Ignore fully opaque alpha channels, preserve alpha when writing grayscale images

crnlib/crn_image_utils.cpp

@@ -174,27 +174,66 @@ bool write_to_file(const char* pFilename, const image_u8& img, uint write_flags,
     if (grayscale_comp_index > 3)
       grayscale_comp_index = 3;
 
-    temp.resize(img.get_total_pixels());
+    if ((img.get_comp_flags() & pixel_format_helpers::cCompFlagGrayscale) &&
+        img.is_component_valid(3) &&
+        !(write_flags & cWriteFlagIgnoreAlpha)) {
+      // Preserve grayscale images with alpha.
+      //
+      // PNG supports grayscale+alpha directly, while BMP/TGA writers
+      // expect color channels for alpha output. Expand to RGBA for
+      // those formats.
+      const bool expand_to_rgba = (ext == "bmp") || (ext == "tga");
+      const uint num_channels = expand_to_rgba ? 4 : 2;
+
+      temp.resize(img.get_total_pixels() * num_channels);
+
+      for (uint y = 0; y < img.get_height(); y++) {
+        const color_quad_u8* pSrc = img.get_scanline(y);
+        const color_quad_u8* pSrc_end = pSrc + img.get_width();
+        uint8* pDst = &temp[y * img.get_width() * num_channels];
+
+        while (pSrc != pSrc_end) {
+          const color_quad_u8 c(*pSrc++);
+
+          if (expand_to_rgba) {
+            pDst[0] = c[1];
+            pDst[1] = c[1];
+            pDst[2] = c[1];
+            pDst[3] = c.a;
+            pDst += 4;
+          } else {
+            pDst[0] = c[1];
+            pDst[1] = c.a;
+            pDst += 2;
+          }
+        }
+      }
 
-    for (uint y = 0; y < img.get_height(); y++) {
-      const color_quad_u8* pSrc = img.get_scanline(y);
-      const color_quad_u8* pSrc_end = pSrc + img.get_width();
-      uint8* pDst = &temp[y * img.get_width()];
-
-      if (img.get_comp_flags() & pixel_format_helpers::cCompFlagGrayscale) {
-        while (pSrc != pSrc_end)
-          *pDst++ = (*pSrc++)[1];
-      } else if (grayscale_comp_index < 0) {
-        while (pSrc != pSrc_end)
-          *pDst++ = static_cast<uint8>((*pSrc++).get_luma());
-      } else {
-        while (pSrc != pSrc_end)
-          *pDst++ = (*pSrc++)[grayscale_comp_index];
+      pSrc_img = &temp[0];
+      num_src_chans = num_channels;
+    } else {
+      temp.resize(img.get_total_pixels());
+
+      for (uint y = 0; y < img.get_height(); y++) {
+        const color_quad_u8* pSrc = img.get_scanline(y);
+        const color_quad_u8* pSrc_end = pSrc + img.get_width();
+        uint8* pDst = &temp[y * img.get_width()];
+
+        if (img.get_comp_flags() & pixel_format_helpers::cCompFlagGrayscale) {
+          while (pSrc != pSrc_end)
+            *pDst++ = (*pSrc++)[1];
+        } else if (grayscale_comp_index < 0) {
+          while (pSrc != pSrc_end)
+            *pDst++ = static_cast<uint8>((*pSrc++).get_luma());
+        } else {
+          while (pSrc != pSrc_end)
+            *pDst++ = (*pSrc++)[grayscale_comp_index];
+        }
       }
-    }
 
-    pSrc_img = &temp[0];
-    num_src_chans = 1;
+      pSrc_img = &temp[0];
+      num_src_chans = 1;
+    }
   } else if ((!img.is_component_valid(3)) || (write_flags & cWriteFlagIgnoreAlpha)) {
     temp.resize(img.get_total_pixels() * 3);
 

crnlib/crn_jpgd.cpp

@@ -2330,8 +2330,12 @@ void jpeg_decoder::init_frame() {
   int i;
 
   if (m_comps_in_frame == 1) {
-    if ((m_comp_h_samp[0] != 1) || (m_comp_v_samp[0] != 1))
-      stop_decoding(JPGD_UNSUPPORTED_SAMP_FACTORS);
+    if ((m_comp_h_samp[0] != 1) || (m_comp_v_samp[0] != 1)) {
+      // Some grayscale JPEGs contain non-1×1 sampling factors.
+      // There is no chroma plane to subsample, so normalize to the expected layout.
+      m_comp_h_samp[0] = 1;
+      m_comp_v_samp[0] = 1;
+    }
 
     m_scan_type = JPGD_GRAYSCALE;
     m_max_blocks_per_mcu = 1;

crnlib/crn_mipmapped_texture.cpp

@@ -2653,6 +2653,10 @@ bool mipmapped_texture::read_regular_image(data_stream_serializer& serializer) {
     return false;
   }
 
+  // Ignore alpha channel when it exists but is fully opaque.
+  const bool has_alpha = image_utils::has_alpha(*pImg);
+  pImg->set_component_valid(3, has_alpha);
+
   mip_level* pLevel = crnlib_new<mip_level>();
   pLevel->assign(pImg);