Fix some incorrect alpha blending; may help with #1155.

diff --git a/src/lib/image.cc b/src/lib/image.cc
index b85451fb32f11ef4101363f27544eba9961c04e4..827ece5fdc97e25fd1aeecbde31547db0827aa99 100644 (file)
--- a/src/lib/image.cc
+++ b/src/lib/image.cc
@@ -439,7 +439,7 @@ Image::make_transparent ()
 void
 Image::alpha_blend (shared_ptr<const Image> other, Position<int> position)
 {
-       /* We're blending RGBA images; first byte is blue, second byte is green, third byte blue, fourth byte alpha */
+       /* We're blending RGBA images; first byte is red, second byte is green, third byte blue, fourth byte alpha */
        DCPOMATIC_ASSERT (other->pixel_format() == AV_PIX_FMT_RGBA);
        int const other_bpp = 4;
 
@@ -464,6 +464,24 @@ Image::alpha_blend (shared_ptr<const Image> other, Position<int> position)
        {
                /* Going onto RGB24.  First byte is red, second green, third blue */
                int const this_bpp = 3;
+               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] + 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] = op[0] * alpha + tp[0] * (1 - alpha);
+                               tp[1] = op[1] * alpha + tp[1] * (1 - alpha);
+                               tp[2] = op[2] * alpha + tp[2] * (1 - alpha);
+
+                               tp += this_bpp;
+                               op += other_bpp;
+                       }
+               }
+               break;
+       }
+       case AV_PIX_FMT_BGRA:
+       {
+               int const this_bpp = 4;
                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] + start_tx * this_bpp;
                        uint8_t* op = other->data()[0] + oy * other->stride()[0];
@@ -472,6 +490,7 @@ Image::alpha_blend (shared_ptr<const Image> other, Position<int> position)
                                tp[0] = op[2] * alpha + tp[0] * (1 - alpha);
                                tp[1] = op[1] * alpha + tp[1] * (1 - alpha);
                                tp[2] = op[0] * alpha + tp[2] * (1 - alpha);
+                               tp[3] = op[3] * alpha + tp[3] * (1 - alpha);
 
                                tp += this_bpp;
                                op += other_bpp;
@@ -479,7 +498,6 @@ Image::alpha_blend (shared_ptr<const Image> other, Position<int> position)
                }
                break;
        }
-       case AV_PIX_FMT_BGRA:
        case AV_PIX_FMT_RGBA:
        {
                int const this_bpp = 4;
@@ -507,10 +525,10 @@ Image::alpha_blend (shared_ptr<const Image> other, Position<int> position)
                        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;
-                               /* Blend high bytes; the RGBA in op appears to be BGRA */
-                               tp[1] = op[2] * alpha + tp[1] * (1 - alpha);
+                               /* Blend high bytes */
+                               tp[1] = op[0] * alpha + tp[1] * (1 - alpha);
                                tp[3] = op[1] * alpha + tp[3] * (1 - alpha);
-                               tp[5] = op[0] * alpha + tp[5] * (1 - alpha);
+                               tp[5] = op[2] * alpha + tp[5] * (1 - alpha);
 
                                tp += this_bpp;
                                op += other_bpp;