CODEWITHSUNDEEP
c++imageopencvbitmap
mans
mans

Reputation: 18148

Converting opencv image to gdi bitmap doesn't work depends on image size

I have this code that converts an opencv image to a bitmap:

  void processimage(MAT imageData)
  {
      Gdiplus::Bitmap bitmap(imageData.cols,imageData.rows,stride, PixelFormat24bppRGB,imageData.data);

   // do some work with bitmap
  }

It is working well when the size of image is 2748 X 3664. But I am tring to process an image wth size 1374 X 1832, it doesn't work.

The error is invalid parameter(2).

I checked and can confirm that:

in 2748 *3664:

in 1374 X 1832

So everything seems correct to me, but it generate error.

What is the problem and how can I fix it?

Edit

Based on answer which explained why I can not create bitmap. I finally implemented it in this way:

   Mat newImage;
   cvtColor(imageData, newImage, CV_BGR2BGRA);
   Gdiplus::Bitmap bitmap(newImage.cols,newImage.rows,newImage.step1(), PixelFormat32bppRGB,newImage.data);

So effectively, I convert input image to a 4 byte per pixel and then use the convert it to bitmap.

All credits to Roger Rowland for his answer.

Upvotes: 2

Views: 2114

Answers (2)

Elmue
Elmue

Reputation: 8138

You can use my class CGdiPlus that implements all you need to convert from cv::Mat to Gdiplus::Bitmap and vice versa:

OpenCV / Tesseract: How to replace libpng, libtiff etc with GDI+ Bitmap (Load into cv::Mat via GDI+)

Upvotes: 0

Roger Rowland
Roger Rowland

Reputation: 26259

I think the problem is that a BMP format must have a stride that is a multiple of 4.

Your larger image has a stride of 8244, which is valid (8244/4 = 2061) but your smaller image has a stride of 4122, which is not (4122/4 = 1030.5).

As it says on MSDN for the stride parameter (with my emphasis):

Integer that specifies the byte offset between the beginning of one scan line and the next. This is usually (but not necessarily) the number of bytes in the pixel format (for example, 2 for 16 bits per pixel) multiplied by the width of the bitmap. The value passed to this parameter must be a multiple of four.

Assuming your stride is correct, I think you're only option is to copy it row by row. So, something like:

  1. Great a Gdiplus::Bitmap of the required size and format
  2. Use LockBits to get the bitmap pixel data.
  3. Copy the OpenCV image one row at a time.
  4. Call UnlockBits to release the bitmap data.

Upvotes: 6

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