Rendering at Half Resolution

[trashcaneron]

Kinda weird, but found an odd behavior when rendering half resolution PNG Multi Machine 2x Render Engines (BG Render).

So my final delivery comp is:

11520x2160

I have previews at 1/4 resolution:

2880x540

So I pre-comped the original asset and scaled it down to 1/4. It takes 30 seconds per frame.

However, and here's the confusing thing, if I precomp and scale it down to half resolution (5760x1080), and then choose half resolution as my output module, I render at about 12 seconds a frame, which is the same resolution of the 1/4 rez. More than twice the speed of rendering it as a scaled precomp at full resolution. Comparing the frames side by side, and overlayed, I can't see a difference.

I have also tested this with a single instance of BG render and get the same (scaled) results.

Any ideas about this? Perhaps I've misunderstood how rendering works.

[OussK]

Yes i think it's logic on AE, where rendered a composition with 1/4 resolution isn't same as pre-comp the composition to the 1/4 resolution and rendered on full quality. e.g

1920x1080 on quarter quality it's not same as pre-comp to 960x540 at full quality since in the pre-comp option (second one) AE will render every pixel than reduce the size to 960x540 to give you the best quality but in the first option where you tell AE to render on quarter resolution AE will render each 4 pixel and save the file on 960x540 and here you notice the difference in render speed.

Now depending to quality it will be almost same if you don't use any image or video file (rasterize assets), so if you render only shapes or text  generated on AE it self you will get same output quality in both scenario.

[trashcaneron]

I figured it would be something like that, but full disclosure, I am using video and images and I don't see a discernible difference. That's what's so confusing to me! With that mindset, I could hypothetically double my final comp size and then render at half resolution and get better render times with no discernible difference. Crazy.

[imeilfx]

Yes. But question is why. If you need smaller comp/final clip - just work with smaller comp.

[trashcaneron]

Right, my final delivery is 11520x2160

The point I'm making is that: If I double the comp size in a pre-comp at 23,040x4,320 and then render at half resolution it'll render faster than the OG comp size (11520) without a discernible difference.

[imeilfx]

OussK you overtook my answer. I was just writing exactly the same thing If you precomp and rescale a composition - AE will interprete every each pixel from original resolution and then resize it. Just like if you work with 1080p composition and you put photos straight from camer f.eg with 16megapixels  - even if you rescale it in AE to fit your comp - render will be looong because AE will read and interpret all 16megapixels from original photo during render.

When you put half or 1/4 resolution on output AE will read and interpret only every 2 or every 4 th pixel so will render 2 or 4 times faster - ok that is over simplification but much faster for sure.