Currently I need to add the RenderTargetTexture into scene.customRenderTargets to make it work during rendering, and I think it will render every frame.
But what if I do not need to render/update the RenderTargetTexture in every frame, I wonder if there if a manual way to only render/update the RenderTargetTexture only when I need.
You can manually tell it to render RenderTargetTexture | Babylon.js Documentation (babylonjs.com) or set the refreshrate RenderTargetTexture | Babylon.js Documentation (babylonjs.com)
But when I simply use render method of RTT, the separate active camera I set for this RTT seems not work.
Could you show us a playground repro of this?
You can check out this playground
Seems this was a bug, have a PR up to fix it: Fix render method of RTT not using its active camera by carolhmj · Pull Request #12794 · BabylonJS/Babylon.js (github.com)