From 5a0dba103893e6b8084be13945a826663917d00a Mon Sep 17 00:00:00 2001
From: Lode <lvandeve@gmail.com>
Date: Mon, 20 Jan 2020 00:27:52 +0100
Subject: [PATCH] fix get-filtertype utility for 1-pixel wide or high
 interlaced images

---
 lodepng_unittest.cpp | 19 +++++++++++++++++++
 lodepng_util.cpp     | 24 +++++++++++++++---------
 2 files changed, 34 insertions(+), 9 deletions(-)

diff --git a/lodepng_unittest.cpp b/lodepng_unittest.cpp
index 90c8323d..4a21e29d 100644
--- a/lodepng_unittest.cpp
+++ b/lodepng_unittest.cpp
@@ -679,6 +679,24 @@ void doCodecTestNoLZ77(Image& image) {
   doCodecTestWithEncState(image, state);
 }
 
+void testGetFilterTypes() {
+  std::cout << "testGetFilterTypes" << std::endl;
+  // Test that getFilterTypes works on the special case of 1-pixel wide interlaced image
+  std::string png64 = "iVBORw0KGgoAAAANSUhEUgAAAAEAAAAHCAIAAAExKYBVAAAAHUlEQVR4ASXHAQoAAAjCwPX/R9tK4ZBN4EHKcPcLXCgGAQa0TV8AAAAASUVORK5CYII=";
+  std::vector<unsigned char> png;
+  fromBase64(png, png64);
+  std::vector<unsigned char> types;
+  lodepng::getFilterTypes(types, png);
+  ASSERT_EQUALS(7, types.size());
+  ASSERT_EQUALS(1, types[0]);
+  ASSERT_EQUALS(1, types[1]);
+  ASSERT_EQUALS(1, types[2]);
+  ASSERT_EQUALS(0, types[3]);
+  ASSERT_EQUALS(1, types[4]);
+  ASSERT_EQUALS(1, types[5]);
+  ASSERT_EQUALS(1, types[6]);
+}
+
 //Test LodePNG encoding and decoding the encoded result, using the C++ interface, with interlace
 void doCodecTestInterlaced(Image& image) {
   std::vector<unsigned char> encoded;
@@ -3663,6 +3681,7 @@ void doMain() {
 
   //lodepng_util
   testChunkUtil();
+  testGetFilterTypes();
 
   std::cout << "\ntest successful" << std::endl;
 }
diff --git a/lodepng_util.cpp b/lodepng_util.cpp
index 5c5d876f..899a49d8 100644
--- a/lodepng_util.cpp
+++ b/lodepng_util.cpp
@@ -198,8 +198,7 @@ unsigned getFilterTypesInterlaced(std::vector<std::vector<unsigned char> >& filt
     for(size_t j = 0; j < 7; j++) {
       unsigned w2 = (w - ADAM7_IX[j] + ADAM7_DX[j] - 1) / ADAM7_DX[j];
       unsigned h2 = (h - ADAM7_IY[j] + ADAM7_DY[j] - 1) / ADAM7_DY[j];
-      if(ADAM7_IX[j] >= w) w2 = 0;
-      if(ADAM7_IY[j] >= h) h2 = 0;
+      if(ADAM7_IX[j] >= w || ADAM7_IY[j] >= h) continue;
       size_t linebytes = 1 + lodepng_get_raw_size(w2, 1, &state.info_png.color);
       for(size_t i = 0; i < h2; i++) {
         filterTypes[j].push_back(data[pos]);
@@ -219,17 +218,24 @@ unsigned getFilterTypes(std::vector<unsigned char>& filterTypes, const std::vect
   if(passes.size() == 1) {
     filterTypes.swap(passes[0]);
   } else {
+    // Simplify interlaced filter types to get a single filter value per scanline:
+    // put pass 6 and 7 alternating in the one vector, these filters
+    // correspond to the closest to what it would be for non-interlaced
+    // image. If the image is only 1 pixel wide, pass 6 doesn't exist so the
+    // alternative values column0 are used. The shift values are to match
+    // the y position in the interlaced sub-images.
+    // NOTE: the values 0-6 match Adam7's passes 1-7.
+    const unsigned column0[8] = {0, 6, 4, 6, 2, 6, 4, 6};
+    const unsigned column1[8] = {5, 6, 5, 6, 5, 6, 5, 6};
+    const unsigned shift0[8] = {3, 1, 2, 1, 3, 1, 2, 1};
+    const unsigned shift1[8] = {1, 1, 1, 1, 1, 1, 1, 1};
     lodepng::State state;
     unsigned w, h;
     lodepng_inspect(&w, &h, &state, &png[0], png.size());
-    /*
-    Interlaced. Simplify it: put pass 6 and 7 alternating in the one vector so
-    that one filter per scanline of the uninterlaced image is given, with that
-    filter corresponding the closest to what it would be for non-interlaced
-    image.
-    */
+    const unsigned* column = w > 1 ? column1 : column0;
+    const unsigned* shift = w > 1 ? shift1 : shift0;
     for(size_t i = 0; i < h; i++) {
-      filterTypes.push_back(i % 2 == 0 ? passes[5][i / 2] : passes[6][i / 2]);
+      filterTypes.push_back(passes[column[i & 7u]][i >> shift[i & 7u]]);
     }
   }
   return 0; /* OK */