Notes about LERPlerp-ops

2 files changed, 90 insertions, 44 deletions

diff --git a/lerp-notes b/lerp-notes
new file mode 100644
index 00000000..883c6e93
--- /dev/null
+++ b/lerp-notes
@@ -0,0 +1,42 @@
+PIXMAN_TYPE_A means the A channel is at the bottom of the pixel. Ie.,
+x4a4 is xxxxaaaa and not aaaaxxxx.
+
+PIXMAN_TYPE_W means the W channel is at the top of the pixel. Ie.,
+x4w4 would be meaningless. However, w8x24 means
+
+     wwwwwwwwxxxxxxxxxxxxxxxxxxxxxxxx
+
+
+The interpretation of TYPE_W formats is that they are shades of
+white. Basically, the alpha channel is replicated in the r, g, and b
+channels so that the result is white with an alpha of w.
+
+The interpretation for TYPE_A formats is that the rgb channels are
+0. Ie., TYPE_A pixels are black with an alpha of a.
+
+The TYPE_W interpretation is convenient because in conjunction with
+the LERP operators it means component alpha becomes completely
+regular. 
+
+LERP is always interpreted across all channels, so if you use a TYPE_W
+format, the result is that all channels are modulated with the alpha
+value. Component alpha then is just a matter of using an RGB mask for
+the LERP.
+
+We need to be careful with strength reducing. Basically, the OVER_LERP
+operator can be reduced from
+
+	 OVER_LERP with TYPE_W mask
+
+to
+
+	OVER with TYPE_A mask
+
+For regular OVER TYPE_A and TYPE_W masks are the same.
+
+However, OVER_LERP with TYPE_A mask is something entirely different
+that cannot be implemented in terms of regular OVER. (That one simply
+throws away the RGB channels of the operation).
+
+For completeness, we should probably specify what happens if you ask
+for component alpha with an A mask.
diff --git a/pixman/pixman.h b/pixman/pixman.h
index ec95fcc2..75969a2d 100644
--- a/pixman/pixman.h
+++ b/pixman/pixman.h
@@ -664,6 +664,7 @@ struct pixman_indexed
 #define PIXMAN_TYPE_YUY2	6
 #define PIXMAN_TYPE_YV12	7
 #define PIXMAN_TYPE_BGRA	8
+#define PIXMAN_TYPE_W		9
 
 #define PIXMAN_FORMAT_COLOR(f)				\
 	(PIXMAN_FORMAT_TYPE(f) == PIXMAN_TYPE_ARGB ||	\
@@ -672,67 +673,70 @@ struct pixman_indexed
 
