From 42a509705e63a88b88ddefb51db7441994f3df2d Mon Sep 17 00:00:00 2001
From: Christian Seiler <christian@iwakd.de>
Date: Thu, 2 Jul 2026 23:14:08 +0200
Subject: [PATCH] C++: fix line rendering logic

---
 cpp/src/monoformat_parseonly.cpp  |  8 ++++++++
 cpp/src/monoformat_structured.cpp | 22 ++++++++++++++++++----
 2 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/cpp/src/monoformat_parseonly.cpp b/cpp/src/monoformat_parseonly.cpp
index 3f3c992..7456b2c 100644
--- a/cpp/src/monoformat_parseonly.cpp
+++ b/cpp/src/monoformat_parseonly.cpp
@@ -946,10 +946,18 @@ std::expected<void, ParseError> handleLine(std::span<std::byte const>& buffer, O
     bool value = !lineFlags.dark;
 
     if (dx == 0) {
+        if (dy < 0) {
+            std::swap(originY, targetY);
+            dy = -dy;
+        }
         for (std::int32_t i = 0; i <= dy; ++i) {
             screen->setPixel(*originX, *originY + i, value);
         }
     } else if (dy == 0) {
+        if (dx < 0) {
+            std::swap(originX, targetX);
+            dx = -dx;
+        }
         for (std::int32_t i = 0; i <= dx; ++i) {
             screen->setPixel(*originX + i, *originY, value);
         }
diff --git a/cpp/src/monoformat_structured.cpp b/cpp/src/monoformat_structured.cpp
index 7f2e02d..88f348e 100644
--- a/cpp/src/monoformat_structured.cpp
+++ b/cpp/src/monoformat_structured.cpp
@@ -1145,18 +1145,32 @@ void LineElement::drawTo(OneBitBufferInterface* imageBuffer, std::size_t animati
     std::int32_t dy = static_cast<std::int32_t>(m_targetY) - static_cast<std::int32_t>(m_originY);
     bool value = true;
 
+    std::uint16_t originX = m_originX;
+    std::uint16_t originY = m_originY;
+    std::uint16_t targetX = m_targetX;
+    std::uint16_t targetY = m_targetY;
+
+
     if (dx == 0) {
+        if (dy < 0) {
+            std::swap(originY, targetY);
+            dy = -dy;
+        }
         for (std::int32_t i = 0; i <= dy; ++i) {
-            imageBuffer->setPixel(m_originX, m_originY + i, value);
+            imageBuffer->setPixel(originX, originY + i, value);
         }
     } else if (dy == 0) {
+        if (dx < 0) {
+            std::swap(originX, targetX);
+            dx = -dx;
+        }
         for (std::int32_t i = 0; i <= dx; ++i) {
-            imageBuffer->setPixel(m_originX + i, m_originY, value);
+            imageBuffer->setPixel(originX + i, originY, value);
         }
     } else {
         for (std::int32_t i = 0; i <= dx; ++i) {
-            std::int32_t x = m_originX + i;
-            std::int32_t y = i * dy / dx + m_originY;
+            std::int32_t x = originX + i;
+            std::int32_t y = i * dy / dx + originY;
             imageBuffer->setPixel(static_cast<std::uint16_t>(x), static_cast<std::uint16_t>(y), value);
         }
     }