Crop Video

This article explains how you can crop a 16:9 video to a 4:3 video.

Source Video

For a source video we use the MP4 file from the TED talk video What’s the next window into our universe? by Andrew Connolly. The original video format is Wide 480p or 16:9, 854 x 480.

Sample Code

This code takes an MP4 file with 16:9 480p (854x480) video and AAC audio, and crops the video to 4:3 640x480. The audio stream is copied from the source as is. The input and output file paths are hardcoded in the source code, no command line parsing is used.

The snippets in this section are from the simple_video_crop macOS sample.

Linux and Windows samples are also available:

• simple_video_crop Linux sample

• simple_video_crop Windows sample

Initialize AVBlocks

The first step in any AVBlocks application is to initialize the library. This must be done before using any other AVBlocks functionality. The Library::initialize() method sets up the internal state and loads necessary codecs. Always remember to call Library::shutdown() at the end of your program to properly clean up resources and release any allocated memory.

int main(int argc, const char *argv[]) {
    Library::initialize();

    auto inputFile = primo::ustring(L"AndrewConnolly_2014.mp4");
    auto outputFile = primo::ustring(L"AndrewConnolly_2014_640x480.mp4");

    // ... cropping code ...

    Library::shutdown();
    return 0;
}

Configure Transcoder and Crop

We probe the input file with a MediaInfo object, and create an input socket from it. The output socket is created by cloning the input socket, so it starts out with an identical configuration, and then we set the output file path on it. The transcoder is the core component that performs the actual cropping - it takes the 854x480 video from the input and crops it to 640x480.

Probe Input and Create Sockets

First, we probe the input file and create the input socket. Then we clone it to create the output socket and set the output file path.

auto inputInfo = primo::make_ref(Library::createMediaInfo());
inputInfo->inputs()->at(0)->setFile(inputFile);

if (inputInfo->open()) {
    auto inputSocket = primo::make_ref(
        Library::createMediaSocket(inputInfo.get())
    );

    // Start with the same output as the input
    auto outputSocket = primo::make_ref(inputSocket->clone());
    outputSocket->setFile(outputFile);
} else {
    std::cerr << "inputInfo->open() failed: "
               << primo::ustring(inputInfo->error()->message())
               << std::endl;
}

Configure Output Video Settings

Next, we get the output video pin and configure the frame dimensions and display ratio. We set the new frame size to 640x480 and change the display ratio from 16:9 to 4:3.

auto outVideoPin = outputSocket->pins()->at(0);
auto outVideoStream = (VideoStreamInfo*)outVideoPin->streamInfo();

// Set the new frame width and height to 640 x 480
outVideoStream->setFrameWidth(640);
outVideoStream->setFrameHeight(480);

// Set the display ratio to 4:3
outVideoStream->setDisplayRatioWidth(4);
outVideoStream->setDisplayRatioHeight(3);

Apply Crop Parameters

To crop the video, we need to calculate how many pixels to remove from each side. Since the input is 854x480 and we want 640x480, we cut (854 - 640) / 2 = 107 pixels from the left and right.

auto outVideoPinParams = primo::make_ref(Library::createParameterList());

// The input video is 854x480.
// To make it 640x480 we have to cut (854 - 640) / 2 pixels from each side.
auto cropLeft = primo::make_ref(Library::createIntParameter());
cropLeft->setName(Crop::Left);
cropLeft->setValue((854 - 640) / 2);
outVideoPinParams->add(cropLeft.get());

auto cropRight = primo::make_ref(Library::createIntParameter());
cropRight->setName(Crop::Right);
cropRight->setValue((854 - 640) / 2);
outVideoPinParams->add(cropRight.get());

outVideoPin->setParams(outVideoPinParams.get());

Create and Run Transcoder

Finally, we create the transcoder, add the input and output sockets, and run the operation. The setAllowDemoMode(true) call allows the transcoder to work even without a valid license (useful for testing, but not recommended for production).

auto transcoder = primo::make_ref(Library::createTranscoder());
transcoder->inputs()->add(inputSocket.get());
transcoder->outputs()->add(outputSocket.get());

// Allow demo mode for the transcoder when
// using the demo version of the library
transcoder->setAllowDemoMode(true);

// Run the transcoder
if (transcoder->open()) {
    transcoder->run();
    transcoder->close();
} else {
    std::cerr << "transcoder->open() failed: "
              << primo::ustring(transcoder->error()->message())
              << std::endl;
}

Complete C++ Code

Here’s the complete working example that demonstrates video cropping using AVBlocks. The input and output file paths are hardcoded, so no command line parsing is required.

#include <primo/avblocks/avb.h>
#include <primo/platform/reference++.h>
#include <primo/platform/ustring.h>

using namespace primo;
using namespace primo::codecs;
using namespace primo::avblocks;

int main(int argc, const char *argv[]) {
    Library::initialize();

    auto inputFile = primo::ustring(L"AndrewConnolly_2014.mp4");
    auto outputFile = primo::ustring(L"AndrewConnolly_2014_640x480.mp4");

    using namespace Param::Video;

    auto inputInfo = primo::make_ref(Library::createMediaInfo());
    inputInfo->inputs()->at(0)->setFile(inputFile);

    if (inputInfo->open()) {
        auto inputSocket = primo::make_ref(
            Library::createMediaSocket(inputInfo.get())
        );

        // Start with the same output as the input
        auto outputSocket = primo::make_ref(inputSocket->clone());
        outputSocket->setFile(outputFile);

        // Get the output video pin
        auto outVideoPin = outputSocket->pins()->at(0);

        auto outVideoStream = (VideoStreamInfo*)outVideoPin->streamInfo();

        // Set the new frame width and height to 640 x 480
        outVideoStream->setFrameWidth(640);
        outVideoStream->setFrameHeight(480);

        // Set the display ratio to 4:3
        outVideoStream->setDisplayRatioWidth(4);
        outVideoStream->setDisplayRatioHeight(3);

        // set Crop.Left and Crop.Right
        auto outVideoPinParams = primo::make_ref(Library::createParameterList());

        // The input video is 854x480.
        // To make it 640x480 we have to cut (854 - 640) / 2 pixels from each side.
        auto cropLeft = primo::make_ref(Library::createIntParameter());
        cropLeft->setName(Crop::Left);
        cropLeft->setValue((854 - 640) / 2);
        outVideoPinParams->add(cropLeft.get());

        auto cropRight = primo::make_ref(Library::createIntParameter());
        cropRight->setName(Crop::Right);
        cropRight->setValue((854 - 640) / 2);
        outVideoPinParams->add(cropRight.get());

        outVideoPin->setParams(outVideoPinParams.get());

        // Create Transcoder and configure it with
        // the input and output sockets
        auto transcoder = primo::make_ref(Library::createTranscoder());
        transcoder->inputs()->add(inputSocket.get());
        transcoder->outputs()->add(outputSocket.get());

        // Allow demo mode for the transcoder when
        // using the demo version of the library
        transcoder->setAllowDemoMode(true);

        // Run the transcoder
        if (transcoder->open()) {
            transcoder->run();
            transcoder->close();
        } else {
            std::cerr << "transcoder->open() failed: "
                      << primo::ustring(transcoder->error()->message())
                      << std::endl;
        }
    } else {
        std::cerr << "inputInfo->open() failed: "
                   << primo::ustring(inputInfo->error()->message())
                   << std::endl;
    }

    Library::shutdown();
    return 0;
}