 /* 32bpp formats */
 typedef enum {
-    PIXMAN_a8r8g8b8 =	 PIXMAN_FORMAT(32,PIXMAN_TYPE_ARGB,8,8,8,8),
-    PIXMAN_x8r8g8b8 =	 PIXMAN_FORMAT(32,PIXMAN_TYPE_ARGB,0,8,8,8),
-    PIXMAN_a8b8g8r8 =	 PIXMAN_FORMAT(32,PIXMAN_TYPE_ABGR,8,8,8,8),
-    PIXMAN_x8b8g8r8 =	 PIXMAN_FORMAT(32,PIXMAN_TYPE_ABGR,0,8,8,8),
-    PIXMAN_b8g8r8a8 =	 PIXMAN_FORMAT(32,PIXMAN_TYPE_BGRA,8,8,8,8),
-    PIXMAN_b8g8r8x8 =	 PIXMAN_FORMAT(32,PIXMAN_TYPE_BGRA,0,8,8,8),
-    PIXMAN_x2r10g10b10 = PIXMAN_FORMAT(32,PIXMAN_TYPE_ARGB,0,10,10,10),
-    PIXMAN_a2r10g10b10 = PIXMAN_FORMAT(32,PIXMAN_TYPE_ARGB,2,10,10,10),
-    PIXMAN_x2b10g10r10 = PIXMAN_FORMAT(32,PIXMAN_TYPE_ABGR,0,10,10,10),
-    PIXMAN_a2b10g10r10 = PIXMAN_FORMAT(32,PIXMAN_TYPE_ABGR,2,10,10,10),
+    PIXMAN_a8r8g8b8 =		PIXMAN_FORMAT(32,PIXMAN_TYPE_ARGB,8,8,8,8),
+    PIXMAN_x8r8g8b8 =		PIXMAN_FORMAT(32,PIXMAN_TYPE_ARGB,0,8,8,8),
+    PIXMAN_a8b8g8r8 =		PIXMAN_FORMAT(32,PIXMAN_TYPE_ABGR,8,8,8,8),
+    PIXMAN_x8b8g8r8 =		PIXMAN_FORMAT(32,PIXMAN_TYPE_ABGR,0,8,8,8),
+    PIXMAN_b8g8r8a8 =		PIXMAN_FORMAT(32,PIXMAN_TYPE_BGRA,8,8,8,8),
+    PIXMAN_b8g8r8x8 =		PIXMAN_FORMAT(32,PIXMAN_TYPE_BGRA,0,8,8,8),
+    PIXMAN_x2r10g10b10 =	PIXMAN_FORMAT(32,PIXMAN_TYPE_ARGB,0,10,10,10),
+    PIXMAN_a2r10g10b10 =	PIXMAN_FORMAT(32,PIXMAN_TYPE_ARGB,2,10,10,10),
+    PIXMAN_x2b10g10r10 =	PIXMAN_FORMAT(32,PIXMAN_TYPE_ABGR,0,10,10,10),
+    PIXMAN_a2b10g10r10 =	PIXMAN_FORMAT(32,PIXMAN_TYPE_ABGR,2,10,10,10),
+    PIXMAN_w8x24 =		PIXMAN_FORMAT(32,PIXMAN_TYPE_W,8,0,0,0),
 
 /* 24bpp formats */
-    PIXMAN_r8g8b8 =	 PIXMAN_FORMAT(24,PIXMAN_TYPE_ARGB,0,8,8,8),
-    PIXMAN_b8g8r8 =	 PIXMAN_FORMAT(24,PIXMAN_TYPE_ABGR,0,8,8,8),
+    PIXMAN_r8g8b8 =		PIXMAN_FORMAT(24,PIXMAN_TYPE_ARGB,0,8,8,8),
+    PIXMAN_b8g8r8 =		PIXMAN_FORMAT(24,PIXMAN_TYPE_ABGR,0,8,8,8),
 
 /* 16bpp formats */
-    PIXMAN_r5g6b5 =	 PIXMAN_FORMAT(16,PIXMAN_TYPE_ARGB,0,5,6,5),
-    PIXMAN_b5g6r5 =	 PIXMAN_FORMAT(16,PIXMAN_TYPE_ABGR,0,5,6,5),
-
-    PIXMAN_a1r5g5b5 =	 PIXMAN_FORMAT(16,PIXMAN_TYPE_ARGB,1,5,5,5),
-    PIXMAN_x1r5g5b5 =	 PIXMAN_FORMAT(16,PIXMAN_TYPE_ARGB,0,5,5,5),
-    PIXMAN_a1b5g5r5 =	 PIXMAN_FORMAT(16,PIXMAN_TYPE_ABGR,1,5,5,5),
-    PIXMAN_x1b5g5r5 =	 PIXMAN_FORMAT(16,PIXMAN_TYPE_ABGR,0,5,5,5),
-    PIXMAN_a4r4g4b4 =	 PIXMAN_FORMAT(16,PIXMAN_TYPE_ARGB,4,4,4,4),
-    PIXMAN_x4r4g4b4 =	 PIXMAN_FORMAT(16,PIXMAN_TYPE_ARGB,0,4,4,4),
-    PIXMAN_a4b4g4r4 =	 PIXMAN_FORMAT(16,PIXMAN_TYPE_ABGR,4,4,4,4),
-    PIXMAN_x4b4g4r4 =	 PIXMAN_FORMAT(16,PIXMAN_TYPE_ABGR,0,4,4,4),
+    PIXMAN_r5g6b5 =		PIXMAN_FORMAT(16,PIXMAN_TYPE_ARGB,0,5,6,5),
+    PIXMAN_b5g6r5 =		PIXMAN_FORMAT(16,PIXMAN_TYPE_ABGR,0,5,6,5),
+
+    PIXMAN_a1r5g5b5 =		PIXMAN_FORMAT(16,PIXMAN_TYPE_ARGB,1,5,5,5),
+    PIXMAN_x1r5g5b5 =		PIXMAN_FORMAT(16,PIXMAN_TYPE_ARGB,0,5,5,5),
+    PIXMAN_a1b5g5r5 =		PIXMAN_FORMAT(16,PIXMAN_TYPE_ABGR,1,5,5,5),
+    PIXMAN_x1b5g5r5 =		PIXMAN_FORMAT(16,PIXMAN_TYPE_ABGR,0,5,5,5),
+    PIXMAN_a4r4g4b4 =		PIXMAN_FORMAT(16,PIXMAN_TYPE_ARGB,4,4,4,4),
+    PIXMAN_x4r4g4b4 =		PIXMAN_FORMAT(16,PIXMAN_TYPE_ARGB,0,4,4,4),
+    PIXMAN_a4b4g4r4 =		PIXMAN_FORMAT(16,PIXMAN_TYPE_ABGR,4,4,4,4),
+    PIXMAN_x4b4g4r4 =		PIXMAN_FORMAT(16,PIXMAN_TYPE_ABGR,0,4,4,4),
 
