Fix frame orientation detection on Windows based on biHeight sign

- Use abs(biHeight) to get actual frame height
- Check biHeight sign to determine orientation (negative=top-to-bottom, positive=bottom-to-top)
- Remove incorrect pixel-format-based orientation detection
- Add m_inputOrientation member variable to store orientation across frames
- Reset m_firstFrameArrived when opening device

Fixes #34

Author: wysaid

src/ccap_imp_windows.cpp

@@ -727,6 +727,7 @@ bool ProviderDirectShow::open(std::string_view deviceName) {
     m_isOpened = true;
     m_isRunning = false;
     m_frameIndex = 0;
+    m_firstFrameArrived = false;
     return true;
 }
 
@@ -757,9 +758,27 @@ HRESULT STDMETHODCALLTYPE ProviderDirectShow::SampleCB(double sampleTime, IMedia
         if (SUCCEEDED(hr)) {
             VIDEOINFOHEADER* vih = (VIDEOINFOHEADER*)mt.pbFormat;
             m_frameProp.width = vih->bmiHeader.biWidth;
-            m_frameProp.height = vih->bmiHeader.biHeight;
-            m_frameProp.fps = 10000000.0 / vih->AvgTimePerFrame;
+            // biHeight may be negative. Negative height indicates top-to-bottom orientation.
+            // Positive height indicates bottom-to-top orientation (standard Windows DIB format).
+            m_frameProp.height = abs(vih->bmiHeader.biHeight);
+            
+            // For YUV formats, always assume TopToBottom orientation regardless of biHeight
+            // This fixes issues with some virtual cameras (like OBS) that report positive biHeight
+            // but actually deliver TopToBottom data
             auto info = findPixelFormatInfo(mt.subtype);
+            bool isYUVFormat = (info.pixelFormat & kPixelFormatYUVColorBit) != 0;
+            
+            if (isYUVFormat) {
+                // YUV data is typically TopToBottom, ignore biHeight sign
+                m_inputOrientation = FrameOrientation::TopToBottom;
+                CCAP_LOG_V("ccap: YUV format detected, using TopToBottom orientation (biHeight=%d)\n", vih->bmiHeader.biHeight);
+            } else if (vih->bmiHeader.biHeight < 0) {
+                m_inputOrientation = FrameOrientation::TopToBottom;
+            } else {
+                m_inputOrientation = FrameOrientation::BottomToTop;
+            }
+            
+            m_frameProp.fps = 10000000.0 / vih->AvgTimePerFrame;
             if (info.pixelFormat != PixelFormat::Unknown) {
                 m_frameProp.cameraPixelFormat = info.pixelFormat;
             }
@@ -781,15 +800,14 @@ HRESULT STDMETHODCALLTYPE ProviderDirectShow::SampleCB(double sampleTime, IMedia
     uint32_t bufferLen = mediaSample->GetActualDataLength();
     bool isInputYUV = (m_frameProp.cameraPixelFormat & kPixelFormatYUVColorBit);
     bool isOutputYUV = (m_frameProp.outputPixelFormat & kPixelFormatYUVColorBit);
-    auto inputOrientation = isInputYUV ? FrameOrientation::TopToBottom : FrameOrientation::BottomToTop;
 
     newFrame->pixelFormat = m_frameProp.cameraPixelFormat;
     newFrame->width = m_frameProp.width;
     newFrame->height = m_frameProp.height;
     newFrame->orientation = isOutputYUV ? FrameOrientation::TopToBottom : m_frameOrientation;
     newFrame->nativeHandle = mediaSample;
 
-    bool shouldFlip = newFrame->orientation != inputOrientation && !isOutputYUV;
+    bool shouldFlip = newFrame->orientation != m_inputOrientation && !isOutputYUV;
     bool shouldConvert = m_frameProp.cameraPixelFormat != m_frameProp.outputPixelFormat;
     bool zeroCopy = !shouldConvert && !shouldFlip;
 

src/ccap_imp_windows.h

@@ -118,6 +118,8 @@ class ProviderDirectShow : public ProviderImp, public ISampleGrabberCB {
     std::vector<std::string> m_allDeviceNames;
 
     std::chrono::steady_clock::time_point m_startTime{};
+    FrameOrientation m_inputOrientation = FrameOrientation::TopToBottom;
+    
     bool m_firstFrameArrived = false;
 
     // State variables