The Problem with Seeking One Frame Forward

Oct 13, 2025 by wbrum.org 111 views

Introduction

FFmpeg is a powerful, open-source multimedia framework that can be used for a wide range of tasks, including video processing, encoding, and decoding. When working with FFmpeg, it's often necessary to seek to specific frames within a video stream. However, seeking to a specific frame can be a complex process, especially when trying to move forward by a single frame. In this article, we'll explore how to seek one frame forward in FFmpeg, and provide a step-by-step guide on how to achieve this using C++ and Qt.

Understanding FFmpeg's Seeking Mechanism

Before we dive into the code, it's essential to understand how FFmpeg's seeking mechanism works. When you call the av_seek_frame function, FFmpeg attempts to seek to the specified frame number. However, this function can be slow and may not always return the exact frame number you requested. This is because FFmpeg uses a complex algorithm to determine the best way to seek to the specified frame, taking into account factors such as the video's frame rate, resolution, and encoding.

The Problem with Seeking One Frame Forward

When you call the seekFrameUp function, you might expect it to seek to the next frame in the video stream. However, as you've observed, the function gets stuck on the first frame sought and does not move forward. This is because FFmpeg's seeking mechanism is designed to seek to a specific frame number, not to move forward by a single frame.

A Solution to the Problem

To seek one frame forward in FFmpeg, you can use the av_read_frame function in conjunction with the av_seek_frame function. Here's an example of how you can modify the seekFrameUp function to achieve this:

void seekFrameUp() {
    if (!fmt_ctx || video_stream_index == -1) {
        // Handle error
        return;
    }
// Get the current frame number
int64_t current_frame = av_frame_get_best_effort_timestamp(fmt_ctx-&gt;streams[video_stream_index]-&gt;frames[fmt_ctx-&gt;streams[video_stream_index]-&gt;index]);

// Seek to the next frame
int64_t next_frame = current_frame + 1;
av_seek_frame(fmt_ctx, video_stream_index, next_frame, AVSEEK_FLAG_BACKWARD);

// Read the next frame
int ret = av_read_frame(fmt_ctx, &amp;packet);
if (ret &lt; 0) {
    // Handle error
    return;
}

// Check if the next frame is the one we&#39;re looking for
if (packet.pts == next_frame) {
    // Decode the next frame
    avcodec_decode_video2(c, &amp;frame, &amp;got_frame, packet.data, packet.size);
    if (got_frame) {
        // Process the next frame
        // ...
    }
} else {
    // Seek to the next frame again
    seekFrameUp();
}

}

How the Solution Works

Here's a step-by-step explanation of how the solution works:

Get the current frame number using av_frame_get_best_effort_timestamp.
Seek to the next frame using av_seek_frame with the AVSEEK_FLAG_BACKWARD flag.
Read the next frame using av_read_frame.
Check if the next frame is the one we're looking for by comparing its pts value with the next frame number.
If the next frame is the one we're looking for, decode it using avcodec_decode_video2 and process it.
If the next frame is not the one we're looking for, seek to the next frame again using seekFrameUp.

Conclusion

Seeking one frame forward in FFmpeg can be a complex process, but by using the av_read_frame function in conjunction with the av_seek_frame function, you can achieve this using C++ and Qt. By following the solution outlined in this article, you should be able to seek one frame forward in FFmpeg and move forward by a single frame.

Additional Tips and Considerations

Here are some additional tips and considerations to keep in mind when seeking one frame forward in FFmpeg:

Make sure to handle errors properly when using av_seek_frame and av_read_frame.
Use the AVSEEK_FLAG_BACKWARD flag when seeking to the next frame to ensure that you move forward by a single frame.
Use the av_frame_get_best_effort_timestamp function to get the current frame number.
Use the avcodec_decode_video2 function to decode the next frame.
Process the next frame as needed.

Introduction

In our previous article, we explored how to seek one frame forward in FFmpeg using C++ and Qt. However, we know that there are many questions and concerns that readers may have about this topic. In this article, we'll answer some of the most frequently asked questions about seeking one frame forward in FFmpeg.

Q: What is the difference between `av_seek_frame` and `av_read_frame`?

A: av_seek_frame is used to seek to a specific frame number in a video stream, while av_read_frame is used to read the next frame from a video stream. When you call av_seek_frame, FFmpeg attempts to seek to the specified frame number, but this function can be slow and may not always return the exact frame number you requested. On the other hand, av_read_frame is used to read the next frame from a video stream, and it's generally faster and more reliable than av_seek_frame.

Q: Why do I need to use `AVSEEK_FLAG_BACKWARD` when seeking to the next frame?

A: When you call av_seek_frame with the AVSEEK_FLAG_BACKWARD flag, FFmpeg will seek to the next frame in the video stream, rather than seeking to the specified frame number. This is because AVSEEK_FLAG_BACKWARD tells FFmpeg to seek to the next frame, rather than seeking to a specific frame number.

Q: How do I handle errors when using `av_seek_frame` and `av_read_frame`?

A: When using av_seek_frame and av_read_frame, you should always check the return value of these functions to ensure that they were successful. If the return value is negative, it means that an error occurred, and you should handle the error accordingly.

Q: Can I use `av_seek_frame` and `av_read_frame` together to seek to a specific frame number?

A: Yes, you can use av_seek_frame and av_read_frame together to seek to a specific frame number. However, this approach can be complex and may not always work as expected. A better approach is to use av_read_frame to read the next frame from the video stream, and then check if the next frame is the one you're looking for.

Q: How do I decode the next frame after seeking to it using `av_seek_frame`?

A: After seeking to the next frame using av_seek_frame, you can decode the next frame using avcodec_decode_video2. This function takes the next frame as input and decodes it into a AVFrame structure.

Q: Can I use `av_seek_frame` and `av_read_frame` with other FFmpeg functions, such as `avcodec_decode_video2`?

A: Yes, you can use av_seek_frame and av_read_frame with other FFmpeg functions, such as avcodec_decode_video2. However, you should always check the documentation for each function to ensure that it's compatible with av_seek_frame and av_read_frame.

Q: What are some common pitfalls to avoid when using `av_seek_frame` and `av_read_frame`?

A: Some common pitfalls to avoid when using av_seek_frame and av_read_frame include:

Not checking the return value of these functions to ensure that they were successful.
Not handling errors properly when using these functions.
Using av_seek_frame and av_read_frame together to seek to a specific frame number, without checking if the next frame is the one you're looking for.
Not decoding the next frame after seeking to it using av_seek_frame.

Conclusion

Seeking one frame forward in FFmpeg can be a complex process, but by understanding the differences between av_seek_frame and av_read_frame, and by following the tips and considerations outlined in this article, you should be able to successfully seek one frame forward in FFmpeg and move forward by a single frame.