How to Extract a Still Image from an MP4 at a Specific Time with Media Foundation - A Single .cpp File You Can Paste In

Bottom line

To grab one still image from an MP4 at a specific time, use IMFSourceReader. It is not a simple “seek once, read once, done” - watch out for these pitfalls:

• SetCurrentPosition does not guarantee an exact seek (it usually lands on a key frame before the target)
• Even when ReadSample succeeds, pSample can be NULL
• Setting the output to MFVideoFormat_RGB32 makes saving easier
• The fourth byte of RGB32 is not necessarily alpha, so force it to 0xFF before saving the PNG
• Account for stride and image orientation when repacking into top-down BGRA

The key: seek -> compare timestamps before and after -> copy with stride in mind -> save as PNG. Implement it in that order and it stays stable.

Processing flow

Frame selection rule

Rules for picking “the frame closest to the target time”:

1. After seeking, keep calling ReadSample
2. Hold on to the last sample where timestamp < target
3. When the first sample with timestamp >= target arrives, compare its distance to the target against the held sample’s distance
4. Take whichever is closer

Pitfalls

1. SetCurrentPosition is not an exact seek

It lands a bit before the requested position - typically on a key frame. Doing only SetCurrentPosition -> one ReadSample -> save will drift considerably for videos with long GOPs. Always compare timestamps before and after.

2. ReadSample can succeed and still hand you a NULL sample

Don’t just check HRESULT. Also inspect flags (MF_SOURCE_READERF_ENDOFSTREAM at the end of stream) and pSample.

3. Stride and orientation

The image buffer is not necessarily a flat width * bytesPerPixel. There may be padding at the end of each row. Use the actual stride returned by IMF2DBuffer::Lock2D, and remember that bottom-up images can have a negative stride.

4. The fourth byte of RGB32

RGB32 is laid out in memory as B, G, R, X (undefined). That X is not necessarily alpha. Save it to PNG as-is and you may end up with a transparent image, so set the fourth byte to 0xFF before saving.

Implementation flow

Creating the Source Reader

• MF_SOURCE_READER_ENABLE_VIDEO_PROCESSING = TRUE
• Set the output type to MFVideoFormat_RGB32
• Synchronous mode is fine (you only need a single frame)

Acquiring the frame

• Seek with SetCurrentPosition
• Loop on ReadSample, comparing timestamps before and after
• Pick the sample closest to the target

Buffer reshaping

• Convert to a contiguous buffer with ConvertToContiguousBuffer
• Get the actual stride via IMF2DBuffer::Lock2D
• Copy row by row into a top-down buffer (flip vertically for bottom-up)
• Force alpha to 0xFF

Saving the PNG

• Encode as PNG with WIC (IWICBitmapEncoder)
• Use GUID_WICPixelFormat32bppBGRA as the pixel format

Field checklist

How to run it

ExtractFrameFromMp4.exe <input.mp4> <seconds> <output.png>

Example:

ExtractFrameFromMp4.exe C:\work\input.mp4 12.345 C:\work\frame.png

Wrap-up

• Seek is not exact - compare before and after to pick the closest frame
• A successful ReadSample can still give you no sample
• Absorb stride and orientation before saving
• Don’t assume the fourth byte of RGB32 is alpha
• Drops straight into thumbnail generation, representative-frame capture for surveillance footage, and evidence trails in inspection logs

References

• Microsoft Learn: Using the Source Reader to Process Media Data
• Microsoft Learn: IMFSourceReader::SetCurrentPosition
• Microsoft Learn: IMFSourceReader::ReadSample
• Microsoft Learn: IMFSourceReader::SetCurrentMediaType
• Microsoft Learn: IMF2DBuffer
• Microsoft Learn: IMF2DBuffer::Lock2D
• Microsoft Learn: Uncompressed Video Buffers
• Microsoft Learn: Image Stride
• Microsoft Learn: MF_MT_FRAME_SIZE attribute
• Microsoft Learn: MF_MT_DEFAULT_STRIDE attribute
• Microsoft Learn: Native pixel formats overview (WIC)
• Microsoft Learn: Uncompressed RGB Video Subtypes