Convert FreeImage FIBITMAP format to OpenCV Mat

Question

I've been searching on an answer but all I can find is the inverse of what I need (Mat to FIBITMAP). I have a short code that loads images as FIBITMAP objects. I need FreeImage because OpenCV doesn't allow me to load pfm and some other extension files. What I'm trying to do is convert the FIBTMAP image into an OpenCV Mat so that I can use this in a much longer code I already have. How do I do?

Answer 1

The funtion FI2MAT will perform the conversion from FreeImage to OpenCV Mat.

I cannot thoroughly test each type of FreeImage format right now, but it should work as expected. Let me know if there are some issues with some formats.

Remember that not all types may be rendered correctly by imshow, so you may need to convert the image before.

Here the code with an example:

#include <FreeImage.h>
#include <opencv2\opencv.hpp>
using namespace cv;

void FI2MAT(FIBITMAP* src, Mat& dst)
{
    //FIT_BITMAP    //standard image : 1 - , 4 - , 8 - , 16 - , 24 - , 32 - bit
    //FIT_UINT16    //array of unsigned short : unsigned 16 - bit
    //FIT_INT16     //array of short : signed 16 - bit
    //FIT_UINT32    //array of unsigned long : unsigned 32 - bit
    //FIT_INT32     //array of long : signed 32 - bit
    //FIT_FLOAT     //array of float : 32 - bit IEEE floating point
    //FIT_DOUBLE    //array of double : 64 - bit IEEE floating point
    //FIT_COMPLEX   //array of FICOMPLEX : 2 x 64 - bit IEEE floating point
    //FIT_RGB16     //48 - bit RGB image : 3 x 16 - bit
    //FIT_RGBA16    //64 - bit RGBA image : 4 x 16 - bit
    //FIT_RGBF      //96 - bit RGB float image : 3 x 32 - bit IEEE floating point
    //FIT_RGBAF     //128 - bit RGBA float image : 4 x 32 - bit IEEE floating point

    int bpp = FreeImage_GetBPP(src);
    FREE_IMAGE_TYPE fit = FreeImage_GetImageType(src);

    int cv_type = -1;
    int cv_cvt = -1;

    switch (fit)
    {
    case FIT_UINT16: cv_type = DataType<ushort>::type; break;
    case FIT_INT16: cv_type = DataType<short>::type; break;
    case FIT_UINT32: cv_type = DataType<unsigned>::type; break;
    case FIT_INT32: cv_type = DataType<int>::type; break;
    case FIT_FLOAT: cv_type = DataType<float>::type; break;
    case FIT_DOUBLE: cv_type = DataType<double>::type; break;
    case FIT_COMPLEX: cv_type = DataType<Complex<double>>::type; break;
    case FIT_RGB16: cv_type = DataType<Vec<ushort, 3>>::type; cv_cvt = COLOR_RGB2BGR; break;
    case FIT_RGBA16: cv_type = DataType<Vec<ushort, 4>>::type; cv_cvt = COLOR_RGBA2BGRA; break;
    case FIT_RGBF: cv_type = DataType<Vec<float, 3>>::type; cv_cvt = COLOR_RGB2BGR; break;
    case FIT_RGBAF: cv_type = DataType<Vec<float, 4>>::type; cv_cvt = COLOR_RGBA2BGRA; break;
    case FIT_BITMAP:
        switch (bpp) {
        case 8: cv_type = DataType<Vec<uchar, 1>>::type; break;
        case 16: cv_type = DataType<Vec<uchar, 2>>::type; break;
        case 24: cv_type = DataType<Vec<uchar, 3>>::type; break;
        case 32: cv_type = DataType<Vec<uchar, 4>>::type; break;
        default:
            // 1, 4 // Unsupported natively
            cv_type = -1;
        }
        break;
    default:
        // FIT_UNKNOWN // unknown type
        dst = Mat(); // return empty Mat
        return;
    }

    int width = FreeImage_GetWidth(src);
    int height = FreeImage_GetHeight(src);
    int step = FreeImage_GetPitch(src);

    if (cv_type >= 0) {
        dst = Mat(height, width, cv_type, FreeImage_GetBits(src), step);
        if (cv_cvt > 0)
        {
            cvtColor(dst, dst, cv_cvt);
        }
    }
    else {

        vector<uchar> lut;
        int n = pow(2, bpp);
        for (int i = 0; i < n; ++i)
        {
            lut.push_back(static_cast<uchar>((255 / (n - 1))*i));
        }

        FIBITMAP* palletized = FreeImage_ConvertTo8Bits(src);
        BYTE* data = FreeImage_GetBits(src);
        for (int r = 0; r < height; ++r) {
            for (int c = 0; c < width; ++c) {
                dst.at<uchar>(r, c) = saturate_cast<uchar>(lut[data[r*step + c]]);
            }
        }
    }

    flip(dst, dst, 0);
}

int main()
{
    FreeImage_Initialise();
    FREE_IMAGE_FORMAT format = FreeImage_GetFileType("path_to_image", 0);
    FIBITMAP* fi_image = FreeImage_Load(format, "path_to_image");

    Mat cv_img;
    FI2MAT(fi_image, cv_img);

    return 0;
}