Is there a way to generate a random contrast level on html canvas?

I am trying to present this shape as a stimulus in a behavioral experiment. I wantthe image to have a random contrast level between 0 and 1. I am trying to use Math.random() for that but when I run this in Chrome the shape flickers when it is presented on the screen. Is there a way to present a stable shape with randomly generated contrast levels?

drawFunc: function gabor() {
            context = jsPsych.currentTrial().context;
            context.beginPath();
            const gradLength = 100;
            const my_gradient  = context.createLinearGradient(850, 0, 1050, 0);
            my_gradient.addColorStop(0,'rgb(0, 0, 0)');
            my_gradient.addColorStop(0.05,'rgb(255, 255, 255)');
            my_gradient.addColorStop(0.1,'rgb(0, 0, 0)');
            my_gradient.addColorStop(0.15,'rgb(255, 255, 255)');
            my_gradient.addColorStop(0.2,'rgb(0, 0, 0)');
            my_gradient.addColorStop(0.25,"rgb(255, 255, 255)");
            my_gradient.addColorStop(0.3,"rgb(0, 0, 0)");
            my_gradient.addColorStop(0.35,"rgb(255, 255, 255)");
            my_gradient.addColorStop(0.4,"rgb(0, 0, 0)");
            my_gradient.addColorStop(0.45,"rgb(255, 255, 255)");
            my_gradient.addColorStop(0.5,"rgb(0, 0, 0)");
            my_gradient.addColorStop(0.55,"rgb(255, 255, 255)");
            my_gradient.addColorStop(0.6,"rgb(0, 0, 0)");
            my_gradient.addColorStop(0.65,"rgb(255, 255, 255)");
            my_gradient.addColorStop(0.7,"rgb(0, 0, 0)");
            my_gradient.addColorStop(0.75,"rgb(255, 255, 255)");
            my_gradient.addColorStop(0.8,"rgb(0, 0, 0)");
            my_gradient.addColorStop(0.85,"rgb(255, 255, 255)");
            my_gradient.addColorStop(0.9,"rgb(0, 0, 0)");
            my_gradient.addColorStop(0.95,"rgb(255, 255, 255)");
            my_gradient.addColorStop(1,"rgb(0, 0, 0)");
            var result1 = Math.random();
            context.filter = 'contrast('+ result1 +')';
            context.fillStyle=my_gradient;
            context.fillRect(950,300,gradLength,gradLength);
            context.stroke();