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When stream transcoding is enabled, a stream published decoded picture frames in YUV format can be intercepted and changed pixel by pixel on server side. The frame changed will be then encoded and transferred to transcoder output stream as usual frame.

Interceptor implementation

To intercept the decoded frames, Java class implementing IDecodedFrameInterceptor interface should be developed. The function frameDecoded() of this class will receive decoded frames in YUV format, for example
// Package name should be strictly defined as com.flashphoner.frameInterceptor
package com.flashphoner.frameInterceptor;
// Import decoded frame interceptor interface
// Import YUV frame description
 * Custom decoded frames interceptor implementation example
 * The example daws a cross over the picture
public class TestInterceptor implements IDecodedFrameInterceptor {
    // Constants to parse pixel
    private final int Y = 0;
    private final int U = 1;
    private final int V = 2;
    // Dark colored pixel
    private final byte[] DarkPixel = new byte []{42, -128, -128};
     * Function to handle decoded frame
     * @param streamName - stream name
     * @param frame - decoded YUV frame
    public void frameDecoded(String streamName, YUVFrame frame) {
        // Get frame height
        int frameHeight = frame.getHeight();
        // Get frame width
        int frameWidth = frame.getWidth();
        // Declare cross lines padding
        int PADDING = 4;
        // Define frame center
        int frameCenterX = frameWidth / 2;
        int frameCenterY = frameHeight / 2;
        // Define vertical line bounds
        int leftBound = frameCenterX - PADDING;
        int rightBound = frameCenterX + PADDING;
        // Define horizontal line bounds
        int topBound = frameCenterY - PADDING;
        int bottomBound = frameCenterY + PADDING;
        // Walk through the frame pixels and draw a cross
        for (int x = 0; x < frameWidth; x++) {
             for (int y = 0; y < frameHeight; y++) {
                  if (validateCoord(x, leftBound, rightBound) || validateCoord(y, topBound, bottomBound)) {
                      // Read the pixel
                      byte[] pixel = frame.readPixel(x, y);
                      // Modify the pixel
                      pixel[Y] = DarkPixel[Y];
                      pixel[U] = DarkPixel[U];
                      pixel[V] = DarkPixel[V];
                      // Write the pixel back
                      frame.writePixel(x, y, pixel);
     * Helper function to validate pixel drawing
     * @param coord - pixel coordinate
     * @param low - low coordinate bound 
     * @param high - high coordinate bound 
     * @return true if coordinate is valid
     private boolean validateCoord(int coord, int low, int high) {
        return (coord > low && coord < high);

Then the class should be complied into byte code. To do this, create folder tree accordind to TestInterceptor class package name

mkdir -p com/flashphoner/frameInterceptor

and execute the command

javac -cp /usr/local/FlashphonerWebCallServer/lib/wcs-core.jar ./com/flashphoner/frameInterceptor/

Now, pack the code compiled to jar file

jar -cf testlayout.jar ./com/flashphoner/frameInterceptor/TestInterceptor.class

and copy this file to WCS libraries folder

cp testinterceptor.jar /usr/local/FlashphonerWebCallServer/lib

To use custom frames interceptor class, set its package name to the following parameter in file


and restart WCS.

A separate folder for custom Java libraries

Since build 5.2.1512, custom layout Java libraries (jar files) should be placed to the folder /usr/local/FlashphonerWebCallServer/lib/custom 

cp testlayout.jar /usr/local/FlashphonerWebCallServer/lib/custom

This folder is kept while updating WCS to a newer builds. A jar files do not need to be copied again after updating.


1. Publish a test stream in Two Way Streaming example, where is WCS server address

2. Play the stream in Player example with explicit resolution setting to enable transcoding, for example, where is WCS server address

The pixels changed will be in the stream picture.

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