# Why is my half resolution render taking a quarter of the time of the full one?

I render an animation at 200 samples; $1920$ x $1080$, using Cycles.

A still takes about 2min44s to be rendered. Tiles are $480$ x $360$.

I noticed that when doing half resolution renders, the same still would take only 43s, which means only 26 % of the full resolution time.

Is it a normal behaviour ?

• a 50% resolution of a 1920 x 1080 will give a 960 x 540 picture, and you need 4 pictures of 960 x 540 to fulfill a 1920 x 1080... actually it"s half the length or height size, not half the dimension – moonboots Jul 9 '18 at 16:36
• I think it has something to do with the exponential growth of tiles as the render percentage increases. – Legoman Jul 9 '18 at 16:36

That is simple math, area doesn't scale linearly, it has a quadratic relation to the dimensions of the shape, if you double the size you quadruple the area hence a four times the render time.

If the area of a $4 \times 4 = 16$ and you halve it then $2 \times 2 = 4$ which is one fourth

Hence you get a quarter of the area and consequently one fourth of pixels to calculate; that's about four times less computation power required leading to roughly one fourth of render time, give or take, for the same scene and settings.

• Of course ! :/ Bad thinking I had ! – gordie Jul 9 '18 at 16:48
• Unrelated to the question, but, in Mathjax, I think it would be better to denote multiplication either by using \times or \cdot (For instance, 4 \cdot 4 > $4 \cdot 4$, 4 \times 4 > $4 \times 4$) – Omar Ahmad Jul 9 '18 at 20:03
• Ah many thanks @OmarAhmad . I wondered if there was a better way to represent that, didn't have the opportunity to investigate. \times works like a charm. Answer updated! – Duarte Farrajota Ramos Jul 9 '18 at 23:16

Following on Duarte's answer this may help to visualize it better.

If you look at the purple (Full HD) square you would need 4 of them to fill up the (4K blue square) So the render would take 4 times as long even though you are only doubling the resolution (vice versa).

Here is a birdseye view of resolutions that I use as a guide.

• I should note, I have no clue where this chart came from. I have had it printed for years and the above is just a picture of it from an iPhone. I just find it helpful, Thought others might too.