From 62f0b9fc6779d1dd33c0adff776542bf56412a91 Mon Sep 17 00:00:00 2001
From: Carl Hetherington <cth@carlh.net>
Date: Fri, 13 Jun 2014 12:01:39 +0100
Subject: [PATCH] Various fixes to alpha blending for pre-multipled alpha (as
 Cairo uses).  Fix merge() to work with non-zero positions.  Optimise merge()
 to return a single image straight away if only one is passed in.

---
 src/lib/image.cc | 33 +++++++++++++++++++++------------
 1 file changed, 21 insertions(+), 12 deletions(-)

diff --git a/src/lib/image.cc b/src/lib/image.cc
index 8e7a51fd8..2ce3738b9 100644
--- a/src/lib/image.cc
+++ b/src/lib/image.cc
@@ -359,16 +359,19 @@ Image::make_transparent ()
 void
 Image::alpha_blend (shared_ptr<const Image> other, Position<int> position)
 {
-	int this_bpp = 0;
-	int other_bpp = 0;
+	assert (other->pixel_format() == PIX_FMT_RGBA);
+	int const other_bpp = 4;
 
-	if (_pixel_format == PIX_FMT_BGRA && other->pixel_format() == PIX_FMT_RGBA) {
+	int this_bpp = 0;
+	switch (_pixel_format) {
+	case PIX_FMT_BGRA:
+	case PIX_FMT_RGBA:
 		this_bpp = 4;
-		other_bpp = 4;
-	} else if (_pixel_format == PIX_FMT_RGB24 && other->pixel_format() == PIX_FMT_RGBA) {
+		break;
+	case PIX_FMT_RGB24:
 		this_bpp = 3;
-		other_bpp = 4;
-	} else {
+		break;
+	default:
 		assert (false);
 	}
 
@@ -389,13 +392,15 @@ Image::alpha_blend (shared_ptr<const Image> other, Position<int> position)
 	}
 
 	for (int ty = start_ty, oy = start_oy; ty < size().height && oy < other->size().height; ++ty, ++oy) {
-		uint8_t* tp = data()[0] + ty * stride()[0] + position.x * this_bpp;
+		uint8_t* tp = data()[0] + ty * stride()[0] + start_tx * this_bpp;
 		uint8_t* op = other->data()[0] + oy * other->stride()[0];
 		for (int tx = start_tx, ox = start_ox; tx < size().width && ox < other->size().width; ++tx, ++ox) {
 			float const alpha = float (op[3]) / 255;
-			tp[0] = (tp[0] * (1 - alpha)) + op[0] * alpha;
-			tp[1] = (tp[1] * (1 - alpha)) + op[1] * alpha;
-			tp[2] = (tp[2] * (1 - alpha)) + op[2] * alpha;
+			tp[0] = op[0] + (tp[0] * (1 - alpha));
+			tp[1] = op[1] + (tp[1] * (1 - alpha));
+			tp[2] = op[2] + (tp[2] * (1 - alpha));
+			tp[3] = op[3] + (tp[3] * (1 - alpha));
+			
 			tp += this_bpp;
 			op += other_bpp;
 		}
@@ -656,6 +661,10 @@ merge (list<PositionImage> images)
 		return PositionImage ();
 	}
 
+	if (images.size() == 1) {
+		return images.front ();
+	}
+
 	dcpomatic::Rect<int> all (images.front().position, images.front().image->size().width, images.front().image->size().height);
 	for (list<PositionImage>::const_iterator i = images.begin(); i != images.end(); ++i) {
 		all.extend (dcpomatic::Rect<int> (i->position, i->image->size().width, i->image->size().height));
@@ -664,7 +673,7 @@ merge (list<PositionImage> images)
 	shared_ptr<Image> merged (new Image (images.front().image->pixel_format (), dcp::Size (all.width, all.height), true));
 	merged->make_transparent ();
 	for (list<PositionImage>::const_iterator i = images.begin(); i != images.end(); ++i) {
-		merged->alpha_blend (i->image, i->position);
+		merged->alpha_blend (i->image, i->position - all.position());
 	}
 
 	return PositionImage (merged, all.position ());
-- 
2.30.2