BMP row 長度限制

Usually pixels are stored "bottom-up", starting in the lower left corner, going from left to right, and then row by row from the bottom to the top of the image.

Padding bytes (not necessarily 0) must be appended to the end of the rows in order to bring up the length of the rows to a multiple of four bytes. When the pixel array is loaded into memory, each row must begin at a memory address that is a multiple of 4.

以 26 x 26 的圖片來說，要在第 2 個 for loop 尾巴補值，補到 4 的倍數止。但是表頭還是描述 26 x 26，不過實際 size 是 26 x 28 的大小。

for row in range(26):
    for col in range(26):
        add R value
        add G value
        add B value
    # padding
    add value
    add value