 /* 8bpp formats */
-    PIXMAN_a8 =		 PIXMAN_FORMAT(8,PIXMAN_TYPE_A,8,0,0,0),
-    PIXMAN_r3g3b2 =	 PIXMAN_FORMAT(8,PIXMAN_TYPE_ARGB,0,3,3,2),
-    PIXMAN_b2g3r3 =	 PIXMAN_FORMAT(8,PIXMAN_TYPE_ABGR,0,3,3,2),
-    PIXMAN_a2r2g2b2 =	 PIXMAN_FORMAT(8,PIXMAN_TYPE_ARGB,2,2,2,2),
-    PIXMAN_a2b2g2r2 =	 PIXMAN_FORMAT(8,PIXMAN_TYPE_ABGR,2,2,2,2),
+    PIXMAN_a8 =			PIXMAN_FORMAT(8,PIXMAN_TYPE_A,8,0,0,0),
+    PIXMAN_w8 =			PIXMAN_FORMAT(8,PIXMAN_TYPE_W,8,0,0,0),
+    PIXMAN_r3g3b2 =		PIXMAN_FORMAT(8,PIXMAN_TYPE_ARGB,0,3,3,2),
+    PIXMAN_b2g3r3 =		PIXMAN_FORMAT(8,PIXMAN_TYPE_ABGR,0,3,3,2),
+    PIXMAN_a2r2g2b2 =		PIXMAN_FORMAT(8,PIXMAN_TYPE_ARGB,2,2,2,2),
+    PIXMAN_a2b2g2r2 =		PIXMAN_FORMAT(8,PIXMAN_TYPE_ABGR,2,2,2,2),
 
-    PIXMAN_c8 =		 PIXMAN_FORMAT(8,PIXMAN_TYPE_COLOR,0,0,0,0),
-    PIXMAN_g8 =		 PIXMAN_FORMAT(8,PIXMAN_TYPE_GRAY,0,0,0,0),
+    PIXMAN_c8 =			PIXMAN_FORMAT(8,PIXMAN_TYPE_COLOR,0,0,0,0),
+    PIXMAN_g8 =			PIXMAN_FORMAT(8,PIXMAN_TYPE_GRAY,0,0,0,0),
 
