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# Mask And Alpha Workflows

## Mask-Based Exposure

Use `ProcessTextureWithMask()` when only part of the image should affect exposure.

This is useful when:

* A person is standing in front of a bright window
* The subject should be exposed correctly but the background should be ignored
* A segmentation mask is available
* A chroma-key or custom mask texture defines the foreground

```csharp
RenderTexture result = autoExposure.ProcessTextureWithMask(inputTexture, maskTexture);
```

## Mask Rules

* White pixels are included in brightness analysis
* Black pixels are ignored
* Mask values above the internal threshold are included
* Mask values at or below the threshold are ignored
* The current implementation treats the mask as included/excluded, not as weighted partial coverage

The internal threshold is `10` on the `0` to `255` readback scale. In normalized terms, that is roughly `0.04`.

If no mask pixels pass the threshold, the plugin keeps the previous exposure target.

## Where Masks Come From

Common mask sources include:

* Person segmentation from AR or vision systems
* Chroma-key output
* Manually authored grayscale textures
* UI alpha masks
* RenderTextures generated by a custom shader

The mask does not crop the final image. It only changes which pixels are measured.

## Alpha-Aware UI Processing

`AutoExposureRawImage` and `AutoExposureImage` can account for alpha.

When `Consider Alpha Channel` is enabled:

* Opaque pixels drive the brightness calculation
* Transparent pixels are ignored
* This prevents invisible borders from affecting exposure

### RawImage Alpha

`AutoExposureRawImage` extracts alpha with the bundled shader:

```
Assets/AsyncAutoExposure/Runtime/Resources/Shaders/AlphaExtract.shader
```

Supported source formats:

* `Texture2D`: `ARGB32`, `RGBA32`, `BGRA32`
* `RenderTexture`: `ARGB32`, `BGRA32`

If the texture format does not expose alpha in one of those formats, the component falls back to standard full-frame metering.

### Image Alpha

`AutoExposureImage` extracts sprite alpha through `Texture2D.GetPixels()`. For alpha-aware sprite metering, the sprite texture must be readable in Unity import settings.

If a sprite texture is not readable, either enable Read/Write on that texture or disable `Consider Alpha Channel`.

## Recommended Mask Workflow

For live subject workflows:

1. Generate or provide a subject mask.
2. Pass the source texture and mask to `ProcessTextureWithMask()`.
3. Display the returned `RenderTexture`.
4. Tune `targetBrightness`, `minExposure`, and `maxExposure`.

```csharp
RenderTexture corrected = autoExposure.ProcessTextureWithMask(source, subjectMask);
display.texture = corrected;
```

## Tuning Tips

| Problem                                              | Adjustment                                |
| ---------------------------------------------------- | ----------------------------------------- |
| Subject is still too dark                            | Raise `targetBrightness` or `maxExposure` |
| Subject is blown out                                 | Lower `targetBrightness` or `maxExposure` |
| Background changes cause flicker                     | Use a tighter foreground mask             |
| Transparent UI padding affects exposure              | Enable `Consider Alpha Channel`           |
| Static sprite does not refresh after settings change | Call `ForceUpdate()`                      |


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