add efficient roi resize rotate doc

docs/how-to-use-and-FAQ/efficient-roi-resize-rotate.md

@@ -0,0 +1,156 @@
+
+### image roi crop + convert to ncnn::Mat
+
+```
++--------------+
+|   y          |           /-------/
+| x +-------+  |          +-------+|
+|   |     roih |im_h  =>  |      roih
+|   +-roiw--+  |          +-roiw--+/
+|              |
++-----im_w-----+
+```
+```
+const unsigned char* data = im.data + (y * im_w + x) * 3;
+ncnn::Mat in = ncnn::Mat::from_pixels(data, ncnn::PIXEL_RGB, roiw, roih, im_w * 3);
+```
+
+### image roi crop + resize + convert to ncnn::Mat
+
+```
++--------------+
+|   y          |           /----/
+| x +-------+  |          +----+|
+|   |     roih |im_h  =>  |  target_h
+|   +-roiw--+  |          |    ||
+|              |          +----+/
++-----im_w-----+         target_w
+```
+```
+const unsigned char* data = im.data + (y * im_w + x) * 3;
+ncnn::Mat in = ncnn::Mat::from_pixels_resize(data, ncnn::PIXEL_RGB, roiw, roih, im_w * 3, target_w, target_h);
+```
+
+### ncnn::Mat export image + offset paste
+
+```
+                +--------------+
+ /-------/      |   y          |
++-------+|      | x +-------+  |
+|       h|  =>  |   |       h  |im_h
++---w---+/      |   +---w---+  |
+                |              |
+                +-----im_w-----+
+```
+```
+const unsigned char* data = im.data + (y * im_w + x) * 3;
+out.to_pixels(data, ncnn::PIXEL_RGB, im_w * 3);
+```
+
+### ncnn::Mat export image + resize + roi paste
+
+```
+            +--------------+
+ /----/     |   y          |
++----+|     | x +-------+  |
+|    h| =>  |   |      roih|im_h
+|    ||     |   +-roiw--+  |
++-w--+/     |              |
+            +-----im_w-----+
+```
+```
+const unsigned char* data = im.data + (y * im_w + x) * 3;
+out.to_pixels_resize(data, ncnn::PIXEL_RGB, roiw, roih, im_w * 3);
+```
+
+### image roi crop + resize
+```
++--------------+
+|   y          |
+| x +-------+  |          +----+
+|   |      roih|im_h  =>  |  target_h
+|   +-roiw--+  |          |    |
+|              |          +----+
++-----im_w-----+         target_w
+```
+```
+const unsigned char* data = im.data + (y * im_w + x) * 3;
+ncnn::resize_bilinear_c3(data, roiw, roih, im_w * 3, outdata, target_w, target_h, target_w * 3);
+```
+
+### image resize + offset paste
+```
+            +--------------+
+            |   y          |
++----+      | x +-------+  |
+|    h  =>  |   |     roih |im_h
+|    |      |   +-roiw--+  |
++-w--+      |              |
+            +-----im_w-----+
+```
+```
+unsigned char* outdata = im.data + (y * im_w + x) * 3;
+ncnn::resize_bilinear_c3(data, w, h, w * 3, outdata, roiw, roih, im_w * 3);
+```
+
+### image roi crop + resize + roi paste
+```
++--------------+         +-----------------+
+|   y          |         |  roiy           |
+| x +-------+  |         |roix----------+  |
+|   |       h  |im_h  => |   |     target_h|outim_h
+|   +---w---+  |         |   |          |  |
+|              |         |   +-target_w-+  |
++-----im_w-----+         +-----outim_w-----+
+```
+```
+const unsigned char* data = im.data + (y * im_w + x) * 3;
+unsigned char* outdata = outim.data + (roiy * outim_w + roix) * 3;
+ncnn::resize_bilinear_c3(data, w, h, im_w * 3, outdata, target_w, target_h, outim_w * 3);
+```
+
+### image roi crop + rotate
+```
++--------------+
+|   y          |
+| x +-------+  |          +---+
+|   |  < <  h  |im_h  =>  | ^ |w
+|   +---w---+  |          | ^ |
+|              |          +---+
++-----im_w-----+            h
+```
+```
+const unsigned char* data = im.data + (y * im_w + x) * 3;
+ncnn::kanna_rotate_c3(data, w, h, im_w * 3, outdata, h, w, h * 3, 6);
+```
+
+### image rotate + offset paste
+```
+             +--------------+
+             |   y          |
+ +---+       | x +-------+  |
+ | ^ |h  =>  |   |  < <  w  |im_h
+ | ^ |       |   +---h---+  |
+ +---+       |              |
+   w         +-----im_w-----+
+```
+```
+unsigned char* outdata = im.data + (y * im_w + x) * 3;
+ncnn::kanna_rotate_c3(data, w, h, w * 3, outdata, h, w, im_w * 3, 7);
+```
+
+### image roi crop + rotate + roi paste
+```
++--------------+         +-----------------+
+|   y          |         |        roiy     |
+| x +-------+  |         |   roix  +---+   |
+|   |  < <  h  |im_h  => |         | ^ w   |outim_h
+|   +---w---+  |         |         | ^ |   |
+|              |         |         +-h-+   |
++-----im_w-----+         +-----outim_w-----+
+```
+```
+const unsigned char* data = im.data + (y * im_w + x) * 3;
+unsigned char* outdata = outim.data + (roiy * outim_w + roix) * 3;
+ncnn::kanna_rotate_c3(data, w, h, im_w * 3, outdata, h, w, outim_w * 3, 6);
+```