A mathematical calculation method for an image-capturing device includes steps: estimating a green value of a red, blue pixel; calculating four differences between an estimated red value and four neighboring green pixels, weighing proportions of the four neighboring green pixels, and summing up the differences between the four products induced by multiplying the four differences and the four proportions respectively, as a green value G3' in the red pixel. A green value of a blue pixel will be obtained by mentioned steps. A blue value of the red pixel occurs when the following steps: figuring out differences between four green values and four blue values of four neighboring blue pixels, averaging the four differences, and subtracting the average of the four differences from the green value of the red pixel. A red value of the blue pixel and a red, blue value of the green pixel can be estimated, too.