From 99ff99a8f42e7f3d1eefb6f260f88f16f7665d61 Mon Sep 17 00:00:00 2001
From: "Sascha L. Teichmann" <sascha.teichmann@intevation.de>
Date: Sun, 26 Oct 2014 11:01:36 +0100
Subject: [PATCH] Added cascading transparent annuation algorithm based on
 spans.

---
 common/colors.go | 41 +++++++++++++++++++++++++++++++++++++----
 1 file changed, 37 insertions(+), 4 deletions(-)

diff --git a/common/colors.go b/common/colors.go
index caeb984..eaf1b3f 100644
--- a/common/colors.go
+++ b/common/colors.go
@@ -24,10 +24,6 @@ type namedColor struct {
 	color color.RGBA
 }
 
-func (colors *Colors) IsTransparent(index int32) bool {
-	return index < colors.NumTransparent
-}
-
 type sortByAlpha []namedColor
 
 func (colors sortByAlpha) Less(i, j int) bool {
@@ -88,3 +84,40 @@ func ParseColors(filename string) (colors *Colors, err error) {
 		NumTransparent: numTransparent}
 	return
 }
+
+func (colors *Colors) IsTransparent(index int32) bool {
+	return index < colors.NumTransparent
+}
+
+func min(a, b int32) int32 {
+	if a < b {
+		return a
+	}
+	return b
+}
+
+func BlendColor(c1, c2 color.RGBA, a float32) color.RGBA {
+	b := float32(1) - a
+	return color.RGBA{
+		R: uint8(float32(c1.R)*a + float32(c2.R)*b),
+		G: uint8(float32(c1.G)*a + float32(c2.G)*b),
+		B: uint8(float32(c1.B)*a + float32(c2.B)*b),
+		A: 0xff}
+}
+
+func (colors *Colors) BlendColors(span *Span, col color.RGBA, pos int32) color.RGBA {
+	curr := span
+	// Ignore colors below pos.
+	for ; curr != nil && pos >= curr.To; curr = curr.Next {
+	}
+	if curr == nil {
+		return col
+	}
+	const scale = float32(1) / float32(100)
+	for ; curr != nil; curr = curr.Next {
+		// At least 20% attenuation + 5% extra for each depth meter.
+		factor := float32(min(100, 20+(curr.To-curr.From)*5)) * scale
+		col = BlendColor(colors.Colors[curr.Value], col, factor)
+	}
+	return col
+}