commit d9922d2e965144682f617c687dac9003f5a24e92
author Boris Gjenero <boris.gjenero@gmail.com> Tue Sep 13 00:03:45 2016 -0700
committer Boris Gjenero <boris.gjenero@gmail.com> Tue Sep 13 00:03:45 2016 -0700
tree e7bb37ad166cd1ca5bce07bf280fc9676f8e738f
parent 43d3d98a2343d98de1f264a5bafa6104536f26a4

Accumulate subpixel mouse motion so motion is not lost.

Previously when the canvas was scaled up and the pointer was locked,
motion corresponding to less than one pixel was lost. Therefore,
slow mouse motion resulted in no motion. This fixes that.

src/video/emscripten/SDL_emscriptenevents.c

diff --git a/src/video/emscripten/SDL_emscriptenevents.c b/src/video/emscripten/SDL_emscriptenevents.c
index 77eb20e..883b599 100644
--- a/src/video/emscripten/SDL_emscriptenevents.c
+++ b/src/video/emscripten/SDL_emscriptenevents.c
@@ -301,25 +301,33 @@
 Emscripten_HandleMouseMove(int eventType, const EmscriptenMouseEvent *mouseEvent, void *userData)
 {
     SDL_WindowData *window_data = userData;
-    int mx = mouseEvent->canvasX, my = mouseEvent->canvasY;
+    int mx, my;
+    static double residualx = 0, residualy = 0;
     EmscriptenPointerlockChangeEvent pointerlock_status;
 
+    /* rescale (in case canvas is being scaled)*/
+    double client_w, client_h, xscale, yscale;
+    emscripten_get_element_css_size(NULL, &client_w, &client_h);
+    xscale = window_data->window->w / (client_w * window_data->pixel_ratio);
+    yscale = window_data->window->h / (client_h * window_data->pixel_ratio);
+
     /* check for pointer lock */
     int isPointerLockSupported = emscripten_get_pointerlock_status(&pointerlock_status);
     int isPointerLocked = isPointerLockSupported == EMSCRIPTEN_RESULT_SUCCESS  ? pointerlock_status.isActive : SDL_FALSE;
 
     if (isPointerLocked) {
-        mx = mouseEvent->movementX;
-        my = mouseEvent->movementY;
+        residualx += mouseEvent->movementX * xscale;
+        residualy += mouseEvent->movementY * yscale;
+        /* Let slow sub-pixel motion accumulate. Don't lose it. */
+        mx = residualx;
+        residualx -= mx;
+        my = residualy;
+        residualy -= my;
+    } else {
+        mx = mouseEvent->canvasX * xscale;
+        my = mouseEvent->canvasY * yscale;
     }
 
-    /* rescale (in case canvas is being scaled)*/
-    double client_w, client_h;
-    emscripten_get_element_css_size(NULL, &client_w, &client_h);
-
-    mx = mx * (window_data->window->w / (client_w * window_data->pixel_ratio));
-    my = my * (window_data->window->h / (client_h * window_data->pixel_ratio));
-
     SDL_SendMouseMotion(window_data->window, 0, isPointerLocked, mx, my);
     return 0;
 }