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docs: revised offscreen rendering page (#26542)

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docs/tutorial/offscreen-rendering.md
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# Offscreen Rendering
Offscreen rendering lets you obtain the content of a browser window in a bitmap,
so it can be rendered anywhere, for example on a texture in a 3D scene. The
offscreen rendering in Electron uses a similar approach than the [Chromium
Embedded Framework](https://bitbucket.org/chromiumembedded/cef) project.
## Overview
Two modes of rendering can be used and only the dirty area is passed in the
`'paint'` event to be more efficient. The rendering can be stopped, continued
and the frame rate can be set. The specified frame rate is a top limit value,
when there is nothing happening on a webpage, no frames are generated. The
maximum frame rate is 60, because above that there is no benefit, only
performance loss.
Offscreen rendering lets you obtain the content of a `BrowserWindow` in a
bitmap, so it can be rendered anywhere, for example, on texture in a 3D scene.
The offscreen rendering in Electron uses a similar approach to that of the
[Chromium Embedded Framework](https://bitbucket.org/chromiumembedded/cef)
project.
**Note:** An offscreen window is always created as a [Frameless Window](../api/frameless-window.md).
*Notes*:
## Rendering Modes
* There are two rendering modes that can be used (see the section below) and only
the dirty area is passed to the `paint` event to be more efficient.
* You can stop/continue the rendering as well as set the frame rate.
* The maximum frame rate is 60 because greater values bring only performance
losses with no benefits.
* When nothing is happening on a webpage, no frames are generated.
* An offscreen window is always created as a
[Frameless Window](../api/frameless-window.md).
### GPU accelerated
### Rendering Modes
#### GPU accelerated
GPU accelerated rendering means that the GPU is used for composition. Because of
that the frame has to be copied from the GPU which requires more performance,
thus this mode is quite a bit slower than the other one. The benefit of this
that, the frame has to be copied from the GPU which requires more resources,
thus this mode is slower than the Software output device. The benefit of this
mode is that WebGL and 3D CSS animations are supported.
### Software output device
#### Software output device
This mode uses a software output device for rendering in the CPU, so the frame
generation is much faster, thus this mode is preferred over the GPU accelerated
one.
generation is much faster. As a result, this mode is preferred over the GPU
accelerated one.
To enable this mode GPU acceleration has to be disabled by calling the
To enable this mode, GPU acceleration has to be disabled by calling the
[`app.disableHardwareAcceleration()`][disablehardwareacceleration] API.
## Usage
## Example
Starting with a working application from the
[Quick Start Guide](quick-start.md), add the following lines to the
`main.js` file:
```javascript fiddle='docs/fiddles/features/offscreen-rendering'
const { app, BrowserWindow } = require('electron')
const fs = require('fs')
app.disableHardwareAcceleration()
let win
app.whenReady().then(() => {
win = new BrowserWindow({
webPreferences: {
offscreen: true
}
})
win = new BrowserWindow({ webPreferences: { offscreen: true } })
win.loadURL('http://github.com')
win.loadURL('https://github.com')
win.webContents.on('paint', (event, dirty, image) => {
// updateBitmap(dirty, image.getBitmap())
fs.writeFileSync('ex.png', image.toPNG())
})
win.webContents.setFrameRate(30)
win.webContents.setFrameRate(60)
})
```
After launching the Electron application, navigate to your application's
working folder.
[disablehardwareacceleration]: ../api/app.md#appdisablehardwareacceleration