A video picture coding or decoding method, the method comprising: determining a distance between control points of an affine picture block; determining (1100, 2100) a motion vector difference of the affine picture block based on motion vectors of the control points; and performing (1400, 2400) coding processing on the affine picture block including an affine motion compensation picture subblock, wherein a size of the affine motion compensation picture subblock varies based on the motion vector difference, a motion vector precision, and the distance between the control points of the affine picture block, wherein the size includes a horizontal length and a vertical length, wherein the motion vector difference includes a horizontal component and a vertical component, the control points include a first control point, a second control point, a third control point, and a fourth control point; wherein the determining (1100, 2100) the motion vector difference of the affine picture block comprises: determining (1101, 2101) the horizontal component of the motion vector difference according to a difference between motion vectors of the first control point and the second control point, wherein the first control point and the second control point are located on a same horizontal line; and determining (1102, 2102) the vertical component of the motion vector difference according to a difference between motion vectors of the third control point and the fourth control point, wherein the third control point and the fourth control point are located on a same vertical line. determining a distance between control points of an affine picture block; determining (1100, 2100) a motion vector difference of the affine picture block based on motion vectors of the control points; and performing (1400, 2400) coding processing on the affine picture block including an affine motion compensation picture subblock, wherein a size of the affine motion compensation picture subblock varies based on the motion vector difference, a motion vector precision, and the distance between the control points of the affine picture block, wherein the size includes a horizontal length and a vertical length, wherein the motion vector difference includes a horizontal component and a vertical component, the control points include a first control point, a second control point, a third control point, and a fourth control point; wherein the determining (1100, 2100) the motion vector difference of the affine picture block comprises: determining (1101, 2101) the horizontal component of the motion vector difference according to a difference between motion vectors of the first control point and the second control point, wherein the first control point and the second control point are located on a same horizontal line; and determining (1102, 2102) the vertical component of the motion vector difference according to a difference between motion vectors of the third control point and the fourth control point, wherein the third control point and the fourth control point are located on a same vertical line. determining (1101, 2101) the horizontal component of the motion vector difference according to a difference between motion vectors of the first control point and the second control point, wherein the first control point and the second control point are located on a same horizontal line; and determining (1102, 2102) the vertical component of the motion vector difference according to a difference between motion vectors of the third control point and the fourth control point, wherein the third control point and the fourth control point are located on a same vertical line.