diff --git a/contrib/libtests/pngvalid.c b/contrib/libtests/pngvalid.c
index 5fa363a66..84402904c 100644
--- a/contrib/libtests/pngvalid.c
+++ b/contrib/libtests/pngvalid.c
@@ -10960,6 +10960,11 @@ static const color_encoding test_encodings[] =
 /*red:  */ { 0.716500716779386, 0.258728243040113, 0.000000000000000 },
 /*green:*/ { 0.101020574397477, 0.724682314948566, 0.051211818965388 },
 /*blue: */ { 0.146774385252705, 0.016589442011321, 0.773892783545073} },
+/* Fake encoding which selects just the green channel */
+/*gamma:*/ { 1.45/2.2, /* the 'Mac' gamma */
+/*red:  */ { 0.716500716779386, 0.000000000000000, 0.000000000000000 },
+/*green:*/ { 0.101020574397477, 1.000000000000000, 0.051211818965388 },
+/*blue: */ { 0.146774385252705, 0.000000000000000, 0.773892783545073} },
 };
 
 /* signal handler
diff --git a/pngpriv.h b/pngpriv.h
index dbce57a6e..0a4d16026 100644
--- a/pngpriv.h
+++ b/pngpriv.h
@@ -1193,10 +1193,10 @@ PNG_INTERNAL_FUNCTION(void,png_init_transform_control,(
 
 #ifdef PNG_READ_TRANSFORMS_SUPPORTED
 PNG_INTERNAL_FUNCTION(unsigned int,png_run_this_transform_list_forwards,
-   (png_transform_controlp tc, png_transformp *start, png_transformp *end),
+   (png_transform_controlp tc, png_transformp *start, png_transformp end),
    PNG_EMPTY);
    /* Used by the transform cache code to run a sub-list, from *start to the
-    * transform containing *end.
+    * transform end.
     */
 #endif /* READ_TRANSFORMS */
 
diff --git a/pngrtran.c b/pngrtran.c
index df317d2fc..ea05709fd 100644
--- a/pngrtran.c
+++ b/pngrtran.c
@@ -6048,7 +6048,7 @@ update_palette(png_structp png_ptr, png_cache_paramsp cp,
       cp->tend.dp = cache.b8;
 
       check_depth =
-         png_run_this_transform_list_forwards(&cp->tend, cp->start, cp->end);
+         png_run_this_transform_list_forwards(&cp->tend, cp->start, *cp->end);
 
       /* If we get here these two things must be true or there are been some
        * buggy difference of opinion between the INIT code and the actual run:
@@ -6308,7 +6308,7 @@ make_cache(png_structp png_ptr, png_cache_paramsp cp, unsigned int max_depth)
 
    {
       unsigned int check_depth =
-         png_run_this_transform_list_forwards(&cp->tend, cp->start, cp->end);
+         png_run_this_transform_list_forwards(&cp->tend, cp->start, *cp->end);
 
       /* This must not change: */
       affirm(PNG_TC_PIXEL_DEPTH(cp->tend) == opd && check_depth == max_depth);
@@ -6590,7 +6590,7 @@ png_read_init_transform_mech(png_structp png_ptr, png_transform_controlp tc)
     */
 {
    png_transformp *list = &png_ptr->transform_list;
-   unsigned int max_depth = 0U;
+   unsigned int max_depth;
    png_cache_params cp;
 
    /* PNG color-mapped data must be handled here so that the palette is updated
@@ -6610,6 +6610,7 @@ png_read_init_transform_mech(png_structp png_ptr, png_transform_controlp tc)
    cp.end = cp.start = list;
    cp.tend = cp.tstart = *tc;
    init_caching(png_ptr, &cp);
+   max_depth = PNG_TC_PIXEL_DEPTH(cp.tend);
 
    while (*cp.end != NULL)
    {
@@ -6660,7 +6661,9 @@ png_read_init_transform_mech(png_structp png_ptr, png_transform_controlp tc)
     */
    if (cp.tend.caching)
    {
+      png_transformp tr = *cp.end;
       handle_cache(png_ptr, &cp, max_depth);
+      affirm(tr == *cp.end);
       max_depth = PNG_TC_PIXEL_DEPTH(cp.tend);
    }
 
diff --git a/pngtrans.c b/pngtrans.c
index 330f83b61..84a1bfb2a 100644
--- a/pngtrans.c
+++ b/pngtrans.c
@@ -458,7 +458,7 @@ run_transform_list_forwards(png_transform_controlp tc, png_transformp *start,
    /* Called from the init code and below, the caller must initialize 'tc' */
 {
    png_const_structp png_ptr = tc->png_ptr;
-   unsigned int max_depth = 0;
+   unsigned int max_depth = PNG_TC_PIXEL_DEPTH(*tc);
 
