Set the background color for PNG with transparency

Question

I am loading PNG images that have a transparency plane. When converted to grayscale, the transparent areas of the image appear black, which seems to be the default background. I need them to be white. What can I do?

[This is not an ordinary question about how to maintain transparency.]

Answer 1

If you are reading PNG with imreadno transfer IMREAD_UNCHANGED, then you will have a 3-channel BGR image. If there was a fourth alpha channel (0 = completely transparent, 255 = completely visible), then it is cropped as documentation to place it .

, , , BGR . (Vec3b(0, 0, 0)).

, BGR (imread wihout IMREAD_UNCHANGED) (imshow, waitkey :

Gimp, , .

: Michael Jepson overlayImage

#include <opencv2/highgui/highgui.hpp> 
#include <opencv2/imgcodecs.hpp>
int main(int argc, char** argv ) {
   cv::Mat img_4_channels;
   img_4_channels = cv::imread(argv[1], cv::IMREAD_UNCHANGED); // gives 8UC4
   // img_4_channels = cv::imread(argv[1]); // inappropriate: gives 8UC3

   cv::Mat background = cv::Mat(img_4_channels.size(), CV_8UC3, cv::Vec3b(255, 255, 255)); // white background
   overlayImage(background, img_4_channels, img_3_channels, cv::Point2i(0, 0));

   cv::imshow("3 channels", img_3_channels);
}

:

( , ).

Answer 2

( ) - libPNG , png_set_background:

-, png_set_background(), . png_set_strip_alpha(), - .

png_set_background(png_ptr, &background_color,
   PNG_BACKGROUND_GAMMA_SCREEN, 0, 1);

background_color - RGB libpng .

, OpenCV libPNG , - ( imread).

, libPNG .

, BGRA -. , , Gabriel, .

void remove_transparency(cv::Mat const& source
    , cv::Mat& destination
    , uint8_t background_color)
{
    CV_Assert(source.type() == CV_8UC4);

    destination.create(source.rows, source.cols, CV_8UC1);

    auto it_src(source.begin<cv::Vec4b>()), it_src_end(source.end<cv::Vec4b>());
    auto it_dest(destination.begin<uint8_t>());

    std::transform(it_src, it_src_end, it_dest
        , [background_color](cv::Vec4b const& v) -> uchar
        {
            // Conversion constants taken from cvtColor docs...
            float gray(v[0] * 0.114f + v[1] * 0.587f + v[2] * 0.299f);
            float alpha(v[3] / 255.0f);
            return cv::saturate_cast<uchar>(gray * alpha + background_color * (1 - alpha));
        }
        );
}

, , cv::parallel_for_, .

class ParallelRemoveTransparency
    : public cv::ParallelLoopBody
{
public:
    ParallelRemoveTransparency(cv::Mat const& source
        , cv::Mat& destination
        , uint8_t background_color)
        : source_(source)
        , destination_(destination)
        , background_color_(background_color)
    {
        CV_Assert(source.size == destination.size);
    }

    virtual void operator()(const cv::Range& range) const
    {
        cv::Mat4b roi_src(source_.rowRange(range));
        cv::Mat1b roi_dest(destination_.rowRange(range));

        std::transform(roi_src.begin(), roi_src.end(), roi_dest.begin()
            , [this](cv::Vec4b const& v) -> uint8_t {
                float gray(v[0] * 0.114f + v[1] * 0.587f + v[2] * 0.299f);
                float alpha(v[3] / 255.0f);
                return cv::saturate_cast<uint8_t>(gray * alpha + background_color_ * (1 - alpha));
            }
            );
    }

private:
    cv::Mat const& source_;
    cv::Mat& destination_;
    uint8_t background_color_;
};

void remove_transparency(cv::Mat const& source
    , cv::Mat& destination
    , uint8_t background_color)
{
    CV_Assert(source.type() == CV_8UC4);

    destination.create(source.rows, source.cols, CV_8UC1);

    ParallelRemoveTransparency parallel_impl(source, destination, background_color);
    cv::parallel_for_(cv::Range(0, source.rows), parallel_impl);
}

---

, Python. :

import numpy as np
import cv2

def remove_transparency(source, background_color):
    source_img = cv2.cvtColor(source[:,:,:3], cv2.COLOR_BGR2GRAY)
    source_mask = source[:,:,3]  * (1 / 255.0)

    background_mask = 1.0 - source_mask

    bg_part = (background_color * (1 / 255.0)) * (background_mask)
    source_part = (source_img * (1 / 255.0)) * (source_mask)

    return np.uint8(cv2.addWeighted(bg_part, 255.0, source_part, 255.0, 0.0))


img = cv2.imread('smile.png', -1)
result = remove_transparency(img, 255)

cv2.imshow('', result)
cv2.waitKey()