Thinking about custom_red_filter

Now that we have defined the function for the custom red filter as below, let us analyze its execution on few inputs:

The output, lightly formatted, is:

[[(240, 240, 200), (200, 0, 0)],
 [(160, 240, 120), (120, 0, 0)]]

Let us analyze how we arrive at this output when we call the function custom_red_filter with the above given inputs.

• The outer for loop goes from top_left[0] which is 0 to top_left[0] + height which is 0+2 = 2. So the outer for loop range would be 0 to 2 (not inclusive). This means that the outer for loop runs for two iterations covering two rows: row index 0 and row index 1 (since range starts from 0 and goes till 2 which is the end point and the end point specified in range function is not inclusive.

• The inner for loop goes from top_left[1] which is 1 to top_left[1] + width which is 1+1 = 2. So the inner for loop range would be 1 to 2 (not inclusive). This means that the inner for loop runs for one iteration covering one column: column index 1 (since range starts from 1 and goes till 2 which is the end point and the end point specified in range function is not inclusive)

So we can see in the final output that the pixel at (row index 0, column index 1) and pixel at (row index 1, column index 1) is only modified to have the blue and green components to be 0 while retaining the red component.

Similarly, try to analyze how we arrived at the output for the 4*4 image specified when we introduced red_filter of this lesson.