49 lines
1.8 KiB
ReStructuredText
49 lines
1.8 KiB
ReStructuredText
|
|
FAQ Technical Questions on Library Use
|
|
--------------------------------------
|
|
|
|
How to access image pixels
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
(The coordinates are 0-based and counted from image origin, either top-left (img->origin=IPL_ORIGIN_TL) or bottom-left (img->origin=IPL_ORIGIN_BL)
|
|
|
|
* Suppose, we have 8-bit 1-channel image I (IplImage* img)::
|
|
|
|
I(x,y) ~ ((uchar*)(img->imageData + img->widthStep*y))[x]
|
|
|
|
* Suppose, we have 8-bit 3-channel image I (IplImage* img)::
|
|
|
|
I(x,y)blue ~ ((uchar*)(img->imageData + img->widthStep*y))[x*3]
|
|
I(x,y)green ~ ((uchar*)(img->imageData + img->widthStep*y))[x*3+1]
|
|
I(x,y)red ~ ((uchar*)(img->imageData + img->widthStep*y))[x*3+2]
|
|
|
|
e.g. increasing brightness of point (100,100) by 30 can be done this way::
|
|
|
|
CvPoint pt = {100,100};
|
|
((uchar*)(img->imageData + img->widthStep*pt.y))[pt.x*3] += 30;
|
|
((uchar*)(img->imageData + img->widthStep*pt.y))[pt.x*3+1] += 30;
|
|
((uchar*)(img->imageData + img->widthStep*pt.y))[pt.x*3+2] += 30;
|
|
|
|
or more efficiently::
|
|
|
|
CvPoint pt = {100,100};
|
|
uchar* temp_ptr = &((uchar*)(img->imageData + img->widthStep*pt.y))[pt.x*3];
|
|
temp_ptr[0] += 30;
|
|
temp_ptr[1] += 30;
|
|
temp_ptr[2] += 30;
|
|
|
|
* Suppose, we have 32-bit floating point, 1-channel image I (IplImage* img)::
|
|
|
|
I(x,y) ~ ((float*)(img->imageData + img->widthStep*y))[x]
|
|
|
|
* Now, the general case: suppose, we have N-channel image of type T::
|
|
|
|
I(x,y)c ~ ((T*)(img->imageData + img->widthStep*y))[x*N + c]
|
|
|
|
or you may use macro CV_IMAGE_ELEM( image_header, elemtype, y, x_Nc )::
|
|
|
|
I(x,y)c ~ CV_IMAGE_ELEM( img, T, y, x*N + c )
|
|
|
|
There are functions that work with arbitrary (up to 4-channel) images and matrices (cvGet2D, cvSet2D), but they are pretty slow.
|
|
|