How to do flipping without using the inbuilt function flip in OpenCV? - opencv

Question

Can anyone help me with this problem, how to do flipping of an image without using the inbuilt flipping function i.e. flip(src image, destination image , 1 or 0) in C++ using OpenCV. I am new to this software so please help.

Answer 1

OpenCV's flip function uses internal flipHoriz or flipVert functions.
static void
flipHoriz( const uchar* src, size_t sstep, uchar* dst, size_t dstep, Size size, size_t esz )
{
int i, j, limit = (int)(((size.width + 1)/2)*esz);
AutoBuffer<int> _tab(size.width*esz);
int* tab = _tab;
for( i = 0; i < size.width; i++ )
for( size_t k = 0; k < esz; k++ )
tab[i*esz + k] = (int)((size.width - i - 1)*esz + k);
for( ; size.height--; src += sstep, dst += dstep )
{
for( i = 0; i < limit; i++ )
{
j = tab[i];
uchar t0 = src[i], t1 = src[j];
dst[i] = t1; dst[j] = t0;
}
}
}
static void
flipVert( const uchar* src0, size_t sstep, uchar* dst0, size_t dstep, Size size, size_t esz )
{
const uchar* src1 = src0 + (size.height - 1)*sstep;
uchar* dst1 = dst0 + (size.height - 1)*dstep;
size.width *= (int)esz;
for( int y = 0; y < (size.height + 1)/2; y++, src0 += sstep, src1 -= sstep,
dst0 += dstep, dst1 -= dstep )
{
int i = 0;
if( ((size_t)src0|(size_t)dst0|(size_t)src1|(size_t)dst1) % sizeof(int) == 0 )
{
for( ; i <= size.width - 16; i += 16 )
{
int t0 = ((int*)(src0 + i))[0];
int t1 = ((int*)(src1 + i))[0];
((int*)(dst0 + i))[0] = t1;
((int*)(dst1 + i))[0] = t0;
t0 = ((int*)(src0 + i))[1];
t1 = ((int*)(src1 + i))[1];
((int*)(dst0 + i))[1] = t1;
((int*)(dst1 + i))[1] = t0;
t0 = ((int*)(src0 + i))[2];
t1 = ((int*)(src1 + i))[2];
((int*)(dst0 + i))[2] = t1;
((int*)(dst1 + i))[2] = t0;
t0 = ((int*)(src0 + i))[3];
t1 = ((int*)(src1 + i))[3];
((int*)(dst0 + i))[3] = t1;
((int*)(dst1 + i))[3] = t0;
}
for( ; i <= size.width - 4; i += 4 )
{
int t0 = ((int*)(src0 + i))[0];
int t1 = ((int*)(src1 + i))[0];
((int*)(dst0 + i))[0] = t1;
((int*)(dst1 + i))[0] = t0;
}
}
for( ; i < size.width; i++ )
{
uchar t0 = src0[i];
uchar t1 = src1[i];
dst0[i] = t1;
dst1[i] = t0;
}
}
}
// you can use it with a small modification as below
void myflip( InputArray _src, OutputArray _dst, int flip_mode )
{
CV_Assert( _src.dims() <= 2 );
Size size = _src.size();
if (flip_mode < 0)
{
if (size.width == 1)
flip_mode = 0;
if (size.height == 1)
flip_mode = 1;
}
if ((size.width == 1 && flip_mode > 0) ||
(size.height == 1 && flip_mode == 0) ||
(size.height == 1 && size.width == 1 && flip_mode < 0))
{
return _src.copyTo(_dst);
}
Mat src = _src.getMat();
int type = src.type();
_dst.create( size, type );
Mat dst = _dst.getMat();
size_t esz = CV_ELEM_SIZE(type);
if( flip_mode <= 0 )
flipVert( src.ptr(), src.step, dst.ptr(), dst.step, src.size(), esz );
else
flipHoriz( src.ptr(), src.step, dst.ptr(), dst.step, src.size(), esz );
if( flip_mode < 0 )
flipHoriz( dst.ptr(), dst.step, dst.ptr(), dst.step, dst.size(), esz );
}

Answer 2

Assuming you have a good reason not to use OpenCV flip function, you can write your custom one.
For this example, I'll use CV_8UC3 images. I'll point out at the end how to expand this to different formats.
Let's see first how to flip an image x axis, which corresponds to cv::flip(src, dst, 1). Given an src image, the dst image will have the same y coordinate, and x coordinate as src.cols - 1 - x coordinates. In practice:
void flip_lr(const Mat3b& src, Mat3b& dst)
{
Mat3b _dst(src.rows, src.cols);
for (int r = 0; r < _dst.rows; ++r) {
for (int c = 0; c < _dst.cols; ++c) {
_dst(r, c) = src(r, src.cols - 1 - c);
}
}
dst = _dst;
}
Then, to flip around y axis (corresponding to cv::flip(src, dst, 0)), dst will have the same x coordinate, and y as src.rows - 1 - y. However, you can reuse the above-mentioned function, simply transposing the dst matrix, apply flip on x axis, and then transpose back. In practice:
dst = src.t();
flip_lr(dst, dst);
dst = dst.t();
Then, to flip both axis, corresponding to cv::flip(src, dst, -1), you need simply to combine the flip on x and y axis:
flip_lr(src, dst);
dst = dst.t();
flip_lr(dst, dst);
dst = dst.t();
You can wrap this functionality in a custom flip function that takes the same parameters as cv::flip:
void custom_flip(const Mat3b& src, Mat3b& dst, int code)
{
if (code > 0)
{ // Flip x axis
flip_lr(src, dst);
}
else if (code == 0)
{
// Flip y axis
dst = src.t();
flip_lr(dst, dst);
dst = dst.t();
}
else // code < 0
{
// Flip x and y axis
flip_lr(src, dst);
dst = dst.t();
flip_lr(dst, dst);
dst = dst.t();
}
}
Note that you can adapt this to different format simply modifing the flip_lr function, and taking care to call the appropriate version inside custom_flip, that will now accept Mat instead of Mat3b.
Full code for reference:
void flip_lr(const Mat3b& src, Mat3b& dst)
{
Mat3b _dst(src.rows, src.cols);
for (int r = 0; r < _dst.rows; ++r) {
for (int c = 0; c < _dst.cols; ++c) {
_dst(r, c) = src(r, src.cols - 1 - c);
}
}
dst = _dst;
}
void custom_flip(const Mat3b& src, Mat3b& dst, int code)
{
if (code > 0)
{ // Flip x axis
flip_lr(src, dst);
}
else if (code == 0)
{
// Flip y axis
dst = src.t();
flip_lr(dst, dst);
dst = dst.t();
}
else // code < 0
{
// Flip x and y axis
flip_lr(src, dst);
dst = dst.t();
flip_lr(dst, dst);
dst = dst.t();
}
}
int main(void)
{
Mat3b img = imread("path_to_image");
Mat3b flipped;
flip(img, flipped, -1);
Mat3b custom;
custom_flip(img, custom, -1);
imshow("OpenCV flip", flipped);
imshow("Custom flip", custom);
waitKey();
return 0;
}