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I’m trying to reproduce Photoshop’s multiply blend mode in OpenCV. Equivalents to this would be what you find in GIMP, or when you use the CIMultiplyBlendMode in Apple’s CoreImage framework.

Everything I read online suggests that multiply blending is accomplished simply by multiplying the channels of the two input images (i.e., Blend = AxB). And, this works, except for the case(s) where alpha is < 1.0.

You can test this very simply in GIMP/PhotoShop/CoreImage by creating two layers/images, filling each with a different solid color, and then modifying the opacity of the first layer. (BTW, when you modify alpha, the operation is no longer commutative in GIMP for some reason.)

A simple example: if A = (0,0,0,0) and B = (0.4,0,0,1.0), and C = AxB, then I would expect C to be (0,0,0,0). This is simple multiplication. But this is not how this blend is implemented in practice. In practice, C = (0.4,0,0,1.0), or C = B.

The bottom line is this: I need to figure out the formula for the multiply blend mode (which is clearly more than AxB) and then implement it in OpenCV (which should be trivial once I have the formula).

Would appreciate any insights.

Also, for reference, here are some links which show multiply blend as being simply AxB:

How does photoshop blend two images together

Wikipedia – Blend Modes

Photoshop Blend Modes



  1. Chosen as BEST ANSWER

    I managed to sort this out. Feel free to comment with any suggested improvements.

    First, I found a clue as to how to implement the multiply function in this post:

    multiply blending

    And here's a quick OpenCV implementation in C++.

    Mat MultiplyBlend(const Mat& cvSource, const Mat& cvBackground) {
    // assumption: cvSource and cvBackground are of type CV_8UC4
    // formula: (cvSource.rgb * cvBackground.rgb * cvSource.a) + (cvBackground.rgb * (1-cvSource.a))
    Mat cvAlpha(cvSource.size(), CV_8UC3, Scalar::all(0));
    Mat input[] = { cvSource };
    int from_to[] = { 3,0, 3,1, 3,2 };
    mixChannels(input, 1, &cvAlpha, 1, from_to, 3);
    Mat cvBackgroundCopy;
    Mat cvSourceCopy;
    cvtColor(cvSource, cvSourceCopy, CV_RGBA2RGB);
    cvtColor(cvBackground, cvBackgroundCopy, CV_RGBA2RGB);
    // A = cvSource.rgb * cvBackground.rgb * cvSource.a
    Mat cvBlendResultLeft;
    multiply(cvSourceCopy, cvBackgroundCopy, cvBlendResultLeft, 1.0 / 255.0);
    multiply(cvBlendResultLeft, cvAlpha, cvBlendResultLeft, 1.0 / 255.0);
    // invert alpha
    bitwise_not(cvAlpha, cvAlpha);
    // B = cvBackground.rgb * (1-cvSource.a)
    Mat cvBlendResultRight;
    multiply(cvBackgroundCopy, cvAlpha, cvBlendResultRight, 1.0 / 255.0);
    delete(cvBackgroundCopy, cvAlpha);
    // A + B
    Mat cvBlendResult;
    add(cvBlendResultLeft, cvBlendResultRight, cvBlendResult);
    delete(cvBlendResultLeft, cvBlendResultRight);
    cvtColor(cvBlendResult, cvBlendResult, CV_RGB2RGBA);
    return cvBlendResult;

  2. Here is an OpenCV solution based the source code of GIMP, specifically the function gimp_operation_multiply_mode_process_pixels.


    • Instead of looping on all pixels it can be vectorized, but I followed the steps of GIMP.
    • Input images must be of type CV_8UC3 or CV_8UC4.
    • it supports also the opacity value, that must be in [0, 255]
    • in the original GIMP implementation there is also the support for a mask. It can be trivially added to the code, eventually.
    • This implementation is in fact not symmetrical, and reproduce your strange behaviour.


    #include <opencv2opencv.hpp>
    using namespace cv;
    Mat blend_multiply(const Mat& level1, const Mat& level2, uchar opacity)
        CV_Assert(level1.size() == level2.size());
        CV_Assert(level1.type() == level2.type());
        CV_Assert(level1.channels() == level2.channels());
        // Get 4 channel float images
        Mat4f src1, src2;
        if (level1.channels() == 3)
            Mat4b tmp1, tmp2;
            cvtColor(level1, tmp1, COLOR_BGR2BGRA);
            cvtColor(level2, tmp2, COLOR_BGR2BGRA);
            tmp1.convertTo(src1, CV_32F, 1. / 255.);
            tmp2.convertTo(src2, CV_32F, 1. / 255.);
            level1.convertTo(src1, CV_32F, 1. / 255.);
            level2.convertTo(src2, CV_32F, 1. / 255.);
        Mat4f dst(src1.rows, src1.cols, Vec4f(0., 0., 0., 0.));
        // Loop on every pixel
        float fopacity = opacity / 255.f;
        float comp_alpha, new_alpha;
        for (int r = 0; r < src1.rows; ++r)
            for (int c = 0; c < src2.cols; ++c)
                const Vec4f& v1 = src1(r, c);
                const Vec4f& v2 = src2(r, c);
                Vec4f& out = dst(r, c);
                comp_alpha = min(v1[3], v2[3]) * fopacity;
                new_alpha = v1[3] + (1.f - v1[3]) * comp_alpha;
                if ((comp_alpha > 0.) && (new_alpha > 0.))
                    float ratio = comp_alpha / new_alpha;
                    out[0] = max(0.f, min(v1[0] * v2[0], 1.f)) * ratio + (v1[0] * (1.f - ratio));
                    out[1] = max(0.f, min(v1[1] * v2[1], 1.f)) * ratio + (v1[1] * (1.f - ratio));
                    out[2] = max(0.f, min(v1[2] * v2[2], 1.f)) * ratio + (v1[2] * (1.f - ratio));
                    out[0] = v1[0];
                    out[1] = v1[1];
                    out[2] = v1[2];
                out[3] = v1[3];
        Mat3b dst3b;
        Mat4b dst4b;
        dst.convertTo(dst4b, CV_8U, 255.);
        cvtColor(dst4b, dst3b, COLOR_BGRA2BGR);
        return dst3b;
    int main()
        Mat3b layer1 = imread("path_to_image_1");
        Mat3b layer2 = imread("path_to_image_2");
        Mat blend = blend_multiply(layer1, layer2, 255);
        return 0;
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