Flip and mirror raster when drawing with negative width or height wit…

…h the canvas graphics device (#370)

* Flip and mirror raster for negative width/height

* Update NEWS.md

NEWS.md

@@ -44,6 +44,8 @@
 
 * Fixed spelling errors in some JavaScript errors thrown by webR.
 
+* Rasters with negative width or height are now drawn mirrored or flipped when using the canvas graphics device (#350).
+
 * Include `cex` parameters when calculating font size in canvas graphics device (#348).
 
 # webR 0.2.2

packages/webr/src/canvas.c

@@ -530,9 +530,17 @@ void canvasRaster(unsigned int *raster, int w, int h,
  double rot,
  Rboolean interpolate,
  const pGEcontext gc, pDevDesc dd) {
+ int scale_x = 1, scale_y = -1;
+
+ y += height;
  if (height < 0) {
- y += height;
  height = -height;
+ scale_y = 1;
+ }
+
+ if (width < 0) {
+ width = -width;
+ scale_x = -1;
  }
 
  EM_ASM({Module.canvasCtx.save();});
@@ -554,8 +562,11 @@ void canvasRaster(unsigned int *raster, int w, int h,
  const img = new ImageData(new Uint8ClampedArray(raster), $1, $2);
  const buffer = new OffscreenCanvas($1, $2);
  buffer.getContext('2d').putImageData(img, 0, 0);
+ Module.canvasCtx.save();
+ Module.canvasCtx.scale($5, $6);
  Module.canvasCtx.drawImage(buffer, 0, 0, $3, $4);
- }, raster, w, h, 2*width, 2*height);
+ Module.canvasCtx.restore();
+ }, raster, w, h, 2*width, 2*height, scale_x, scale_y);
 
  EM_ASM({Module.canvasCtx.restore();});
 }