diff --git a/.gitignore b/.gitignore
index 89320d5..b471a7b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -17,4 +17,6 @@ doc
 *.dat
 *.ind
 *.tab
-*.json
\ No newline at end of file
+*.json
+
+*.bmp
\ No newline at end of file
diff --git a/src/common/dxt5.h b/src/common/dxt5.h
index c4b77fe..9e43183 100644
--- a/src/common/dxt5.h
+++ b/src/common/dxt5.h
@@ -24,6 +24,10 @@
 #include "color.h"
 #include "mat.h"
 
+// https://en.wikipedia.org/wiki/S3_Texture_Compression
+// decompressor https://github.com/Benjamin-Dobell/s3tc-dxt-decompression
+// compressor https://github.com/nothings/stb/blob/master/stb_dxt.h
+// or modified https://github.com/Cyan4973/RygsDXTc/blob/master/stb_dxt.h
 struct dxt5_block
 {
     union
@@ -99,7 +103,7 @@ struct dxt5_block
             pixels[p].a = a[(alpha_part >> (16 + p * 3)) & 0b111];
         }
     }
-    color interpolate(color c0, color c1, float m)
+    static color interpolate(color c0, color c1, float m)
     {
         color r;
         for (int i = 0; i < 4; i++)