Issue while loading a Gdiplus::Bitmap of PixelFormat8bppIndexed into an open cv::mat

Question

I am trying to load an Gdiplus bitmap of 8 bit depth to an opencv mat. the image is in greyscale.

Greyscale Image of 8bit:

Greysacle Image of 8bit .

this is the VC++ code snippet I am using to convert it into cv::mat. But using this when I convert I am getting a faded image of the original image in the cv::mat. What could be the reason?

// Creating the m_Bitmap 
Gdiplus::Bitmap bit(152, 152, PixelFormat8bppIndexed);
m_Bitmap = bit.Clone(0, 0, bit.GetWidth(), bit.GetHeight(), PixelFormat8bppIndexed);

// Later Different paths are added to this bitmap

// Converting this bitmap to cv::mat
Gdiplus::PixelFormat format = m_Bitmap->GetPixelFormat();
if (format == PixelFormat8bppIndexed)
{
            Gdiplus::BitmapData bitmapData;
            Gdiplus::Rect rect(0, 0, width, height);
            m_Bitmap->LockBits(&rect, Gdiplus::ImageLockModeRead, format, &bitmapData);
            
            cv::Mat nmat = cv::Mat(height, width, CV_8UC1, static_cast<unsigned char*>(bitmapData.Scan0), bitmapData.Stride).clone();
}

Original bitmap Image (m_Bitmap):

enter image description here

This is the image preview of cv::mat nmat:

enter image description here

I need to have the same image as the source to be loaded into the cv::mat but that is not happening here.

Note: When the bitmap was in PixelFormat32bppARGB

The below approach has made the correct conversion and the data in the cv::mat was proper. (like there was no fading thing)

    // Lock the gdiplus::bitmap object to get direct access to the pixel data
    Gdiplus::BitmapData bitmapData;
    Gdiplus::Rect rect(0, 0, width, height);
    m_Bitmap->LockBits(&rect, Gdiplus::ImageLockModeRead, format, &bitmapData);

    // Create a cv::Mat object with the same dimensions and type as the gdiplus::bitmap object
    cv::Mat nmat = cv::Mat(height, width, CV_8UC4, static_cast<unsigned char*>(bitmapData.Scan0), bitmapData.Stride).clone();

    // Convert to single channel image for performing open::CV operations
    cv::Mat singleChannelImage;
    cv::cvtColor(nmat, singleChannelImage, cv::COLOR_RGBA2GRAY);


    m_Bitmap->UnlockBits(&bitmapData);

enter image description here

Are you sure nmat actually has "faded" pixels (and it's not misrepresentation in the debug preview display) ? you can use cv::imwrite to save to file and check. — wohlstad
– wohlstad, Commented May 16, 2024 at 12:36
You cannot directly copy data from gdi+ bitmap which has indexed format into cv::Mat because you then copy indices instead colors to which indices refer. Your gdi+ bitmap has indices + colors palette (from palette by indices you read colors). [1] Convert gdi+ bitmap into PixelFormat24bppRGB by LockBits. [2] Create from this data cv::Mat. [3] Use cv::cvtColor to convert 3 channels image into one - grayscale. — rafix07
– rafix07, Commented May 16, 2024 at 12:55
@rafix07 isn't the index just a 1 to 1 trivial mapping in case of 8 bit grayscale ? (i.e. the index is the same as the grayscale ?) — wohlstad
– wohlstad, Commented May 16, 2024 at 13:41
@wohlstad I could not find any information you write about in offical Gdi+ reference (it could be the best to work in such way i.e index is value of grayscale), that is why I put my comment :) — rafix07
– rafix07, Commented May 16, 2024 at 14:25
@rafix07 i tried out you approach and it worked! Thanks for clarification. I have added my solution to the main post. — Jishnu JK
– Jishnu JK, Commented May 30, 2024 at 10:43

Jishnu JK · Accepted Answer · 2024-05-30 11:03:34Z

This worked for me

Convert gdi+ bitmap into PixelFormat24bppRGB by LockBits.
Create from this data cv::Mat.
Use cv::cvtColor to convert channels image into one - grayscale

as suggested by @rafix07

Gdiplus::PixelFormat format = m_Bitmap->GetPixelFormat();
if (format == PixelFormat8bppIndexed)
{  
// Step 1: Convert GDI+ bitmap into PixelFormat24bppRGB
        
Gdiplus::Bitmap tempBitmap24(width, height, PixelFormat24bppRGB);
Gdiplus::Graphics g(&tempBitmap24);
Gdiplus::Rect rect(0, 0, width, height);
        
g.DrawImage(m_Bitmap, rect);
            
    
// Step 2: Create cv::Mat from the converted data               
cv::Mat nmat(height, width, CV_8UC3);
        
for (int y = 0; y < height; y++)                
{               
for (int x = 0; x < width; x++)                 
{                       
Gdiplus::Color pixelColor;                      
tempBitmap24.GetPixel(x, y, &pixelColor);                   
cv::Vec3b& pixel = nmat.at<cv::Vec3b>(y, x);                    
pixel[0] = pixelColor.GetBlue();                        
pixel[1] = pixelColor.GetGreen();                   
pixel[2] = pixelColor.GetRed();         
}           
}
        
cv::Mat singleChannelImage;
// Step 3: Convert 3-channel image into grayscale
cv::cvtColor(nmat, singleChannelImage, cv::COLOR_RGBA2GRAY);

}

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