compositor: Fix blending functions

Correctly calculate alpha in a few places by dividing by 255,
not 256.

Fix the argb and bgra blending functions to avoid an off-by-one
error in the calculations, so painting with alpha = 0xff doesn't
ever bleed through from behind

diff --git a/gst/compositor/compositororc.orc b/gst/compositor/compositororc.orc
index 5a348b25391daceb38838965450a4a4a2cd1745a..a9368b7343e097646fb6cc360f900d5fe5ca44c2 100644 (file)
--- a/gst/compositor/compositororc.orc
+++ b/gst/compositor/compositororc.orc
@@ -35,6 +35,7 @@ convsuswb d1, t2
 .source 4 s guint8
 .param 2 alpha
 .temp 4 t
+.temp 4 t2
 .temp 2 tw
 .temp 1 tb
 .temp 4 a
@@ -49,14 +50,26 @@ convwb tb, tw
 splatbl a, tb
 x4 convubw a_wide, a
 x4 mullw a_wide, a_wide, alpha
-x4 shruw a_wide, a_wide, 8
+x4 div255w a_wide, a_wide
+
+# dest pixel into t2
+loadl t2, d
+
+# t = s - d (as bytes)
+x4 subb t, t, t2
+
+# s_wide = (uint16)(t) * alpha
 x4 convubw s_wide, t
-loadl t, d
-x4 convubw d_wide, t
-x4 subw s_wide, s_wide, d_wide
 x4 mullw s_wide, s_wide, a_wide
+
+# s_wide /= 255
 x4 div255w s_wide, s_wide
+
+# d_wide = (uint16)(dest) + s_wide
+x4 convubw d_wide, t2
 x4 addw d_wide, d_wide, s_wide
+
+# Set output alpha to 0xff and store
 x4 convwb t, d_wide
 orl t, t, a_alpha
 storel d, t
@@ -83,13 +96,17 @@ convwb tb, tw
 splatbl a, tb
 x4 convubw a_wide, a
 x4 mullw a_wide, a_wide, alpha
-x4 shruw a_wide, a_wide, 8
+x4 div255w a_wide, a_wide
+
+loadl t2, d
+# t = s - d (as bytes)
+x4 subb t, t, t2
+
 x4 convubw s_wide, t
-loadl t, d
-x4 convubw d_wide, t
-x4 subw s_wide, s_wide, d_wide
 x4 mullw s_wide, s_wide, a_wide
 x4 div255w s_wide, s_wide
+
+x4 convubw d_wide, t2
 x4 addw d_wide, d_wide, s_wide
 x4 convwb t, d_wide
 orl t, t, a_alpha
@@ -114,14 +131,14 @@ storel d, t
 .const 4 a_alpha 0x000000ff
 .const 4 a_alpha_inv 0xffffff00
 
-# calc source alpha as alpha_s = alpha_s * alpha / 256
+# calc source alpha as alpha_s = alpha_s * alpha / 255
 loadl t, s
 convlw tw, t
 convwb tb, tw
 splatbl a, tb
 x4 convubw alpha_s, a
 x4 mullw alpha_s, alpha_s, alpha
-x4 shruw alpha_s, alpha_s, 8
+x4 div255w alpha_s, alpha_s
 x4 convubw s_wide, t
 x4 mullw s_wide, s_wide, alpha_s
 
@@ -175,7 +192,7 @@ storel d, t
 .const 4 a_alpha 0xff000000
 .const 4 a_alpha_inv 0x00ffffff
 
-# calc source alpha as alpha_s = alpha_s * alpha / 256
+# calc source alpha as alpha_s = alpha_s * alpha / 255
 loadl t, s
 shrul t2, t, 24
 convlw tw, t2
@@ -183,7 +200,7 @@ convwb tb, tw
 splatbl a, tb
 x4 convubw alpha_s, a
 x4 mullw alpha_s, alpha_s, alpha
-x4 shruw alpha_s, alpha_s, 8
+x4 div255w alpha_s, alpha_s
 x4 convubw s_wide, t
 x4 mullw s_wide, s_wide, alpha_s