Is there any reason I should not use a macro for fast element access in C++ API (like CV_MAT_ELEM_PTR_FAST in C API)? Here's my implementation
#define MY_MAT_ELEM_PTR_FAST( mat, row, col, elemtype ) \
(assert( (unsigned)(row) < (unsigned)(mat).rows && \
(unsigned)(col) < (unsigned)(mat).cols ), \
(* (elemtype*) ( (mat).data + (mat).step*(row) + (sizeof(elemtype)*(col)) ) ) )
Yes, there is: The image is not always continuous in the memory; so mat.data+(y*width+x)*sizeof(elemtype) won't give you the pixel (x,y). Check if the matrix is continuous with mat.isContinuous().
You can use mat.ptr() of mat.ptr(row) instead mat.data (that you can cast directly to another type).
So the last line of the macro could look like (for a more general application):
(elemtype*) (mat.ptr(row)) + col
I also think (but I might be wrong) that the line pointers are precomputed, so this can save you a multiplication.
To access an element directly you can use the standard mat.at<elemtype>(y,x), it's also fast.
thank you! I will try using parallel_for
p.s. I've accidentally deleted my previous comment :( here's the performance test I've found on the web (maybe someone will find it useful) http://longstryder.com/2014/07/which-...
yes, there are a lot of arguments against your approach:
in general:
avoid per-pixel operations at all cost, in 90% cases, there's a high-level, optimized function for this already in opencv
get a row ptr(), and access the desired type directly, like explained here
don't try to be smart. don't try to be smarter than the devs.
There are 3 methods , to access MAT elements
1-Pointer method (effecint in time , but risky in handling piointers errors)
2-Iterator method (the safe mode)
3-On the fly method
also there is LUT which give best time because Opencv optmization for multithreading and multicore
you can find examples for every method in the following link [link text](http://docs.opencv.org/2.4/doc/tutorials/core/how_to_scan_images/how_to_scan_images.html)
Asked: 2016-03-10 03:34:14 -0600
Seen: 1,475 times
Last updated: Mar 10 '16
can you show us, what you're actually trying ?
I've edited question