From 6846010416479143151cf28962587f5b123888be Mon Sep 17 00:00:00 2001
From: whitequark <whitequark@whitequark.org>
Date: Mon, 25 Jul 2016 10:55:53 +0000
Subject: [PATCH] Fix a flip bug in ReadPngIntoPixmap.

The only user of that was the background image, and it was flipped
again when it was rendered, so the two bugs masked out each other.
This adds a `bool flip` to ReadPng and FromPng, since that's cheap
to do when writing the PNG file, expensive on the pixel arrays,
and sometimes inconvenient in OpenGL due to offsets.
---
 src/resource.cpp | 14 ++++++++------
 src/resource.h   |  4 ++--
 src/style.cpp    |  2 +-
 3 files changed, 11 insertions(+), 9 deletions(-)

diff --git a/src/resource.cpp b/src/resource.cpp
index 6dbcd393..4bd89038 100644
--- a/src/resource.cpp
+++ b/src/resource.cpp
@@ -87,7 +87,8 @@ RgbaColor Pixmap::GetPixel(size_t x, size_t y) const {
     ssassert(false, "Unexpected resource format");
 }
 
-static std::shared_ptr<Pixmap> ReadPngIntoPixmap(png_struct *png_ptr, png_info *info_ptr) {
+static std::shared_ptr<Pixmap> ReadPngIntoPixmap(png_struct *png_ptr, png_info *info_ptr,
+                                                 bool flip) {
     png_read_png(png_ptr, info_ptr, PNG_TRANSFORM_EXPAND | PNG_TRANSFORM_GRAY_TO_RGB, NULL);
 
     std::shared_ptr<Pixmap> pixmap = std::make_shared<Pixmap>();
@@ -106,7 +107,8 @@ static std::shared_ptr<Pixmap> ReadPngIntoPixmap(png_struct *png_ptr, png_info *
     pixmap->data = std::vector<uint8_t>(pixmap->stride * pixmap->height);
     uint8_t **rows = png_get_rows(png_ptr, info_ptr);
     for(size_t y = 0; y < pixmap->height; y++) {
-        memcpy(&pixmap->data[pixmap->stride * y], rows[y],
+        uint8_t *srcRow = flip ? rows[y] : rows[pixmap->height - y - 1];
+        memcpy(&pixmap->data[pixmap->stride * y], srcRow,
                pixmap->width * pixmap->GetBytesPerPixel());
     }
 
@@ -114,7 +116,7 @@ static std::shared_ptr<Pixmap> ReadPngIntoPixmap(png_struct *png_ptr, png_info *
     return pixmap;
 }
 
-std::shared_ptr<Pixmap> Pixmap::FromPng(const uint8_t *data, size_t size) {
+std::shared_ptr<Pixmap> Pixmap::FromPng(const uint8_t *data, size_t size, bool flip) {
     struct Slice { const uint8_t *data; size_t size; };
     Slice dataSlice = { data, size };
     png_struct *png_ptr = NULL;
@@ -139,14 +141,14 @@ std::shared_ptr<Pixmap> Pixmap::FromPng(const uint8_t *data, size_t size) {
             }
         });
 
-    return ReadPngIntoPixmap(png_ptr, info_ptr);
+    return ReadPngIntoPixmap(png_ptr, info_ptr, flip);
 
 exit:
     png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
     return nullptr;
 }
 
-std::shared_ptr<Pixmap> Pixmap::ReadPng(FILE *f) {
+std::shared_ptr<Pixmap> Pixmap::ReadPng(FILE *f, bool flip) {
     png_struct *png_ptr = NULL;
     png_info *info_ptr = NULL;
 
@@ -164,7 +166,7 @@ std::shared_ptr<Pixmap> Pixmap::ReadPng(FILE *f) {
     png_init_io(png_ptr, f);
     png_set_sig_bytes(png_ptr, sizeof(header));
 
-    return ReadPngIntoPixmap(png_ptr, info_ptr);
+    return ReadPngIntoPixmap(png_ptr, info_ptr, flip);
 
 exit:
     png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
diff --git a/src/resource.h b/src/resource.h
index 3249f074..aab95069 100644
--- a/src/resource.h
+++ b/src/resource.h
@@ -33,9 +33,9 @@ public:
     std::vector<uint8_t>       data;
 
     static std::shared_ptr<Pixmap> Create(Format format, size_t width, size_t height);
-    static std::shared_ptr<Pixmap> FromPng(const uint8_t *data, size_t size);
+    static std::shared_ptr<Pixmap> FromPng(const uint8_t *data, size_t size, bool flip = false);
 
-    static std::shared_ptr<Pixmap> ReadPng(FILE *f);
+    static std::shared_ptr<Pixmap> ReadPng(FILE *f, bool flip = false);
     bool WritePng(FILE *f, bool flip = false);
 
     size_t GetBytesPerPixel() const;
diff --git a/src/style.cpp b/src/style.cpp
index 9d8ffaed..8e0c9b9f 100644
--- a/src/style.cpp
+++ b/src/style.cpp
@@ -371,7 +371,7 @@ void TextWindow::ScreenBackgroundImage(int link, uint32_t v) {
         if(GetOpenFile(&bgImageFile, "", PngFileFilter)) {
             FILE *f = ssfopen(bgImageFile, "rb");
             if(f) {
-                SS.bgImage.pixmap = Pixmap::ReadPng(f);
+                SS.bgImage.pixmap = Pixmap::ReadPng(f, /*flip=*/true);
                 SS.bgImage.scale  = SS.GW.scale;
                 SS.bgImage.origin = SS.GW.offset.ScaledBy(-1);
                 fclose(f);