gis

docs/.vitepress/config.js

@@ -14,6 +14,7 @@ export default {
             {
                 text: '知识', items: [
                     { text: "opencv", link: '/chapter/tech/opencv' },
+                    { text: "gis", link: '/chapter/tech/gis' },
                     { text: "线性代数", link: '/chapter/math/linearAlgebra' },
                 ]
             },

docs/chapter/tech/gis.md

@@ -0,0 +1,30 @@
+# gis
+## postgis
+### sql
+将geometry类型的字段直接转换为svg图片，以显示轮廓。  
+核心做法:
+1. 按最小的x、y坐标负向平移，以实现初始坐标为0
+2. 取宽和高中的较大值来进行放大
+3. 对svg的viewBox进行平移，以实现轮廓完全接边展示
+
+```sql 
+SELECT ST_Scale(ST_Translate(geom, -1 * ST_XMin(geom), -1 * ST_YMin(geom))
+	, CEIL(GREATEST((ST_XMax(geom)-ST_YMin(geom)),(ST_YMax(geom)-ST_XMin(geom))))
+	, CEIL(GREATEST((ST_XMax(geom)-ST_YMin(geom)),(ST_YMax(geom)-ST_XMin(geom))))) AS scale_geom
+	, CEIL(GREATEST((ST_XMax(geom)-ST_YMin(geom)),(ST_YMax(geom)-ST_XMin(geom)))) AS sc
+FROM region
+WHERE geom IS NOT NULL
+LIMIT 10
+),
+p AS (
+SELECT ST_AsSvg(scale_geom,0,4) AS svg
+,sc,ST_XMin(scale_geom) AS x_min
+, ST_YMin(scale_geom) AS y_min
+, ST_XMax(scale_geom) AS x_max
+, ST_YMax(scale_geom) AS y_max
+FROM q
+)
+SELECT CONCAT('<svg width="300" height="300" version="1.1" xmlns="http://www.w3.org/2000/svg" viewBox="',
+            CONCAT_WS(' ', 0, CEIL(-1*y_max), CEIL(( x_max-x_min)), CEIL(( y_max-y_min)) ), '"> <path d="', svg, '"></path></svg>') AS "svg"
+FROM p 
+```