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How to alpha blend with transparency masks

asked 2019-09-02 01:46:15 -0500

I have figured out how to do alpha blend with transparency masks, but as with most things OpenCV, I suspect I'm not doing it as efficiently as could be.

I know how to copy an image with a mask onto another using cv::Mat::copyTo(). But when I do that, the 2nd image is copied 100% onto the destination. What I want is to copy it at 30% or some other value, like in the 3rd "blended" window in this example:

image description

Note how the text "starry night blend test" is partially see-through in that final blended image?

The only way I could figure out how to do that is like this:

// compile with:  g++ test.cpp $(pkg-config --libs opencv)
#include <opencv2/opencv.hpp>

int main()
    const auto fontface         = cv::FONT_HERSHEY_PLAIN;
    const auto fontscale        = 3.0;
    const auto fontthickness    = 3;

    const std::string text("starry night blend test");
    const cv::Scalar purple(255, 0, 255);
    const cv::Scalar white(255, 255, 255);

    cv::Mat background  = cv::imread("starrynight.jpg");

    // Create the layers on which we'll write the text.  Layers start out as
    // pure black.
    cv::Mat font_layer  = cv::Mat::zeros(background.size(), CV_8UC3);
    cv::Mat mask_layer  = cv::Mat::zeros(background.size(), CV_8UC1); // <- note the mask is 1 channel

    // Write the text in colour on the "font" layer.  In my actual application
    // there are many layers, and it is more complicated than just a line of
    // text.
    const cv::Point p(20, 100);
    cv::putText(font_layer, text, p, fontface, fontscale, purple, fontthickness, cv::LINE_8);

    // I also have a mask of the "font" layer.  In this example, the easiest
    // way to produce that mask is to write out the same text, with the exact
    // same options, but onto the single channel mat.
    cv::putText(mask_layer, text, p, fontface, fontscale, white, fontthickness, cv::LINE_8);

    // The function copyTo() has an optional "mask" parameter which we can use
    // to copy parts of an image onto another.  In this case, we copy the text
    // onto a clone of the image.
    cv::Mat tmp = background.clone();
    font_layer.copyTo(tmp, mask_layer);

    // This is the cool part of the code.  We do an alpha blend between the
    // two mats.  One of which is just the background, and the other is the
    // background with bright purple text.  When blended, it will appear as
    // if the text layer is semi-transparent.  0.3 == 30%
    const double alpha = 0.3;
    const double beta = 1.0 - alpha;
    cv::Mat blended;
    cv::addWeighted(tmp, alpha, background, beta, 0, blended);

    cv::imshow("tmp"        , tmp       );
    cv::imshow("blended"    , blended   );
    cv::imshow("background" , background);

    return 0;

The thing I don't like about this solution is the multiple copies:

  1. First I copy the entire background image into "tmp"
  2. Then I copy the font layer onto tmp using the mask
  3. Then I "copy" again when I call into cv::addWeighted().

Is this the way I should be doing this, or is there a better solution?

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1 answer

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answered 2019-09-02 11:00:48 -0500

your question is a bit unclear but maybe this answer will give you new ideas...

take a look at and the code below

image description

image description

#include "opencv2/imgproc.hpp"
#include "opencv2/highgui.hpp"
#include <iostream>

using namespace cv;
using namespace std;

void overlayImage(Mat* src, Mat* overlay, const Point& location)
    for (int y = max(location.y, 0); y < src->rows; ++y)
        int fY = y - location.y;

        if (fY >= overlay->rows)

        for (int x = max(location.x, 0); x < src->cols; ++x)
            int fX = x - location.x;

            if (fX >= overlay->cols)

            double opacity = ((double)overlay->data[fY * overlay->step + fX * overlay->channels() + 3]) / 255;

            for (int c = 0; opacity > 0 && c < src->channels(); ++c)
                unsigned char overlayPx = overlay->data[fY * overlay->step + fX * overlay->channels() + c];
                unsigned char srcPx = src->data[y * src->step + x * src->channels() + c];
                src->data[y * src->step + src->channels() * x + c] = srcPx * (1. - opacity) + overlayPx * opacity;

int main(int argc, char** argv)
    Mat underlay0 = imread("lena.jpg", IMREAD_UNCHANGED);
    Mat overlay = imread("opencv-logo-white.png", IMREAD_UNCHANGED);
    Mat underlay1 = underlay0.clone();

    if (underlay0.empty() || overlay.empty())
        cout << "Could not read input image files " << endl;
        return -1;

    overlayImage(&underlay0, &overlay, Point());
    imshow("result1", underlay0);

    overlay = overlay - Scalar(0, 0, 0, 127);

    overlayImage(&underlay1, &overlay, Point());
    imshow("result2", underlay1);


    return 0;
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Asked: 2019-09-02 01:46:15 -0500

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Last updated: Sep 02