Fix slightly inexplicable RGB/BGR confusion; before this the colour of subtitles...

diff --git a/src/lib/image.cc b/src/lib/image.cc
index a2b3bf519062b9dc656a22a9e1087f5550d5aec7..2a413c83efa118375d73ff179e33e67e549115c5 100644 (file)
--- a/src/lib/image.cc
+++ b/src/lib/image.cc
@@ -426,6 +426,7 @@ 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 */
        DCPOMATIC_ASSERT (other->pixel_format() == AV_PIX_FMT_RGBA);
        int const other_bpp = 4;
 
@@ -448,15 +449,16 @@ Image::alpha_blend (shared_ptr<const Image> other, Position<int> position)
        switch (_pixel_format) {
        case AV_PIX_FMT_RGB24:
        {
+               /* 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[0] = op[2] * alpha + tp[0] * (1 - alpha);
                                tp[1] = op[1] * alpha + tp[1] * (1 - alpha);
-                               tp[2] = op[2] * alpha + tp[2] * (1 - alpha);
+                               tp[2] = op[0] * alpha + tp[2] * (1 - alpha);
 
                                tp += this_bpp;
                                op += other_bpp;