 记个笔记

#include "stdafx.h"
#include <iostream>
#include "stdio.h"

using namespace std;

int img_down_scale(const unsigned char* src_img, unsigned char* dst_img, 
	unsigned int src_width, unsigned int src_height, 
	unsigned int dst_width, unsigned int dst_height)
{
	//bilinear interpolation
	if (!src_img || !dst_img) {
		std::cout << "input array can not be null!" << std::endl;
		return -1;
	}
	if (!dst_width|| !dst_height) {
		std::cout << "image size wrong!" << std::endl;
		return -2;
	}

	double x_ratio = (double)src_width / dst_width;
	double y_ratio = (double)src_height / dst_height;
	double aspect_ratio = 0.0f;
	int border_shift = 0;
	int h = 0, w = 0;
	int x0 = 0, x1 = 0, y0 = 0, y1 = 0;
	int dst_r = 0, dst_g = 0, dst_b = 0;

	if (x_ratio < y_ratio) {
		aspect_ratio = x_ratio;
		border_shift = (int)((src_height - (int)(dst_height * aspect_ratio)) / 2);
		for (h = 0; h < dst_height; h++) {
			for (w = 0; w < dst_width; w++) {
				double src_h = border_shift + h * aspect_ratio;
				double src_w = w * aspect_ratio;
				x0 = (int)src_w;
				x1 = x0 + 1;
				y0 = (int)src_h;
				y1 = y0 + 1;

				int rx0y0 = src_img[3 * (y0*src_width + x0) + 0];
				int gx0y0 = src_img[3 * (y0*src_width + x0) + 1];
				int bx0y0 = src_img[3 * (y0*src_width + x0) + 2];

				int rx1y0 = src_img[3 * (y0*src_width + x1) + 0];
				int gx1y0 = src_img[3 * (y0*src_width + x1) + 1];
				int bx1y0 = src_img[3 * (y0*src_width + x1) + 2];

				int rx0y1 = src_img[3 * (y1*src_width + x0) + 0];
				int gx0y1 = src_img[3 * (y1*src_width + x0) + 1];
				int bx0y1 = src_img[3 * (y1*src_width + x0) + 2];

				int rx1y1 = src_img[3 * (y1*src_width + x1) + 0];
				int gx1y1 = src_img[3 * (y1*src_width + x1) + 1];
				int bx1y1 = src_img[3 * (y1*src_width + x1) + 2];

				dst_r = (y1 - src_h) * ((x1 - src_w)* rx0y0 + (src_w - x0)*rx1y0) + (src_h - y0) *((x1 - src_w)*rx0y1 + (src_w - x0)*rx1y1);
				dst_g = (y1 - src_h) * ((x1 - src_w)* gx0y0 + (src_w - x0)*gx1y0) + (src_h - y0) *((x1 - src_w)*gx0y1 + (src_w - x0)*gx1y1);
				dst_b = (y1 - src_h) * ((x1 - src_w)* bx0y0 + (src_w - x0)*bx1y0) + (src_h - y0) *((x1 - src_w)*bx0y1 + (src_w - x0)*bx1y1);

				dst_r = dst_r > 255 ? 255 : dst_r < 0 ? 0 : dst_r;
				dst_g = dst_g > 255 ? 255 : dst_g < 0 ? 0 : dst_g;
				dst_b = dst_b > 255 ? 255 : dst_b < 0 ? 0 : dst_b;

				dst_img[3 * (h * dst_width + w) + 0] = (unsigned char)dst_r;
				dst_img[3 * (h * dst_width + w) + 1] = (unsigned char)dst_g;
				dst_img[3 * (h * dst_width + w) + 2] = (unsigned char)dst_b;
			}
		}
	} else {
		aspect_ratio = y_ratio;
		border_shift = (int)((src_width - (int)(dst_width * aspect_ratio)) / 2);
		for (h = 0; h < dst_height; h++) {
			double src_h = h * aspect_ratio;
			for (w = 0; w < dst_width; w++) {
				double src_w = border_shift +  w * aspect_ratio;
				x0 = (int)src_w;
				x1 = x0 + 1;
				y0 = (int)src_h;
				y1 = y0 + 1;

