Fix for half pixel offset rasterizing masks

Referenced by issue #39750, Bones disappearing with strange transforms

source/blender/blenkernel/intern/mask_rasterize.c

@@ -1434,6 +1434,10 @@ void BKE_maskrasterize_buffer(MaskRasterHandle *mr_handle,
                               const unsigned int width, const unsigned int height,
                               float *buffer)
 {
+	const float x_inv = 1.0f / (float)width;
+	const float y_inv = 1.0f / (float)height;
+	const float x_px_ofs = x_inv * 0.5f;
+	const float y_px_ofs = y_inv * 0.5f;
 #ifdef _MSC_VER
 	int y;  /* msvc requires signed for some reason */
 
@@ -1449,9 +1453,9 @@ void BKE_maskrasterize_buffer(MaskRasterHandle *mr_handle,
 		unsigned int i = y * width;
 		unsigned int x;
 		float xy[2];
-		xy[1] = (float)y / (float)height;
+		xy[1] = ((float)y * y_inv) + y_px_ofs;
 		for (x = 0; x < width; x++, i++) {
-			xy[0] = (float)x / (float)width;
+			xy[0] = ((float)x * x_inv) + x_px_ofs;
 
 			buffer[i] = BKE_maskrasterize_handle_sample(mr_handle, xy);
 		}

source/blender/compositor/operations/COM_MaskOperation.cpp

@@ -129,8 +129,9 @@ void MaskOperation::determineResolution(unsigned int resolution[2], unsigned int
 
 void MaskOperation::executePixelSampled(float output[4], float x, float y, PixelSampler sampler)
 {
-	const float xy[2] = {x * this->m_maskWidthInv,
-	                     y * this->m_maskHeightInv};
+	const float xy[2] = {
+	    (x * this->m_maskWidthInv)  + this->m_mask_px_ofs[0],
+	    (y * this->m_maskHeightInv) + this->m_mask_px_ofs[1]};
 
 	if (this->m_rasterMaskHandleTot == 1) {
 		if (this->m_rasterMaskHandles[0]) {

source/blender/compositor/operations/COM_MaskOperation.h

@@ -43,6 +43,7 @@ protected:
 	int   m_maskHeight;
 	float m_maskWidthInv;  /* 1 / m_maskWidth  */
 	float m_maskHeightInv; /* 1 / m_maskHeight */
+	float m_mask_px_ofs[2];
 
 	float m_frame_shutter;
 	int   m_frame_number;
@@ -70,11 +71,13 @@ public:
 	{
 		this->m_maskWidth    = width;
 		this->m_maskWidthInv = 1.0f / (float)width;
+		this->m_mask_px_ofs[0] = this->m_maskWidthInv * 0.5f;
 	}
 	void setMaskHeight(int height)
 	{
 		this->m_maskHeight = height;
 		this->m_maskHeightInv = 1.0f / (float)height;
+		this->m_mask_px_ofs[1] = this->m_maskHeightInv * 0.5f;
 	}
 	void setFramenumber(int frame_number) { this->m_frame_number = frame_number; }
 	void setSmooth(bool smooth) { this->m_do_smooth = smooth; }

source/blender/editors/mask/mask_draw.c

@@ -643,15 +643,21 @@ static void mask_rasterize_func(TaskPool *pool, void *taskdata, int UNUSED(threa
 	ThreadedMaskRasterizeState *state = (ThreadedMaskRasterizeState *) BLI_task_pool_userdata(pool);
 	ThreadedMaskRasterizeData *data = (ThreadedMaskRasterizeData *) taskdata;
 	int scanline;
+	const float x_inv = 1.0f / (float)state->width;
+	const float y_inv = 1.0f / (float)state->height;
+	const float x_px_ofs = x_inv * 0.5f;
+	const float y_px_ofs = y_inv * 0.5f;
 
 	for (scanline = 0; scanline < data->num_scanlines; scanline++) {
+		float xy[2];
 		int x, y = data->start_scanline + scanline;
+
+		xy[1] = ((float)y * y_inv) + y_px_ofs;
+
 		for (x = 0; x < state->width; x++) {
 			int index = y * state->width + x;
-			float xy[2];
 
-			xy[0] = (float) x / state->width;
-			xy[1] = (float) y / state->height;
+			xy[0] = ((float)x * x_inv) + x_px_ofs;
 
 			state->buffer[index] = BKE_maskrasterize_handle_sample(state->handle, xy);
 		}