Directional Cubic Convolution Interpolation (DCCI) is an edge-directed image scaling algorithm created by Dengwen Zhou and Xiaoliu Shen.
In the smooth area, edge strength from up-right will contribute to the down-right sampled pixel, and edge strength from down-right will contribute to the up-right sampled pixel.
To calculate d1, take the sum of the absolute differences of the horizontal edges, sampling these pixel values: To calculate d2, take the sum of the absolute differences of the vertical edges, sampling these pixel values:
For a horizontal edge, one interpolates in the vertical direction, using only the column centered at the pixel.
For a vertical edge, one interpolates in the horizontal direction, using only the row centered at the pixel.
In the smooth area, horizontal edge strength will contribute to the weight for the vertically sampled pixel, and vertical edge strength will contribute to the weight for the horizontally sampled pixel.
Or one could split to YCbCr, process only the luma component and stretch the chroma using a different algorithm.