How can I gradient a multicoloured line into another to create a cleanly gradiented rectangle?

Question

I'm trying to blend one vertical line of multicoloured pixels (1px width, 200 px height) on the left, into another vertical line of multicoloured pixels (1px width 200px height) on the right, with a spacing of 500px between them, to create a 500x200 rectangle where each horizontal row of pixels is a smooth gradient between the corresponding end pixels on the left and the right.

Is there a standard way of doing this? I know how to gradient from one pixel into another, but I'm unsure how to do it for a whole set of pixels like this into another set of pixels. I could do it one-by-one but that would be very slow.

I'm using Paint.net; if there is a feature in it that does what I need I'd love to be pointed to it.

Answer 1

I commented earlier that I couldn't find a solution for paint.net, but I couldn't leave it and finally found out how to do it using a transparent gradient.

• Create a document with the dimensions 500x200px (the final size).

• Place one of your lines in the left side and the other in the right side, like this:

  

  Make sure that the lines are on separate layers.

• Now select one of the lines with Rectangle Select and switch tool to Move Selected Pixels:

  

• Pull the middle handle to stretch the line to the width of the canvas:
• Do the same with the other line:
• Select the top layer.
• Select the Gradient tool and make sure that it is set to Transparency Mode:
• With the mouse, click and drag a line from one end of the image to the other. Hold down shift to make sure the line is horizontal:

That's it!