Mapping out FFglitch

Step 2. Preliminary JavaScript File

After having downloaded FFglitch and extracted it using a file-manager (such as 7-Zip) and having a text editing software (Notepad) at the ready, code is necessary in order to create the desired glitch effect. Worry not, all code will be provided, no knowledge of JavaScript is required.

• Copy and paste the entire text contents of the code block below into a new text file

Ramiro Polla’s “Average motion vector” code

// global variable holding forward motion vectors from previous frames
var prev_fwd_mvs = [ ];

// change this value to use a smaller or greater number of frames to
// perform the average of motion vectors
var tail_length = 10;

// calculate average of previous motion vectors
function average_mv(mv, i, j, n, k)
{
    let sum = 0;
    for ( let t = 0; t < n; t++ )
        sum += prev_fwd_mvs[t][i][j][k];
    let val = Math.round(sum / n);
    val = Math.max(val, -64);
    val = Math.min(val,  63);
    return val;
}

function glitch_frame(frame)
{
    // bail out if we have no motion vectors
    let mvs = frame["mv"];
    if ( !mvs )
        return;
    // bail out if we have no forward motion vectors
    let fwd_mvs = mvs["forward"];
    if ( !fwd_mvs )
        return;

    // update variable holding forward motion vectors from previous
    // frames. note that we perform a deep copy of the clean motion
    // vector values before modifying them.
    let json_str = JSON.stringify(fwd_mvs);
    let deep_copy = JSON.parse(json_str);
    // push to the end of array
    prev_fwd_mvs.push(deep_copy);
    // drop values from earliest frames to always keep the same tail
    // length
    if ( prev_fwd_mvs.length > tail_length )
        prev_fwd_mvs = prev_fwd_mvs.slice(1);

    // bail out if we still don't have enough frames
    if ( prev_fwd_mvs.length != tail_length )
        return;

    // replace all motion vectors of current frame with an average
    // of the motion vectors from the previous 10 frames
    for ( let i = 0; i < fwd_mvs.length; i++ )
    {
        // loop through all rows
        let row = fwd_mvs[i];
        for ( let j = 0; j < row.length; j++ )
        {
            // loop through all macroblocks
            let mv = row[j];

            // THIS IS WHERE THE MAGIC HAPPENS

            mv[0] = average_mv(mv, i, j, tail_length, 0);
            mv[1] = average_mv(mv, i, j, tail_length, 1);
        }
    }
}

• Save the text file and name it “mv_average.js”
• Drag the newly created JavaScript(.js) file into your FFglitch folder