How to remove transparency from images downloaded in Chart.js-CodePudding

How can I remove opacity from downloaded images using the toBase64Image() function in Chart.js? If possible, save to JPG instead of PNG. This is the code:

const myChart = new Chart(ctx, {
    type: 'bar',
    data: {
        labels: ['Red', 'Blue', 'Yellow', 'Green', 'Purple', 'Orange'],
        datasets: [{
            label: '# of Votes',
            data: [12, 19, 3, 5, 2, 3],
            backgroundColor: [
                'rgba(255, 99, 132, 0.2)',
                'rgba(54, 162, 235, 0.2)',
                'rgba(255, 206, 86, 0.2)',
                'rgba(75, 192, 192, 0.2)',
                'rgba(153, 102, 255, 0.2)',
                'rgba(255, 159, 64, 0.2)'
            ],
            borderColor: [
                'rgba(255, 99, 132, 1)',
                'rgba(54, 162, 235, 1)',
                'rgba(255, 206, 86, 1)',
                'rgba(75, 192, 192, 1)',
                'rgba(153, 102, 255, 1)',
                'rgba(255, 159, 64, 1)'
            ],
            borderWidth: 1
        }]
    },
    options: {
        scales: {
            y: {
                beginAtZero: true
            }
        },
        animation: {
            onComplete: function() {
                image = graficoConsumo.toBase64Image();
            }
        }
    }
});
document.getElementById('btn-download-chart').onclick = function() {
        // Trigger the download
        var a = document.createElement('a');
        a.href = image;
        a.download = 'myChart.png';
        a.click();
}

CodePudding user response：

No by default canvases dont have a background so it will always be transparent. To fix this you can write a small custom plugin that draws a white background before chart.js draws its things on it:

Chart.register({
  id: 'customBackground',
  beforeDraw: (chart, args, opts) => {
    const ctx = chart.canvas.getContext('2d');
    ctx.save();
    ctx.globalCompositeOperation = 'destination-over';
    ctx.fillStyle = 'white';
    ctx.fillRect(0, 0, chart.width, chart.height);
    ctx.restore();
  }
})

const options = {
  type: 'line',
  data: {
    labels: ["Red", "Blue", "Yellow", "Green", "Purple", "Orange"],
    datasets: [{
        label: '# of Votes',
        data: [12, 19, 3, 5, 2, 3],
        borderColor: 'orange'
      },
      {
        label: '# of Points',
        data: [7, 11, 5, 8, 3, 7],
        borderColor: 'pink'
      }
    ]
  },
  options: {},
}

const ctx = document.getElementById('chartJSContainer').getContext('2d');
new Chart(ctx, options);

body {
  background-color: black;
}

<body>
  <canvas id="chartJSContainer" width="600" height="400"></canvas>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/3.6.2/chart.js"></script>
</body>