IRSA-2894: orient N-E correctly in thumbnail view

Author: stargaser

src/firefly/js/visualize/ui/ThumbnailView.jsx

@@ -223,11 +223,21 @@ function makeDrawing(pv,width,height) {
     //const wptE2= makeWorldPt(wptC.getLon()+(Math.abs(cdelt1)/thumbZoomFact)*(arrowLength/2), wptC.getLat());
     const wptE1= wptC;
     const wpt1= wptC;
+    const wpt3= makeWorldPt(wptC.getLon() +
+                            (Math.abs(cdelt1)/(Math.cos(wptC.getLat()*Math.PI/180.)/thumbZoomFact)*(arrowLength/1.6)),
+                            wptC.getLat());
+
 
     const sptC = cc.getScreenCoords(wptC);
     const sptN = cc.getScreenCoords(wpt2);
     if (!sptC || !sptN) return null;
-    const [x, y] = [(sptN.y - sptC.y) + sptC.x, (-sptN.x + sptC.x)+sptC.y];
+    const spt3= cc.getScreenCoords(wpt3);
+    // don't use spt3 because of funny effects near the celestial poles
+    // the sign of the cross product of compass vectors tells us if the image is mirror-reversed from the sky
+    const cross_product= (spt3.x - sptC.x)*(sptN.y - sptC.y) -
+                       (spt3.y - sptC.y)*(sptN.x - sptC.x);
+    const [x, y] = [Math.sign(cross_product)*(sptN.y - sptC.y) + sptC.x,
+                    Math.sign(cross_product)*(-sptN.x + sptC.x)+sptC.y];
     const wptE2 = cc.getWorldCoords(makeScreenPt(x, y));
 
     const spt1= cc.getDeviceCoords(wpt1, thumbZoomFact, thumbnailTrans);