change README

1 files changed, 20 insertions, 1 deletions

diff --git a/README.md b/README.md
index 6f84cb7..2831672 100644
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+++ b/README.md
@@ -175,6 +175,25 @@ This example board image is generated by following scripts from [these source da
 pcb-tools-extension generate data block stream to focus equivalence of final image, but not focus equivalence of data block sequence.
 There are some difference between input data and output data as below.
 
+- **Aperture definition [RS-274x]**<br>
+    When gerber data is rotated, it's necessory to rotate not only coordinates whilch indicate locations of drawing aperture, but also aperture geometory itself.
+    However, standard aperture templates, such as rectangle, cannot rotate. These standard aperture templates can be placed only horizontally or vertically.<br>
+    Threfore, pcb-tools-extension replace aperture definitions using standard aperture template to aperture macro that represent equivalent shape.<br>
+    For example, In case of rotating folowing aperture definition 20 degrees counter clockwise,
+
+    ```rs-274x
+    %ADD10R,1X0.5X0.2*%
+    ```
+
+    pcb-toolse-extension generate a aperture macro definition and a aperture definition referencing that macro as below.
+
+    ```rs-274x
+    %AMMACR*
+    21,1,$1,$2,0,0,20*
+    1,0,$3,0,0,20*%
+    %ADD10MACR,1X0.5X0.2*%
+    ```
+
 - **File Scope Modifier [RS-274x]**<br>
     Commands that affect entire image and should be specified only once in a file, such as ```MI``` (Mirror Image) command, sometimes cause contradiction when multiple gerber file are merged.<br>
     For example, when mergeing a file containing ```%MIA1B0*%``` command and a file containing ```%MIA0B1*``` command, which command should remain as output?
@@ -233,6 +252,6 @@ pcb-tools-extension extends excellon parser in [pcb-tools](https://github.com/cu
 
 - User defined stored pattern defined by ```M99``` command cannot be handled.
 - Canned text specified by ```M97``` command cannot be handled.
-- Patten defined by ```M25``` command cannot be handled.
+- Pattern defined by ```M25``` command cannot be handled.