Fix comment and alpha test node oopsies from #14852

games/devtest/mods/testnodes/textures.lua

@@ -89,10 +89,12 @@ for a=1,#alphas do
 end
 
 minetest.register_node("testnodes:alpha_compositing", {
-	description = S("Alpha Compositing Test Node") .. "\n" ..
+	description = S("Texture Overlay Test Node") .. "\n" ..
 		S("A regular grid should be visible where each cell contains two " ..
-		"texels with the same colour.") .. "\n" ..
-		S("Alpha compositing is gamma-incorrect for backwards compatibility."),
+		"texels with the same color.") .. "\n" ..
+		S("Texture overlay is gamma-incorrect, " ..
+		"and in general it does not do alpha compositing, " ..
+		"both for backwards compatibility."),
 	drawtype = "glasslike",
 	paramtype = "light",
 	tiles = {"testnodes_alpha_compositing_bottom.png^"  ..

src/client/imagesource.cpp

@@ -371,14 +371,12 @@ void blitBaseImage(video::IImage* &src, video::IImage* &dst)
 
 namespace {
 
-/** Calculate the color of a single pixel drawn on top of another pixel without
- * gamma correction
+/** Calculate the result of the overlay texture modifier (`^`) for a single
+ * pixel
  *
- * The color mixing is a little more complicated than just
- * video::SColor::getInterpolated because getInterpolated does not handle alpha
- * correctly.
- * For example, a pixel with alpha=64 drawn atop a pixel with alpha=128 should
- * yield a pixel with alpha=160, while getInterpolated would yield alpha=96.
+ * This is not alpha blending if both src and dst are semi-transparent. The
+ * reason this is that an old implementation did it wrong, and fixing it would
+ * break backwards compatibility (see #14847).
  *
  * \tparam overlay If enabled, only modify dst_col if it is fully opaque
  * \param src_col Color of the top pixel