diff --git a/apps/openstmap/custom.html b/apps/openstmap/custom.html
index 80ab29c56..6e79a6e9a 100644
--- a/apps/openstmap/custom.html
+++ b/apps/openstmap/custom.html
@@ -116,8 +116,33 @@ TODO:
         if (document.getElementById("3bit").checked) {
           options = {
             compression:false, output:"raw",
-            mode:"3bit", brightness:-64,
+            mode:"3bit",
           };
+          /* If in 3 bit mode, go through all the data beforehand and
+          turn the saturation up to maximum, so when thresholded it
+          works a lot better */
+          var imageData = ctx.getImageData(0,0,width,height);
+          var rgba = imageData.data;
+          var l = width*height*4;
+          for (var i=0;i<l;i+=4) {
+             var min = Math.min(rgba[i+0],rgba[i+1],rgba[i+2]);
+             var max = Math.max(rgba[i+0],rgba[i+1],rgba[i+2]);
+             var d = max-min;
+             if (max<120) { // black
+               rgba[i+0]=0;
+               rgba[i+1]=0;
+               rgba[i+2]=0;
+             } else if (min>240 || d<8) { // white or grey
+               rgba[i+0]=255;
+               rgba[i+1]=255;
+               rgba[i+2]=255;
+             } else { // another colour - use max saturation
+               rgba[i+0] = (rgba[i+0]-min) * 255 / d;
+               rgba[i+1] = (rgba[i+1]-min) * 255 / d;
+               rgba[i+2] = (rgba[i+2]-min) * 255 / d;
+             }
+          }
+          ctx.putImageData(imageData,0,0);
         }
         console.log("Compression options", options);
         var w = Math.round(width / TILESIZE);
@@ -131,7 +156,7 @@ TODO:
             options.width = TILESIZE;
             options.height = TILESIZE;
             var imgstr = imageconverter.RGBAtoString(rgba, options);
-            ctx.putImageData(imageData,x*TILESIZE, y*TILESIZE);
+            ctx.putImageData(imageData,x*TILESIZE, y*TILESIZE); // write preview
             /*var compress = 'require("heatshrink").decompress('
             if (!imgstr.startsWith(compress)) throw "Data in wrong format";
             imgstr = imgstr.slice(compress.length,-1);*/