eeshow/: fix offset calculation and also get rid of rounding artefacts

eeshow/cro.c

@@ -43,7 +43,8 @@
 struct cro_ctx {
 	struct record record;	/* must be first */
 
-	int xo, yo;
+	int xo, yo;		/* offset in target (e.g., canvas) coord */
+	int xe, ye;		/* additional offset in eeschema coord */
 	float scale;
 
 	cairo_t *cr;
@@ -66,19 +67,19 @@ static inline int cd(const struct cro_ctx *cc, int x)
 
 static inline int cx(const struct cro_ctx *cc, int x)
 {
-	return cc->xo + x * cc->scale;
+	return cc->xo + (x + cc->xe) * cc->scale;
 }
 
 
 static inline int xc(const struct cro_ctx *cc, int x)
 {
-	return (x - cc->xo) / cc->scale;
+	return (x - cc->xe) / cc->scale - cc->xe;
 }
 
 
 static inline int cy(const struct cro_ctx *cc, int y)
 {
-	return cc->yo + y * cc->scale;
+	return cc->yo + (y + cc->ye) * cc->scale;
 }
 
 
@@ -261,6 +262,7 @@ static struct cro_ctx *new_cc(void)
 
 	cc = alloc_type(struct cro_ctx);
 	cc->xo = cc->yo = 0;
+	cc->xe = cc->ye = 0;
 	cc->scale = DEFAULT_SCALE;
 
 	cc->sheets = NULL;
@@ -532,6 +534,7 @@ void cro_canvas_draw(struct cro_ctx *cc, cairo_t *cr, int xo, int yo,
 
 
 uint32_t *cro_img(struct cro_ctx *ctx, int xo, int yo, int w, int h,
+    int xe, int ye,
     float scale, cairo_t **res_cr, int *res_stride)
 {
 	int stride;
@@ -558,6 +561,8 @@ uint32_t *cro_img(struct cro_ctx *ctx, int xo, int yo, int w, int h,
 	ctx->s = s;
 	ctx->xo = xo;
 	ctx->yo = yo;
+	ctx->xe = xe;
+	ctx->ye = ye;
 	ctx->scale = scale;
 	ctx->color_override = COLOR_NONE;
 

eeshow/cro.h

@@ -44,6 +44,7 @@ void cro_canvas_draw(struct cro_ctx *cc, cairo_t *cr,
     int x, int y, float scale);
 
 uint32_t *cro_img(struct cro_ctx *ctx, int x0, int yo, int w, int h,
+    int xe, int ye,
     float scale, cairo_t **res_cr, int *res_stride);
 
 #endif /* !CRO_H */

eeshow/diff.c

@@ -337,19 +337,29 @@ void diff_to_canvas(cairo_t *cr, int cx, int cy, float scale,
 	merge_coord(old_ymin, new_ymin, old_h, new_h, &ymin, &h);
 
 	/*
-	 * @@@ we should handle offset differences (xmin - old_xmin, et al.)
+	 * We use two sets of offset differences: one applied to eeschema
-	 * before scaling. Else we risk rounding differences to lead to
+	 * coordinates (the xe, ye pair), and one applied to canvas coordinates
-	 * single-pixel differences, which the differences algorithm will
+	 * (the xo, yo pair).
-	 * be eager to mark with big yellow boxes.
+	 *
+	 * This is to prevent different offsets from leading to rounding
+	 * differences, resulting in single-pixel differences, which in turn
+	 * the differences algorithm will be eager to mark with big yellow
+	 * boxes.
 	 */
 	img_old = cro_img(old,
-	    sw / 2.0 - (cx + 2 * xmin - old_xmin) * scale,
+	    sw / 2.0 - (cx + xmin) * scale,
-	    sh / 2.0 - (cy + 2 * ymin - old_ymin) * scale,
+	    sh / 2.0 - (cy + ymin) * scale,
-	    sw, sh, scale, &old_cr, &stride);
+	    sw, sh,
+	    xmin - old_xmin,
+	    ymin - old_ymin,
+	    scale, &old_cr, &stride);
 	img_new = cro_img(new,
-	    sw / 2.0 - (cx + 2 * xmin - new_xmin) * scale,
+	    sw / 2.0 - (cx + xmin) * scale,
-	    sh / 2.0 - (cy + 2 * ymin - new_ymin) * scale,
+	    sh / 2.0 - (cy + ymin) * scale,
-	    sw, sh, scale, NULL, NULL);
+	    sw, sh,
+	    xmin - new_xmin,
+	    ymin - new_ymin,
+	    scale, NULL, NULL);
 
 	struct diff diff = {
 		.w		= sw,