小图片：
IfranView: 
IrfanView是免费软件但不开源。下面的程序读入small.png，用最简单的方法放大，然后写入big.png:
/*
https://www.nongnu.org/pngpp/ 在libpng上包了C++接口，用起来很方便。
安装与查看libpng++-dev
# apt-get install libpng++-dev
# dpkg -l | grep libpng
...
# dpkg -L libpng++-dev
/usr/include/png++ ...
# dpkg -L libpng-dev
/usr/include/libpng16
/usr/lib/x86_64-linux-gnu/libpng.a ...
$ libpng-config --ldflags --cflags
-lpng16 -I/usr/include/libpng16
16是版本号
*/
#include <png++/image.hpp> // g++ 1.cpp -lpng16
using namespace png;
#define RGB rgb_pixel
#define ratio 3.14
int main() {
image<RGB> small("small.png"); // 打开文件失败会throw
const pixel_buffer<RGB>& s = small.get_pixbuf();
int w = small.get_width(), h = small.get_height();
int ww = int(ratio * w + 0.5), hh = (ratio * h + 0.5);
image<RGB> big(ww, hh);
pixel_buffer<RGB>& b = big.get_pixbuf();
b[0][0] = RGB(255, 128, 0); // 试一下
for (int yy = 0; yy < hh; yy++) {
for (int xx = 0; xx < ww; xx++) {
int x = int(xx / ratio + 0.5), y = int(yy / ratio + 0.5);
if (x >= w) x = w - 1; if (y >= h) y = h - 1;
b[yy][xx] = s[y][x];
}
}
big.write("big.png");
}
效果：
重复IfranView:
lanczos · GitHub Topics | Lanczos resampling - Detailed Pedia
双线性插值 | Bilinear interpolation - Detailed Pedia
Source Code for Smooth Image Resampling [fairly-well-optimized, MMX加普通版, MMX LOOP - about 32% faster]
Plain C Resampling DLL - CodeProject
标签：get,int,big,读写,small,PNG,libpng,放入,png From： https://www.cnblogs.com/funwithwords/p/17034329.html