    /* Caller guarantees that *start is non-NULL */
    debug(*start != NULL);
@@ -494,9 +494,9 @@ run_transform_list_forwards(png_transform_controlp tc, png_transformp *start,
 #ifdef PNG_READ_TRANSFORMS_SUPPORTED
 unsigned int /* PRIVATE */
 png_run_this_transform_list_forwards(png_transform_controlp tc,
-   png_transformp *start, png_transformp *end)
+   png_transformp *start, png_transformp end)
 {
-   return run_transform_list_forwards(tc, start, *end);
+   return run_transform_list_forwards(tc, start, end);
 }
 #endif /* READ_TRANSFORMS */
 
@@ -504,18 +504,11 @@ png_run_this_transform_list_forwards(png_transform_controlp tc,
 unsigned int /* PRIVATE */
 png_run_transform_list_forwards(png_structp png_ptr, png_transform_controlp tc)
 {
-   unsigned int max_depth = PNG_PIXEL_DEPTH(*png_ptr);
-
    if (png_ptr->transform_list != NULL)
-   {
-      unsigned int depth =
-         run_transform_list_forwards(tc, &png_ptr->transform_list, NULL);
+      return run_transform_list_forwards(tc, &png_ptr->transform_list, NULL);
 
-      if (depth > max_depth)
-         max_depth = depth;
-   }
-
-   return max_depth;
+   else
+      return PNG_PIXEL_DEPTH(*png_ptr);
 }
 #endif /* READ */
 
@@ -2284,6 +2277,36 @@ png_init_byte_ops(png_transformp *transform, png_transform_controlp tc)
 #endif /* SWAP poo */
 
 #ifdef PNG_READ_RGB_TO_GRAY_SUPPORTED
+static void
+png_init_rgb_to_gray_byte_ops(png_transformp *transform, 
+   png_transform_controlp tc)
+{
+   /* This just delay initializes the function; all the transform initialization
+    * has been done below.
+    */
+   (*transform)->fn = png_do_byte_ops_up;
+
+   /*  If this happens on a row do the transform immediately: */
+   if (!tc->init)
+      png_do_byte_ops_up(transform, tc);
+
+   else
+   {
+      /* This doing the init - update the row information here */
+#     define png_ptr (tc->png_ptr)
+      png_transform_byte_op *tr =
+         png_transform_cast(png_transform_byte_op, *transform);
+
+      debug(tc->bit_depth == 8U || tc->bit_depth == 16U);
+      debug((tc->format & PNG_FORMAT_FLAG_COLORMAP) == 0U &&
+            (tc->format & PNG_FORMAT_FLAG_COLOR) != 0U);
+
+      tc->format = tr->format;
+      tc->bit_depth = tr->bit_depth;
+#     undef png_ptr
+   }
+}
+
 void /* PRIVATE */
 png_add_rgb_to_gray_byte_ops(png_structrp png_ptr, png_transform_controlp tc,
    unsigned int index, unsigned int order)
@@ -2294,7 +2317,7 @@ png_add_rgb_to_gray_byte_ops(png_structrp png_ptr, png_transform_controlp tc,
 {
    png_transform_byte_op *tr = png_transform_cast(png_transform_byte_op,
       png_add_transform(png_ptr, sizeof (png_transform_byte_op),
-         png_do_byte_ops_up, order));
+         png_init_rgb_to_gray_byte_ops, order));
 
    affirm((tc->format & (PNG_FORMAT_FLAG_COLOR+PNG_FORMAT_FLAG_COLORMAP)) ==
             PNG_FORMAT_FLAG_COLOR &&
@@ -2305,11 +2328,8 @@ png_add_rgb_to_gray_byte_ops(png_structrp png_ptr, png_transform_controlp tc,
 
    /* For 1 byte channel [index] plus, maybe, alpha: */
    if (tc->bit_depth == 8)
-   {
       tr->codes = 8U + index +
          ((tc->format & PNG_FORMAT_FLAG_ALPHA) != 0 ? (8U+3U) << 4 : 0U);
-      UNTESTED
-   }
 
    else
    {
@@ -2320,7 +2340,6 @@ png_add_rgb_to_gray_byte_ops(png_structrp png_ptr, png_transform_controlp tc,
       tr->codes = (8U + index) + ((9U + index) << 4) +
          ((tc->format & PNG_FORMAT_FLAG_ALPHA) != 0 ?
             ((8U+6U) + ((9U+6U) << 4)) << 8 : 0U);
-      UNTESTED
    }
 }
 #endif /* READ_RGB_TO_GRAY */
diff --git a/pngwutil.c b/pngwutil.c
index 2404905a3..8ab7e334c 100644
--- a/pngwutil.c
+++ b/pngwutil.c
@@ -1427,8 +1427,6 @@ png_write_tRNS(png_structrp png_ptr, png_const_bytep trans_alpha,
                   inverted_alpha.u32[i] = ~inverted_alpha.u32[i];
 
                trans_alpha = inverted_alpha.b8;
-
-               UNTESTED
             }
 #        endif /* WRITE_INVERT_ALPHA */