Golang练习片如何处理大价值

Question

I'm going through the Golang tutorial and I'm a little bit confused as to what it is doing with some of the values in the slices exercise. https://tour.golang.org/moretypes/18

Here is the code that I am confused with:

A value of 0 is a perfectly blue pixel and a value of 255 is a perfectly white pixel. So what is happening here when the value displayed is some form of x*y (I did /20 to make the image a little bit bigger and easier to see).

If you follow the image horizontally, you will see that at some point in the process, the ever increasing x and y values seem to revert to blue (0 value) If I type a static value like 256 in the return I get a compile error. So it obviously does not allow the numbers to go off the scale and revert to 0 or anything. So how does it get the blue curves in the picture?

imported source here: https://github.com/golang/tour/blob/master/pic/pic.go#L15

package main

import "golang.org/x/tour/pic"

func Pic(dx, dy int) [][]uint8 {
    //First, the array has to be made so we can put some values in it later
    //This only makes the second dimension of the array ([[uint8 dy]])?
    image := make([][]uint8, dy)

    //The inputs into the function are Int's, so it is ok to have a non uint8
    //loop initializer
    for x := 0; x < dy; x++ {
        //once we are in the loop we have to make the first dimension of the array
        //based on the dx values
        image[x] = make([]uint8, dx)
        for y := 0; y < dx; y++ {
            //This is a function +to assign the pixel values to the array
            image[x][y] = uint8((x * y) /20)
        }
    }
    return image
}

func main() {
    pic.Show(Pic)
}

Answer 1

Imagine i is of type int, uint8(i) returns Least Significant Byte (LSB) of i:

When x is in range [0, 255] , meaning: 0 <= x <= 255
and y is in range [0, 255],
then x*y is in range [0, 255*255] = [0, 65025]
so x*y/20 is in range [0, 255*255/20] = [0, 65025/20] = [0, 3251]
and value of uint8(x*y/20) is equal to (x*y/20)%256 meaning exactly LSB byte:
uint8(3251) = uint8(0XCB3) = 0XB3 = 179
3251 = 12*256 + 179

So every time the x*y/20 is bigger than 255 it counts from 0 again: (x*y/20) % 256 this is why your image is repeated circles.

Try this working sample code:

package main

import "fmt"

func main() {
    for y := 0; y <= 255; y++ {
        for x := 0; x <= 255; x++ {
            v := x * y / 20
            if int(uint8(v)) != v%256 {
                fmt.Println(v, v%256)
            }
        }
    }
    fmt.Println("Done.")
}

output:

Done.

Let's simplify you example, see this working sample code:

package main

import (
    "bytes"
    "image"
    "image/png"
    "os"
)

func main() {
    const dx = 256
    const dy = 256
    m := image.NewNRGBA(image.Rect(0, 0, dx, dy))
    for y := 0; y < dy; y++ {
        for x := 0; x < dx; x++ {
            v := uint8(x * y / 20)
            i := y*m.Stride + x*4
            m.Pix[i] = v     //R
            m.Pix[i+1] = v   //G
            m.Pix[i+2] = 255 //B
            m.Pix[i+3] = 255 //A
        }
    }
    var buf bytes.Buffer
    err := png.Encode(&buf, m)
    if err != nil {
        panic(err)
    }
    os.Stdout.Write(buf.Bytes())
}

And redirect the output to a file like main > b.png or, go run main.go > b.png
see output file b.png: