plug-ins: issue #5556 Interpret transparency as black on DDS DXT1 textures

This adds an import option for DDS DXT1/BC1 images to always use
transparency. This is the default behavior, since this was what always
happened until now and it seems that DDPF_ALPHAPIXELS is very rarely
set for these type of images.

However, as the mentioned issues explains, advanced compression
algorithms can use this transparency data instead to mean a black
pixel. There is however, no certain way to determine this.

For that reason, we add an option here, that, if disabled, will
interpret fully transparent values as a black pixel.

plug-ins/file-dds/dds.c

@@ -176,6 +176,14 @@ dds_create_procedure (GimpPlugIn  *plug_in,
                              FALSE,
                              G_PARAM_READWRITE);
 
+      GIMP_PROC_ARG_BOOLEAN (procedure, "bc1-use-transparency",
+                             _("Always use _transparency for BC1/DXT1"),
+                             _("Interprets transparency for BC1/DXT1 images even when "
+                               "DDPF_ALPHAPIXELS is not set; if disabled, sets it "
+                               "to opaque black"),
+                             TRUE,
+                             G_PARAM_READWRITE);
+
       GIMP_PROC_ARG_BOOLEAN (procedure, "decode-images",
                              _("Automatically decode YCoCg/AE_xp images when detected"),
                              _("Decode YCoCg/AExp images when detected"),

plug-ins/file-dds/dds.h

@@ -51,6 +51,7 @@ typedef enum
   DDS_COMPRESS_AEXP,       /* DXT5  */
   DDS_COMPRESS_YCOCG,      /* DXT5  */
   DDS_COMPRESS_YCOCGS,     /* DXT5  */
+  _DDS_COMPRESS_BC1_NO_ALPHA,  /* For internal use only. */
   DDS_COMPRESS_MAX
 } DDS_COMPRESSION_TYPE;
 

plug-ins/file-dds/ddsread.c

@@ -139,6 +139,7 @@ read_dds (GFile          *file,
   gint               i, j;
   guint              computed_pitch_or_linsize;
   gboolean           flip_import;
+  gboolean           bc1_use_transparency;
 
   if (interactive)
     {
@@ -151,6 +152,7 @@ read_dds (GFile          *file,
   g_object_get (config,
                 "load-mipmaps",         &read_mipmaps,
                 "flip-image",           &flip_import,
+                "bc1-use-transparency", &bc1_use_transparency,
                 "decode-images",        &decode_images,
                 NULL);
 
@@ -307,7 +309,8 @@ read_dds (GFile          *file,
   if (hdr.pixelfmt.flags & DDPF_FOURCC)
     {
       /* fourcc is dXt* or rXgb */
-      if (hdr.pixelfmt.fourcc[1] == 'X')
+      if (hdr.pixelfmt.fourcc[1] == 'X' &&
+          (hdr.pixelfmt.fourcc[3] != '1' || bc1_use_transparency))
         hdr.pixelfmt.flags |= DDPF_ALPHAPIXELS;
     }
 
@@ -1841,6 +1844,12 @@ load_layer (FILE             *fp,
               dst[y * (width * 4) + (x * 4) + 3] = 255;
         }
 
+      if (format == DDS_COMPRESS_BC1 &&
+          ! (hdr->pixelfmt.flags & DDPF_ALPHAPIXELS))
+        {
+          format = _DDS_COMPRESS_BC1_NO_ALPHA;
+        }
+
       dxt_decompress (dst, buf, format, size, width, height, d->gimp_bpp,
                       hdr->pixelfmt.flags & DDPF_NORMAL);
 
@@ -2013,6 +2022,7 @@ load_dialog (GimpProcedure *procedure,
                                          "dds-read-box",
                                          "load-mipmaps",
                                          "flip-image",
+                                         "bc1-use-transparency",
                                          "decode-images",
                                          NULL);
   gtk_box_set_spacing (GTK_BOX (vbox), 8);

plug-ins/file-dds/dxt.c

@@ -1329,7 +1329,18 @@ decode_color_block (unsigned char *block,
           d[1] = colors[idx][1];
           d[2] = colors[idx][0];
           if (format == DDS_COMPRESS_BC1)
+            {
               d[3] = ((c0 <= c1) && idx == 3) ? 0 : 255;
+            }
+          else if (format == _DDS_COMPRESS_BC1_NO_ALPHA)
+            {
+              /* BC1, but interpret transparent as black */
+              if ((c0 <= c1) && idx == 3)
+                {
+                  d[0] = d[1] = d[2] = 0;
+                }
+              d[3] = 255;
+            }
           indices >>= 2;
           d += 4;
         }
@@ -1486,7 +1497,7 @@ dxt_decompress (unsigned char *dst,
         {
           memset(block, 0, 16 * 4);
 
-          if (format == DDS_COMPRESS_BC1)
+          if (format == DDS_COMPRESS_BC1 || format == _DDS_COMPRESS_BC1_NO_ALPHA)
             {
               decode_color_block(block, s, format);
               s += 8;