				int rx0y0 = src_img[3 * (y0*src_width + x0) + 0];
				int gx0y0 = src_img[3 * (y0*src_width + x0) + 1];
				int bx0y0 = src_img[3 * (y0*src_width + x0) + 2];

				int rx1y0 = src_img[3 * (y0*src_width + x1) + 0];
				int gx1y0 = src_img[3 * (y0*src_width + x1) + 1];
				int bx1y0 = src_img[3 * (y0*src_width + x1) + 2];

				int rx0y1 = src_img[3 * (y1*src_width + x0) + 0];
				int gx0y1 = src_img[3 * (y1*src_width + x0) + 1];
				int bx0y1 = src_img[3 * (y1*src_width + x0) + 2];

				int rx1y1 = src_img[3 * (y1*src_width + x1) + 0];
				int gx1y1 = src_img[3 * (y1*src_width + x1) + 1];
				int bx1y1 = src_img[3 * (y1*src_width + x1) + 2];

				dst_r = (y1 - src_h) * ((x1 - src_w)* rx0y0 + (src_w - x0)*rx1y0) + (src_h - y0) *((x1 - src_w)*rx0y1 + (src_w - x0)*rx1y1);
				dst_g = (y1 - src_h) * ((x1 - src_w)* gx0y0 + (src_w - x0)*gx1y0) + (src_h - y0) *((x1 - src_w)*gx0y1 + (src_w - x0)*gx1y1);
				dst_b = (y1 - src_h) * ((x1 - src_w)* bx0y0 + (src_w - x0)*bx1y0) + (src_h - y0) *((x1 - src_w)*bx0y1 + (src_w - x0)*bx1y1);

				dst_r = dst_r > 255 ? 255 : dst_r < 0 ? 0 : dst_r;
				dst_g = dst_g > 255 ? 255 : dst_g < 0 ? 0 : dst_g;
				dst_b = dst_b > 255 ? 255 : dst_b < 0 ? 0 : dst_b;

				dst_img[3 * (h * dst_width + w) + 0] = (unsigned char)dst_r;
				dst_img[3 * (h * dst_width + w) + 1] = (unsigned char)dst_g;
				dst_img[3 * (h * dst_width + w) + 2] = (unsigned char)dst_b;
			}
		}
	}
		
	

	return 0;
}
int _tmain(int argc, _TCHAR* argv[])
{
	unsigned char *pSrcImg = (unsigned char*)malloc(1000 * 1000 * 3);
	unsigned char *pDstImg = (unsigned char*)malloc(480 * 320 * 3);
	FILE *fp = fopen("input_1000x1000.bmp", "rb");
	fseek(fp, 54, SEEK_SET);
	fread(pSrcImg, 1000 * 1000 * 3, 1, fp);
	fclose(fp);
	img_down_scale(pSrcImg, pDstImg, 1000,1000, 480,320);

	unsigned char bmpHeader[54] = { 0 };
	FILE *fp_bmp_header = fopen("semseg_480x320.bmp", "rb");
	fread(bmpHeader, 54, 1, fp_bmp_header);
	fclose(fp_bmp_header);

	FILE *fp_write = fopen("down_scale_480x320.bmp", "wb");
	if (fp_write)
	{
		fwrite(bmpHeader, 54, 1, fp_write);
		fwrite(pDstImg, 480 * 320 * 3, 1, fp_write);
		fclose(fp_write);
		fp_write = NULL;
	}

	if (pSrcImg) { free(pSrcImg); pSrcImg = NULL; }
	if (pDstImg) { free(pDstImg); pDstImg = NULL; }
	return 0;
}