Question on uniswap v3 Fee growth Below

[fffuuuming]

From https://updraft.cyfrin.io/courses/uniswap-v3/fee-algorithm/fee-growth-below

If I understand correctly: during $t_0 \le t < t_1 $, all fee growth are above the tick $I$, then why there exists fee growth below $f_b$ = $f_{g_0}$ that time range ?

That is, why does a red rectangle appear at the lower left? I thought such a rectangle should exist only when there is an increase in fee growth that occurs below tick $i$.

[EngrPips]

@t4sk, Your insight is needed, sir.

[t4sk]

@fffuuuming

all fee growth are above the tick I, then why there exists fee growth below

All fee growth may not be above tick I. When positions are initialized, fee growth can be above or below the tick.
By convention, fee growth below is initialized with the current fee growth. Magically, the math works out when fee growth inside is calculated.

[theblack2ke]

Read this I think it might get you to understand better :
#28

[EngrPips]

Thanks for the heads-up.