func (d *Decoder) prepareYBR(mbx, mby int) {
	if mbx == 0 {
		for y := 0; y < 17; y++ {
			d.ybr[y][7] = 0x81
		}
		for y := 17; y < 26; y++ {
			d.ybr[y][7] = 0x81
			d.ybr[y][23] = 0x81
		}
	} else {
		for y := 0; y < 17; y++ {
			d.ybr[y][7] = d.ybr[y][7+16]
		}
		for y := 17; y < 26; y++ {
			d.ybr[y][7] = d.ybr[y][15]
			d.ybr[y][23] = d.ybr[y][31]
		}
	}
	if mby == 0 {
		for x := 7; x < 28; x++ {
			d.ybr[0][x] = 0x7f
		}
		for x := 7; x < 16; x++ {
			d.ybr[17][x] = 0x7f
		}
		for x := 23; x < 32; x++ {
			d.ybr[17][x] = 0x7f
		}
	} else {
		for i := 0; i < 16; i++ {
			d.ybr[0][8+i] = d.img.Y[(16*mby-1)*d.img.YStride+16*mbx+i]
		}
		for i := 0; i < 8; i++ {
			d.ybr[17][8+i] = d.img.Cb[(8*mby-1)*d.img.CStride+8*mbx+i]
		}
		for i := 0; i < 8; i++ {
			d.ybr[17][24+i] = d.img.Cr[(8*mby-1)*d.img.CStride+8*mbx+i]
		}
		if mbx == d.mbw-1 {
			for i := 16; i < 20; i++ {
				d.ybr[0][8+i] = d.img.Y[(16*mby-1)*d.img.YStride+16*mbx+15]
			}
		} else {
			for i := 16; i < 20; i++ {
				d.ybr[0][8+i] = d.img.Y[(16*mby-1)*d.img.YStride+16*mbx+i]
			}
		}
	}
	for y := 4; y < 16; y += 4 {
		d.ybr[y][24] = d.ybr[0][24]
		d.ybr[y][25] = d.ybr[0][25]
		d.ybr[y][26] = d.ybr[0][26]
		d.ybr[y][27] = d.ybr[0][27]
	}
}
prepareYBR prepares the {abcdefghij} elements of ybr.
package vp8
const (
	bCoeffBase   = 1*16*16 + 0*8*8
	rCoeffBase   = 1*16*16 + 1*8*8
	whtCoeffBase = 1*16*16 + 2*8*8
)
const (
	ybrYX = 8
	ybrYY = 1
	ybrBX = 8
	ybrBY = 18
	ybrRX = 24
	ybrRY = 18
)