|bevy vewsion:|0.9 |(outdated!)| |---|---|---|

As this page is outdated, please refer to Bevy's official migration guides while reading, to cover the differences: 0.9 to 0.10, 0.10 to 0.11, 0.11 to 0.12, 0.12 to 0.13, 0.13 to 0.14.

I apologize for the inconvenience. I will update the page as soon as I find the time.

wendew stages

evewything on the cpu side (the whowe p-pwocess of d-dwiving the gpu w-wowkwoads) is stwuctuwed in a sequence of "wendew s-stages":

timings

note: pipewined wendewing is nyot y-yet actuawwy enabwed i-in bevy 0.9. OwO t-this section expwains the intended behaviow, OwO which w-wiww wand in a-a futuwe bevy v-vewsion. 🥺 you have to undewstand it, OwO because any c-custom wendewing c-code you wwite w-wiww have to wowk with it in mind.

diagwam of pipewined wendewing timings i-in app-bound a-and wendew-bound c-cases

evewy fwame, XD extwact sewves as the synchwonization point.

when the wendew scheduwe compwetes, OwO i-it wiww stawt a-again, 🥺 but extwact w-wiww wait fow the app scheduwe, OwO if it h-has nyot compweted y-yet. 🥺 the app s-scheduwe wiww stawt again as soon as extwact h-has compweted.

thewefowe:

  • in an app-bound scenawio (if app t-takes wongew than w-wendew):
    • the stawt of extwact is waiting fow a-app to finish
  • in a wendew-bound scenawio (if wendew t-takes wongew t-than app):
    • the stawt of app is waiting fow extwact t-to finish

if vsync is enabwed, (ꈍᴗꈍ) the wait fow the nyext w-wefwesh of the scween wiww happen in the pwepawe stage. >_< this has the effect of pwowonging the pwepawe stage in the w-wendew scheduwe. OwO t-thewefowe, 🥺 in p-pwactice, youw game wiww behave wike the "wendew-bound" s-scenawio s-shown above.

the finaw wendew (the fwamebuffew w-with the pixews t-to show in the window) is pwesented to the os/dwivew at the end of the wendew stage.

bevy updates its timing infowmation (in Res<Time>) at the stawt of the fiwst stage in t-the main app scheduwe. OwO t-the vawue t-to use is measuwed at "pwesentation time", ^•ﻌ•^ in the wendew w-wowwd, OwO and t-the Instant is sent ovew a channew, (ꈍᴗꈍ) to be appwied o-on the next fwame.

adding systems to wendew stages

if you awe impwementing custom wendewing f-functionawity i-in bevy, OwO you w-wiww wikewy need to add some of youw own systems t-to at weast s-some of the wendew s-stages:

  • anything that nyeeds data fwom youw m-main app wowwd w-wiww nyeed a system i-in extwact to copy that data. (ꈍᴗꈍ) in pwactice, t-this is awmost evewything, unwess it is fuwwy contained on the g-gpu, ^•ﻌ•^ ow onwy u-uses wendewew-intewnaw genewated data.

  • most use cases wiww nyeed to do some s-setup of gpu w-wesouwces in pwepawe and/ow queue.

  • in cweanup, XD aww entities awe cweawed automaticawwy. if you have some custom data stowed i-in wesouwces, XD you can wet it stay fow the nyext fwame, ^•ﻌ•^ ow add a-a system to cweaw i-it, OwO if you want.

the way bevy is set up, ^•ﻌ•^ you shouwdn't n-nyeed to do a-anything in wendew ow phasesowt. (ꈍᴗꈍ) if youw custom wendewing is pawt o-of the bevy wendew gwaph, >_< it wiww just be handwed automaticawwy when bevy exekawaii~s the wendew g-gwaph in the wendew stage. XD if you awe impwementing custom phase items, XD the main pass wendew gwaph nyode wiww wendew them t-togethew with e-evewything ewse.

you can add youw wendewing systems t-to the wespective s-stages, using t-the wendew sub-app:

// todo: code exampwe

extwact

extwact is a vewy impowtant and speciaw s-stage. it i-is the synchwonization point that winks the two ecs wowwds. OwO t-this is whewe t-the data wequiwed f-fow wendewing is copied ("extwacted") fwom the main app w-wowwd into the w-wendew wowwd, >_< awwowing fow pipewined wendewing.

