Replace transparency with a colour

Hi,

I’m currently converting PNG (&/BMP/GIF) to JPG. When I convert images, any transparency (alpha channels) turn BLACK. I would like to change this to WHITE.

Is there a way to do this out of the box?

1. Apply image on top of a white background

Jimp.read(file).then(function(image) {
    var width = image.bitmap.width;
    var height = image.bitmap.height;
    var white = 0xFFFFFFFF;
    // create a white background...
    new Jimp(width, height, white, function(err, backgroundImage) {
        if (err) {
            return handleError(err);
        }
        // apply image on top of white background...
        backgroundImage.blit(image, 0, 0)
            .getBuffer(Jimp.MIME_JPEG, function(err2, buffer) {
                // do stuff with image
            });
    });
});

2. Manually alter each pixel

Jimp.read(file).then(function(image) {
    image.scan(0, 0, image.bitmap.width, image.bitmap.height, function(x, y, idx) {
        var alphaModifier = 1 - (this.bitmap.data[idx + 3] / 255);
        var red = (255 - this.bitmap.data[idx + 0]) * alphaModifier;
        var green = (255 - this.bitmap.data[idx + 1]) * alphaModifier;
        var blue = (255 - this.bitmap.data[idx + 2]) * alphaModifier;
        this.bitmap.data[idx + 0] += red;
        this.bitmap.data[idx + 1] += green;
        this.bitmap.data[idx + 2] += blue;
        this.bitmap.data[idx + 3] = 255; // set no transparency
    });
    // do stuff with image...
});

Sample Source Images SourceImages.zip

[itslenny]

You can achieve this with compositing…

const width = 300;
const height = 300;
const image = // a jimp image with transparent background
const outputPath = 'where/to/save/it';

new Jimp(width, height, '#FFFFFF', (err, bgImage) => {
    bgImage.composite(image, 0, 0)
        .write(outputFilePath,
            (err) => {
                console.log('file created!!');
            });
});

[Ojas-Gulati]

You can set a “default color” in Jimp. Let me look into that…

EDIT: try this: image.background(0xFFFFFFFF); and then convert the image.

I don’t have my normal computer, so can’t test 😦