本文介绍opencv给图片加水印的方法。

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1、添加水印

添加水印的原理是调低两张图片的透明度，然后叠加起来。公式如下：
dst = src1 * opacity + src2 * (1 - opacity) + gamma;
opacity是透明度，取值范围为0 - 1。
gamma是增益，用来调整图像亮度。

准备好以下素材，在说明书上添加小鸟水印。

下面是几种添加水印的形式。

1.1、铺满

#include <opencv2/core.hpp>
#include <opencv2/imgcodecs.hpp>
#include <opencv2/highgui.hpp>
#include <opencv2/imgproc.hpp>
using namespace cv;
int main() {
    Mat wordSrc = imread("word.png");
    Mat birdSrc = imread("bird.jpg");

    int rOffset, cOffset;
    float bOca = 0.3, wOca = 1 - bOca;
    int gamma = 0;
    for (int r = 0; r < wordSrc.rows; r++) {
        rOffset = r / birdSrc.rows * birdSrc.rows;
        for (int c = 0; c < wordSrc.cols; c++) {
            cOffset = c / birdSrc.cols * birdSrc.cols;

            Vec3b *wPixel = wordSrc.ptr<Vec3b>(r, c);
            Vec3b *bPixel = birdSrc.ptr<Vec3b>(r - rOffset, c - cOffset);
            int val;
            val = wPixel->val[0] * wOca + bPixel->val[0] * bOca + gamma;//B
            wPixel->val[0] = val > 255 ? 255 : (val < 0 ? 0 : val);
            val = wPixel->val[1] * wOca + bPixel->val[1] * bOca + gamma;//G
            wPixel->val[1] = val > 255 ? 255 : (val < 0 ? 0 : val);
            val = wPixel->val[2] * wOca + bPixel->val[2] * bOca + gamma;//R
            wPixel->val[2] = val > 255 ? 255 : (val < 0 ? 0 : val);
        }
    }

    imshow("word", wordSrc);
    waitKey(0);
}

1.2、在指定区域添加

int main()
{
    Mat wordSrc = imread("word.png");
    Mat birdSrc = imread("bird.jpg");

    int rOffset = wordSrc.rows - birdSrc.rows, cOffset = wordSrc.cols - birdSrc.cols;
    float bOca = 0.3, wOca = 1 - bOca;
    int gamma = -20;
    for (int r = 0; r < birdSrc.rows; r++) {
        for (int c = 0; c < birdSrc.cols; c++) {
            Vec3b *wPixel = wordSrc.ptr<Vec3b>(r + rOffset, c + cOffset);
            Vec3b *bPixel = birdSrc.ptr<Vec3b>(r, c);
            int val;
            val = wPixel->val[0] * wOca + bPixel->val[0] * bOca + gamma;//B
            wPixel->val[0] = val > 255 ? 255 : (val < 0 ? 0 : val);
            val = wPixel->val[1] * wOca + bPixel->val[1] * bOca + gamma;//G
            wPixel->val[1] = val > 255 ? 255 : (val < 0 ? 0 : val);
            val = wPixel->val[2] * wOca + bPixel->val[2] * bOca + gamma;//R
            wPixel->val[2] = val > 255 ? 255 : (val < 0 ? 0 : val);
        }
    }

    imshow("word", wordSrc);
    waitKey(0);
}

1.3、一比一铺满

如果水印大小和图像大小一直，可以使用opencv自带的函数addWeighted(src1, alpha, src2, beta, gamma, dst, dtype = -1)。
用法可以参考：https://blog.csdn.net/weixin_45001971/article/details/129042853
在“图片叠加”一节中有介绍用法。