The task is to display a series of vertical greyscale bars (contrast bars) with a sufficient number of bars to span the entire width of the display.

For the top quarter of the display, the left hand bar should be black, and we then incrementally step through six shades of grey until we have a white bar on the right hand side of the display. (This gives a total of 8 bars)

For the second quarter down, we start with white and step down through 14 shades of gray, getting darker until we have black on the right hand side of the display. (This gives a total of 16 bars).

Halfway down the display, we start with black, and produce 32 bars, ending in white, and for the last quarter, we start with white and step through 62 shades of grey, before finally arriving at black in the bottom right hand corner, producing a total of 64 bars for the bottom quarter.

package main
 
import (
    "github.com/fogleman/gg"
    "math"
)
 
func greyBars(dc *gg.Context) {
    run := 0
    colorComp := 0.0 // component of the color
    for colCount := 8; colCount < 128; colCount *= 2 {
        // by this gap we change the background color
        colorGap := 255.0 / float64(colCount-1)
        colWidth := float64(dc.Width() / colCount)
        colHeight := float64(dc.Height() / 4)
        // switches color directions with each iteration of for loop
        if run%2 == 0 {
            colorComp = 0.0
        } else {
            colorComp = 255.0
            colorGap = -colorGap
        }
        xstart, ystart := 0.0, colHeight*float64(run)
        for i := 0; i < colCount; i++ {
            icolor := int(math.Round(colorComp)) // round to nearer integer
            dc.SetRGB255(icolor, icolor, icolor)
            dc.DrawRectangle(xstart, ystart, colWidth, colHeight)
            dc.Fill()
            xstart += colWidth
            colorComp += colorGap
        }
        run++
    }
}
 
func main() {
    dc := gg.NewContext(640, 320)
    greyBars(dc)
    dc.SavePNG("greybars.png")
}