$Id: 316

$Score: 0

$SOId: 31685

An image represents a rectangular grid of picture elements (pixel). In the image package, the pixel is represented as one of the color defined in image/color package. The 2-D geometry of the image is represented as [image.Rectangle](<https://golang.org/pkg/image/#Rectangle>), while [image.Point](<https://golang.org/pkg/image/#Point>) denotes a position on the grid.

Image and 2-D geometry

The above figure illustrates the basic concepts of an image in the package. An image of size 15x14 pixels has a rectangular bounds started at upper left corner (e.g. co-ordinate (-3, -4) in the above figure), and its axes increase right and down to lower right corner (e.g. co-ordinate (12, 10) in the figure). Note that the bounds do not necessarily start from or contain point (0,0).

Image related type

In Go, an image always implement the following [image.Image](<https://golang.org/pkg/image/#Image>) interface

type Image interface {
    // ColorModel returns the Image's color model.
    ColorModel() color.Model
    // Bounds returns the domain for which At can return non-zero color.
    // The bounds do not necessarily contain the point (0, 0).
    Bounds() Rectangle
    // At returns the color of the pixel at (x, y).
    // At(Bounds().Min.X, Bounds().Min.Y) returns the upper-left pixel of the grid.
    // At(Bounds().Max.X-1, Bounds().Max.Y-1) returns the lower-right one.
    At(x, y int) color.Color
}

in which the [color.Color](<https://golang.org/pkg/image/color/#Color>) interface is defined as

type Color interface {
    // RGBA returns the alpha-premultiplied red, green, blue and alpha values
    // for the color. Each value ranges within [0, 0xffff], but is represented
    // by a uint32 so that multiplying by a blend factor up to 0xffff will not
    // overflow.
    //
    // An alpha-premultiplied color component c has been scaled by alpha (a),
    // so has valid values 0 <= c <= a.
    RGBA() (r, g, b, a uint32)
}

and [color.Model](<https://golang.org/pkg/image/color/#Model>) is an interface declared as

type Model interface {
    Convert(c Color) Color
}

Accessing image dimension and pixel

Suppose we have an image stored as variable img, then we can obtain the dimension and image pixel by:

// Image bounds and dimension
b := img.Bounds()
width, height := b.Dx(), b.Dy()
// do something with dimension ...

// Corner co-ordinates
top := b.Min.Y
left := b.Min.X
bottom := b.Max.Y
right := b.Max.X

// Accessing pixel. The (x,y) position must be
// started from (left, top) position not (0,0)
for y := top; y < bottom; y++ {
    for x := left; x < right; x++ {
        cl := img.At(x, y)
        r, g, b, a := cl.RGBA()
        // do something with r,g,b,a color component
    }
}

Note that in the package, the value of each R,G,B,A component is between 0-65535 (0x0000 - 0xffff), not 0-255.