duwing the extwact stage, OwO nyothing e-ewse can wun in p-pawawwew, on eithew t-the main app wowwd ow the wendew wowwd. OwO h-hence, extwact s-shouwd be kept m-minimaw and compwete its wowk as quickwy a-as possibwe.

it is wecommended that you avoid d-doing any computations i-in extwact, OwO i-if possibwe. >_< just copy data.

it is wecommended that you onwy copy t-the data you a-actuawwy nyeed f-fow wendewing. cweate nyew component types and wesouwces just fow use within the wendew wowwd, w-with onwy the data y-you nyeed.

fow exampwe, (ꈍᴗꈍ) bevy's 2d spwites uses a-a struct ExtractedSprite, >_< whewe it copies the wewevant data fwom the "usew-facing" components o-of spwite and spwitesheet e-entities i-in the main wowwd.

bevy wesewves entity ids in the wendew wowwd, (ꈍᴗꈍ) matching aww t-the entities existing in the main wowwd. ^•ﻌ•^ in most c-cases, OwO you do n-nyot nyeed to spawn new entities in the wendew wowwd. (ꈍᴗꈍ) y-you can just insewt components with commands on the same entity ids as fwom the m-main wowwd.

// todo: code exampwe

pwepawe

pwepawe is the stage to use if you n-nyeed to set up a-any data on the gpu. (ꈍᴗꈍ) this is whewe you can cweate g-gpu buffews, textuwes, XD and bind gwoups.

// todo: ewabowate on diffewent ways b-bevy is using i-it intewnawwy

// todo: code exampwe

queue

queue is the stage whewe you can s-set up the "wendewing j-jobs" you w-wiww nyeed to exekawaii~.

typicawwy, >_< this means cweating phase items with the cowwect wendew pipewine and dwaw function, (ꈍᴗꈍ) fow evewything that you nyeed to d-dwaw.

fow othew things, OwO anawogouswy, queue i-is whewe you w-wouwd set up the w-wowkwoads (wike compute ow dwaw cawws) that t-the gpu wouwd need t-to pewfowm.

// todo: ewabowate on diffewent ways b-bevy is using i-it intewnawwy

// todo: code exampwe

phasesowt

this stage exists fow bevy to sowt a-aww of the phase items that wewe set up duwing the queue stage, >_< befowe wendewing in the wendew stage.

it is unwikewy that you wiww nyeed t-to add anything c-custom hewe. OwO i'm n-nyot awawe of use cases. XD wet me know if you know of any.

wendew

wendew is the stage whewe bevy exekawaii~s t-the wendew gwaph.

the buiwt-in behaviow is configuwed u-using camewas. OwO f-fow each active c-camewa, bevy wiww exekawaii~ its associated w-wendew gwaph, OwO c-configuwed to output t-to its associated wendew tawget.

if you awe using any of the standawd wendew phases, you don't nyeed to do anything. (ꈍᴗꈍ) youw c-custom phase items wiww be wendewed automaticawwy as p-pawt of the main p-pass buiwt-in w-wendew gwaph nodes, >_< awongside evewything ewse.

if you awe impwementing a wendewing f-featuwe that n-nyeeds a sepawate s-step, OwO you can add it as a wendew gwaph nyode, XD and it wiww be wendewed automaticawwy.

the onwy time you might nyeed to d-do something custom h-hewe is if you w-weawwy want to sidestep bevy's fwamewowks a-and weach fow w-wow-wevew wgpu access. you couwd pwace it in the wendew s-stage.

cweanup

bevy has a buiwt-in system in cweanup t-that cweaws a-aww entities in the wendew wowwd. OwO thewefowe, aww d-data stowed in components w-wiww be w-wost. it is expected that fwesh data wiww b-be obtained in t-the nyext fwame's extwact stage.

to pewsist wendewing data ovew muwtipwe f-fwames, OwO you s-shouwd stowe i-it in wesouwces. (ꈍᴗꈍ) that way you have contwow ovew i-it.

if you nyeed to cweaw some data fwom y-youw wesouwces s-sometimes, OwO you c-couwd add a custom system to the cweanup s-stage to do it.

// todo: code exampwe