Fix png little endian alpha

Add a new helper routine _twin_apply_alpha() that handles
big and little endian details of alpha multiplication
for png to ARGB32.

Signed-off-by: Geoff Levand <geoff@infradead.org>

1 files changed, 26 insertions, 10 deletions

diff --git a/libtwin/twin_draw.c b/libtwin/twin_draw.c
index 3b19984..39af904 100644
--- a/libtwin/twin_draw.c
+++ b/libtwin/twin_draw.c
@@ -641,11 +641,34 @@ void twin_composite (twin_pixmap_t	*dst,
 			       operator, width, height);
 }
 
+static twin_argb32_t _twin_apply_alpha(twin_argb32_t v)
+{
+    uint16_t t1, t2, t3;
+    twin_a8_t alpha = twin_get_8(v, (__BYTE_ORDER == __BIG_ENDIAN) ? 24 : 0);
+
+    /* clear RGB data if alpha is zero */
+
+    if (!alpha)
+	return 0;
+
+    /* twin needs ARGB format */
+
+    if (__BYTE_ORDER == __BIG_ENDIAN)
+	return alpha << 24 |
+	    twin_int_mult(twin_get_8(v, 16), alpha, t1) << 16 |
+	    twin_int_mult(twin_get_8(v, 8), alpha, t2) << 8 |
+	    twin_int_mult(twin_get_8(v, 0), alpha, t3) << 0;
+
+    return alpha << 24 |
+	twin_int_mult(twin_get_8(v, 8), alpha, t1) << 16 |
+	twin_int_mult(twin_get_8(v, 16), alpha, t2) << 8 |
+	twin_int_mult(twin_get_8(v, 24), alpha, t3) << 0;
+}
+
 void twin_premultiply_alpha(twin_pixmap_t *px)
 {
     int x, y;
     twin_pointer_t p;
-    uint16_t t1, t2, t3;
 
     if (px->format != TWIN_ARGB32)
 	return;
@@ -653,15 +676,8 @@ void twin_premultiply_alpha(twin_pixmap_t *px)
     for (y = 0; y < px->height; y++) {
 	p.b = px->p.b + y * px->stride;
 
-	for (x = 0; x < px->width; x++) {
-	    twin_argb32_t v = p.argb32[x];
-	    twin_a8_t a = twin_get_8(v, 24);
-
-	    p.argb32[x] = (v & 0xff000000) |
-		twin_int_mult(twin_get_8(v, 16), a, t1) << 16 |
-		twin_int_mult(twin_get_8(v, 8), a, t2) << 8 |
-		twin_int_mult(twin_get_8(v, 0), a, t3);
-	}
+	for (x = 0; x < px->width; x++)
+	    p.argb32[x] = _twin_apply_alpha(p.argb32[x]);
     }
 }