How to generate a gradated visual based on multiple coordinates on the matrix

Hello.

I would like to ask how to generate a gradated visual spectrum look from the multiple coordinates on matrix.

for example,
there are two (x.y) coordinates on a 320x240 matrix
for calculating those from (0,0), it will like this include beginning and ending value of the matrix coordinates.
(0,0) , (x1,y1) , (x2,y2) , (320,0)

by using y axis of those (0,y1,y2,0)
generates a gradated spectrum look on x axis of the matrix
(horizontal B/W gradation with y coord. as a brightness value 0 = white, 240 = black)
with a threshold value for a stiffness of the gradation by finding even value between each y coordinates from the matrix, then smooth the gradation curves.
I attached an image for the description.

thanks