-    PIXMAN_x4a4 =	 PIXMAN_FORMAT(8,PIXMAN_TYPE_A,4,0,0,0),
+    PIXMAN_x4a4 =		PIXMAN_FORMAT(8,PIXMAN_TYPE_A,4,0,0,0),
 
-    PIXMAN_x4c4 =	 PIXMAN_FORMAT(8,PIXMAN_TYPE_COLOR,0,0,0,0),
-    PIXMAN_x4g4 =	 PIXMAN_FORMAT(8,PIXMAN_TYPE_GRAY,0,0,0,0),
+    PIXMAN_x4c4 =		PIXMAN_FORMAT(8,PIXMAN_TYPE_COLOR,0,0,0,0),
+    PIXMAN_x4g4 =		PIXMAN_FORMAT(8,PIXMAN_TYPE_GRAY,0,0,0,0),
 
 /* 4bpp formats */
-    PIXMAN_a4 =		 PIXMAN_FORMAT(4,PIXMAN_TYPE_A,4,0,0,0),
-    PIXMAN_r1g2b1 =	 PIXMAN_FORMAT(4,PIXMAN_TYPE_ARGB,0,1,2,1),
-    PIXMAN_b1g2r1 =	 PIXMAN_FORMAT(4,PIXMAN_TYPE_ABGR,0,1,2,1),
-    PIXMAN_a1r1g1b1 =	 PIXMAN_FORMAT(4,PIXMAN_TYPE_ARGB,1,1,1,1),
-    PIXMAN_a1b1g1r1 =	 PIXMAN_FORMAT(4,PIXMAN_TYPE_ABGR,1,1,1,1),
+    PIXMAN_a4 =			PIXMAN_FORMAT(4,PIXMAN_TYPE_A,4,0,0,0),
+    PIXMAN_r1g2b1 =		PIXMAN_FORMAT(4,PIXMAN_TYPE_ARGB,0,1,2,1),
+    PIXMAN_b1g2r1 =		PIXMAN_FORMAT(4,PIXMAN_TYPE_ABGR,0,1,2,1),
+    PIXMAN_a1r1g1b1 =		PIXMAN_FORMAT(4,PIXMAN_TYPE_ARGB,1,1,1,1),
+    PIXMAN_a1b1g1r1 =		PIXMAN_FORMAT(4,PIXMAN_TYPE_ABGR,1,1,1,1),
 
-    PIXMAN_c4 =		 PIXMAN_FORMAT(4,PIXMAN_TYPE_COLOR,0,0,0,0),
-    PIXMAN_g4 =		 PIXMAN_FORMAT(4,PIXMAN_TYPE_GRAY,0,0,0,0),
+    PIXMAN_c4 =			PIXMAN_FORMAT(4,PIXMAN_TYPE_COLOR,0,0,0,0),
+    PIXMAN_g4 =			PIXMAN_FORMAT(4,PIXMAN_TYPE_GRAY,0,0,0,0),
 
 /* 1bpp formats */
-    PIXMAN_a1 =		 PIXMAN_FORMAT(1,PIXMAN_TYPE_A,1,0,0,0),
+    PIXMAN_a1 =			PIXMAN_FORMAT(1,PIXMAN_TYPE_A,1,0,0,0),
+    PIXMAN_w1 =			PIXMAN_FORMAT(1,PIXMAN_TYPE_W,1,0,0,0),
 
-    PIXMAN_g1 =		 PIXMAN_FORMAT(1,PIXMAN_TYPE_GRAY,0,0,0,0),
+    PIXMAN_g1 =			PIXMAN_FORMAT(1,PIXMAN_TYPE_GRAY,0,0,0,0),
 
 /* YUV formats */
-    PIXMAN_yuy2 =	 PIXMAN_FORMAT(16,PIXMAN_TYPE_YUY2,0,0,0,0),
-    PIXMAN_yv12 =	 PIXMAN_FORMAT(12,PIXMAN_TYPE_YV12,0,0,0,0)
+    PIXMAN_yuy2 =		PIXMAN_FORMAT(16,PIXMAN_TYPE_YUY2,0,0,0,0),
+    PIXMAN_yv12 =		PIXMAN_FORMAT(12,PIXMAN_TYPE_YV12,0,0,0,0)
 } pixman_format_code_t;
 
 /* Querying supported format values. */