From 96729d89611e9b7fbeb681162dcf6896050035de Mon Sep 17 00:00:00 2001
From: morkt <muvluv.ru@gmail.com>
Date: Mon, 22 Jan 2018 21:04:06 +0400
Subject: [PATCH] (DDS): added DXT3 decoder.

---
 ArcFormats/DirectDraw/DxtDecoder.cs | 44 +++++++++++++++++++++++++++++
 ArcFormats/DirectDraw/ImageDDS.cs   |  6 ++++
 2 files changed, 50 insertions(+)

diff --git a/ArcFormats/DirectDraw/DxtDecoder.cs b/ArcFormats/DirectDraw/DxtDecoder.cs
index 051cd338..1d8cd719 100644
--- a/ArcFormats/DirectDraw/DxtDecoder.cs
+++ b/ArcFormats/DirectDraw/DxtDecoder.cs
@@ -56,6 +56,18 @@ namespace GameRes.Formats.DirectDraw
             return m_output;
         }
 
+        public byte[] UnpackDXT3 ()
+        {
+            int src = 0;
+            for (int y = 0; y < m_height; y += 4)
+            for (int x = 0; x < m_width; x += 4)
+            {
+                DecompressDXT3Block (m_input, src, y, x);
+                src += 16;
+            }
+            return m_output;
+        }
+
         public byte[] UnpackDXT5 ()
         {
             int src = 0;
@@ -116,6 +128,38 @@ namespace GameRes.Formats.DirectDraw
             m_dxt_buffer[idx+3] = 0xFF;
         }
 
+        byte[] m_alpha_data = new byte[16];
+
+        void DecompressDXT3Block (byte[] input, int src, int block_y, int block_x)
+        {
+            int alpha_pos = 0;
+            for (int i = 0; i < 8; ++i)
+            {
+                byte a = input[src++];
+                m_alpha_data[alpha_pos++] = (byte)((a & 0xF) * 17);
+                m_alpha_data[alpha_pos++] = (byte)((a >> 4) * 17);
+            }
+            ReadDXT1Color (input, src, 0);
+            ReadDXT1Color (input, src+2, 4);
+            for (int i = 0; i < 4; ++i)
+            {
+                m_dxt_buffer[8+i]  = (byte)(((m_dxt_buffer[i] << 1) + m_dxt_buffer[4+i]) / 3);
+                m_dxt_buffer[12+i] = (byte)(((m_dxt_buffer[4+i] << 1) + m_dxt_buffer[i]) / 3);
+            }
+            uint map = LittleEndian.ToUInt32 (input, src+4);
+            for (int y = 0; y < 4 && (block_y + y) < m_height; ++y)
+            for (int x = 0; x < 4 && (block_x + x) < m_width; ++x)
+            {
+                int color = (int)(map & 3) << 2;
+                int dst = m_output_stride * (block_y + y) + (block_x + x) * 4;
+                m_output[dst]   = m_dxt_buffer[color];
+                m_output[dst+1] = m_dxt_buffer[color+1];
+                m_output[dst+2] = m_dxt_buffer[color+2];
+                m_output[dst+3] = m_alpha_data[y*4+x];
+                map >>= 2;
+            }
+        }
+
         void DecompressDXT5Block (byte[] input, int src, int block_y, int block_x)
         {
             byte alpha0 = input[src];
diff --git a/ArcFormats/DirectDraw/ImageDDS.cs b/ArcFormats/DirectDraw/ImageDDS.cs
index b4bfe717..32c61797 100644
--- a/ArcFormats/DirectDraw/ImageDDS.cs
+++ b/ArcFormats/DirectDraw/ImageDDS.cs
@@ -109,6 +109,12 @@ namespace GameRes.Formats.DirectDraw
                 var dxt = new DxtDecoder (input, meta);
                 pixels = dxt.UnpackDXT5();
             }
+            else if ("DXT3" == meta.FourCC)
+            {
+                var input = stream.ReadBytes ((int)meta.Width * (int)meta.Height);
+                var dxt = new DxtDecoder (input, meta);
+                pixels = dxt.UnpackDXT3();
+            }
             else if ("DXT1" == meta.FourCC)
             {
                 var input = stream.ReadBytes ((int)meta.Width * (int)meta.Height / 2);