Update to 2.0.0 tree from current Fremantle build

[opencv] / doc / plastex / faq_technical.rst

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+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. 
+