How to Concatenate Images in Golang?

Concatenating Images in Golang

Problem

Although Go provides robust image manipulation capabilities, users often face challenges in creating a single image from multiple smaller ones. Specifically, how can two PNG or JPEG files be concatenated to form a comprehensive image?

Analysis

The provided code reveals the reading of PNG files into an RGBA format:

imgFile, err := os.Open(path)
if err != nil {
    return Image{}, err
}
img, _, err := image.Decode(imgFile)
if err != nil {
    return Image{}, err
}

rgba := image.NewRGBA(img.Bounds())
if rgba.Stride != rgba.Rect.Size().X*4 {
    return Image{}, fmt.Errorf("unsupported stride")
}
draw.Draw(rgba, rgba.Bounds(), img, image.Point{0, 0}, draw.Src)

However, the crux of the issue lies in understanding how to combine multiple RGBA datasets into a single image.

Solution

The solution involves creating a new, empty image (NewRGBA) of sufficient size to accommodate both original images. Subsequently, the Draw method can be employed to overlay each image onto designated portions of the extended image:

// Load two images
imgFile1, err := os.Open("test1.jpg")
if err != nil {
    fmt.Println(err)
}
imgFile2, err := os.Open("test2.jpg")
if err != nil {
    fmt.Println(err)
}
img1, _, err := image.Decode(imgFile1)
if err != nil {
    fmt.Println(err)
}
img2, _, err := image.Decode(imgFile2)
if err != nil {
    fmt.Println(err)
}

// Starting position of the second image
sp2 := image.Point{img1.Bounds().Dx(), 0}

// New rectangle for the second image
r2 := image.Rectangle{sp2, sp2.Add(img2.Bounds().Size())}

// Rectangle for the big image
r := image.Rectangle{image.Point{0, 0}, r2.Max}

// Create a new image
rgba := image.NewRGBA(r)

// Draw the two images
draw.Draw(rgba, img1.Bounds(), img1, image.Point{0, 0}, draw.Src)
draw.Draw(rgba, r2, img2, image.Point{0, 0}, draw.Src)

// Export the image
out, err := os.Create("./output.jpg")
if err != nil {
    fmt.Println(err)
}

var opt jpeg.Options
opt.Quality = 80

jpeg.Encode(out, rgba, &opt)

Note: This method preserves the height of the second image. If the first image is taller, it will be cropped.

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