Update FORMATS.md

docs/FORMATS.md

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+<br/>
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-TODO: Explanation on how to use `getAvailableCameraDevices()` and `Camera`'s `format={}`, `fps={}` and `hdr={}` props
+<h1 align="center">Formats</h1>
+
+<div>
+  <img align="right" width="35%" src="../img/example.png">
+</div>
+
+### What are camera formats?
+
+Each camera device (see [DEVICES.md](./DEVICES.md)) provides a number of capture formats that have different specifications. There are formats specifically designed for high-resolution photo capture, which have very high photo output quality but in return only support frame-rates of up to 30 FPS. On the other side, there might be formats that are designed for slow-motion video capture which have frame-rates up to 240 FPS.
+
+### What if I don't want to choose a format?
+
+If you don't want to specify the best format for your camera device, you don't have to. The Camera _automatically chooses the best matching format_ for the current camera device. This is why the Camera's `format` property is _optional_.
+
+If you don't want to do a lot of filtering, but still want to let the camera know what your intentions are, you can use the Camera's `preset` property. (🔗 See the [CameraPreset.ts](../src/CameraPreset.ts) type for more information about presets)
+
+For example, use the `'medium'` preset if you want to create a video-chat application that shouldn't excessively use mobile data:
+
+```tsx
+function App() {
+  const devices = useCameraDevices()
+  const device = devices.back
+
+  return (
+    <Camera
+      style={StyleSheet.absoluteFill}
+      device={device}
+      preset="medium"
+    />
+  )
+}
+```
+
+### What you need to know about cameras
+
+To understand a bit more about camera formats, you first need to understand a few "general camera basics":
+
+* Each camera device is built differently, e.g. _Telephoto devices_ often don't provide frame-rates as high as _Wide-Angle devices_.
+* Formats are designed for specific use-cases, so formats with high resolution photo output don't support frame-rates as high as formats with lower resolution.
+* Different formats provide different field-of-views (FOV), maximum zoom factors, color spaces (iOS only), resolutions, frame rate ranges, and systems to assist with capture (auto-focus systems, video stabilization systems, ...)
+
+### Get started
+
+Each application has different needs, so the format filtering is up to you.
+
+To get all available formats, simply use the `CameraDevice`'s `.formats` property. See how to get a camera device in the [DEVICES.md](./DEVICES.md) doc.
+
+> Note: You can also manually get all camera devices and decide which device to use based on the available `formats`. In fact, this is how we do it in the [Cuvent](https://cuvent.com) app.
+
+This example shows how you would pick the format with the _highest frame rate_:
+
+```tsx
+function getMaxFps(format: CameraDeviceFormat): number {
+  return format.frameRateRanges.reduce((prev, curr) => {
+    if (curr.maxFrameRate > prev) return curr.maxFrameRate;
+    else return prev;
+  }, 0);
+}
+
+function App() {
+  const devices = useCameraDevices('wide-angle-camera')
+  const device = devices.back
+
+  const format = useMemo(() => {
+    return device?.formats.reduce((prev, curr) => {
+      if (prev == null) return curr;
+      if (getMaxFps(curr) > getMaxFps(prev)) return curr;
+      else return prev;
+    }, undefined);
+  }, [device?.formats]);
+
+  return (
+    <Camera
+      style={StyleSheet.absoluteFill}
+      device={device}
+      format={format}
+    />
+  )
+}
+```
+
+Note that you don't want to simply pick the highest frame rate, as those formats often have incredibly low resolutions. You want to find a balance between high frame rate and high resolution, so instead you might want to use the `.sort` function.
+
+### Sort
+
+To sort your formats, create a custom comparator function which will be used as the `.sort` function's argument. The custom comparator then compares formats, preferring ones with higher frame rate AND higher resolution.
+
+Implement this however you want, I personally use a "point-based system":
+
+```ts
+export const sortFormatsByResolution = (left: CameraDeviceFormat, right: CameraDeviceFormat): number => {
+  // in this case, points aren't "normalized" (e.g. higher resolution = 1 point, lower resolution = -1 points)
+  let leftPoints = left.photoHeight * left.photoWidth;
+  let rightPoints = right.photoHeight * right.photoWidth;
+
+  if (left.videoHeight != null && left.videoWidth != null && right.videoHeight != null && right.videoWidth != null) {
+    leftPoints += left.videoWidth * left.videoHeight;
+    rightPoints += right.videoWidth * right.videoHeight;
+  }
+
+  // you can also add points for FPS, etc
+
+  return rightPoints - leftPoints;
+};
+
+// and then call it:
+const formats = useMemo(() => device?.formats.sort(sortFormatsByResolution), [device?.formats])
+```
+
+Be careful that you don't `filter` out a lot of formats since you might end up having no format to use at all. (_Remember; not all devices support e.g. 240 FPS._) Always sort them and pick the best format, that way you are guaranteed to have a format available, even if your desired specifications aren't fully met.
+
+### Props
+
+The `Camera` View provides a few props that depend on the specified `format`. For example, you can only set the `fps` prop to a value that is supported by the current `format`. So if you have a format that supports 240 FPS, you can set the `fps` to `240`:
+
+```tsx
+function App() {
+  return (
+    <Camera
+      style={StyleSheet.absoluteFill}
+      device={device}
+      format={format}
+      fps={240}
+    />
+  )
+}
+```
+
+> Note: You should always verify that the format supports the desired FPS, and fall back to `undefined` (or a value that is supported, like `30`) if it doesn't.
+
+Other props that depend on the `format`:
+
+* `hdr`: Enables HDR photo or video capture and preview
+* `lowLightBoost`: Enables a night-mode/low-light-boost for photo or video capture and preview
+* `colorSpace`: Uses the specified color-space for photo or video capture and preview (iOS only since Android only uses `YUV`)