1. A method comprising: determining, with a video processing device, for a plurality of coded units arranged in a plurality of vectors from a portion of a video frame, a first set of horizontal parameter candidates for a first horizontal parameter of a global motion model for the portion of the video frame without requiring access to pixel values of the video frame, wherein each of the horizontal parameter candidates of the first set of horizontal parameter candidates corresponds to one of a first subset of the plurality of vectors; selecting, with the video processing device, a median value from the first set of horizontal parameter candidates as the first horizontal parameter of the global motion model for the portion of the video frame; determining, with the video processing device, a first set of vertical parameter candidates for a first vertical parameter of the global motion model for the portion of the video frame without requiring access to the pixel values of the video frame, wherein each of the vertical parameter candidates of the first set of vertical parameter candidates corresponds to one of a second subset of the plurality of vectors; selecting, with the video processing device, a median value from the first set of vertical parameter candidates as the first vertical parameter of the global motion model for the portion of the video frame; determining a second set of horizontal parameter candidates for a second horizontal parameter of the global motion model, wherein each of the horizontal parameter candidates of the second set of horizontal parameter candidates corresponds to one of the first subset of the plurality of vectors; selecting a median value from the second set of horizontal parameter candidates as the second horizontal parameter of the global motion model for the portion of the video frame; determining a second set of vertical parameter candidates for a second vertical parameter of the global motion model, wherein each of the vertical parameter candidates of the second set of vertical parameter candidates corresponds to one of the second subset of the plurality of vectors; selecting a median value from the second set of vertical parameter candidates as the second vertical parameter of the global motion model for the portion of the video frame; and processing, with the video processing device, at least one of the plurality of coded units according to the first and second horizontal parameters and the first and second vertical parameters, wherein the global motion model comprises an affine model that is expressed as: 
 MV — x i =b 2 *x i +b 1 , 
 and 
 MV — y j =c 2 *y j +c 1 , wherein the first horizontal parameter comprises b 1 , the first vertical parameter comprises c 1 , the second horizontal parameter comprises b 2 , the second vertical parameter comprises c 2 , x i comprises a row index, y j comprises a column index, MV_x i comprises a horizontal motion vector component, and MV_y j comprises a vertical motion vector component, wherein determining the first set of horizontal parameter candidates and determining the second set of horizontal parameter candidates comprises calculating, for each vector i, a value for B i , where B i comprises a matrix [ b 2 vector - i b 1 vector - i ] , wherein the first set of horizontal parameter candidates comprises b 1 — vector-i for each vector i, wherein the second set of horizontal parameter candidates comprises b 2 — vector-i for each vector i, and wherein calculating the value of B i comprises calculating (X T X) −1 X T MV x , wherein X comprises a matrix [ 1 i 2 i … … C i ] , wherein C corresponds to a number of vectors in the second subset of the plurality of vectors, wherein X T comprises a transpose matrix of X, and wherein MV x comprises a matrix [ 1 * ⁢ b 2 ⁢ _ ⁢ vector - i + b 1 ⁢ _ ⁢ vector - i 2 * ⁢ b 2 ⁢ _ ⁢ vector - i + b 1 ⁢ _ ⁢ vector - i … C * ⁢ b 2 ⁢ _ ⁢ vector - i + b 1 ⁢ _ ⁢ vector - i ] , and wherein determining the first set of vertical parameter candidates and determining the second set of vertical parameter candidates comprises calculating, for each vector j, a value for C j , where C j comprises a matrix [ c 2 vector - j c 1 column - j ] , wherein the first set of vertical parameter candidates comprises c i — vector-j for each vector j, wherein the second set of vertical parameter candidates comprises c 2 — vector-j for each vector j, and wherein calculating the value of C j comprises calculating (Y T Y) −1 Y T MV y , wherein Y comprises a matrix [ 1 i 2 i … … R i ] , wherein R corresponds to a number of vectors in the first subset of the plurality of vectors, wherein Y T comprises a transpose matrix of Y, and wherein MV y comprises a matrix [ 1 * ⁢ c 2 ⁢ _ ⁢ vector - j + c 1 ⁢ _ ⁢ vector - j 2 * ⁢ c 2 ⁢ _ ⁢ vector - j + c 1 ⁢ _ ⁢ vector - j … R * ⁢ c 2 ⁢ _ ⁢ vector - j + c 1 ⁢ _ ⁢ vector - j ] .