> For the complete documentation index, see [llms.txt](https://docs.roboflow.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.roboflow.com/workflows/blocks/blocks/classical-computer-vision/contrast-enhancement.md).

# Contrast Enhancement

Enhance image contrast using histogram normalization (the algorithm from GIMP's Auto Levels). This block stretches the image histogram to use the full available range \[0-255], improving visibility of features with low contrast.

## How This Block Works

1. **Channel Analysis**: For grayscale images, find min/max directly. For color images, analyze each channel independently.
2. **Histogram Normalization**: For each channel, stretch values from \[min, max] to \[0, 255] using linear scaling: `output = (input - min) / (max - min) * 255`
3. **Clipping**: Values outside \[0, 255] are clipped to the valid range

## Common Use Cases

* **Low-contrast medical imaging**: Normalize tissue visibility across varying acquisition parameters
* **Industrial inspection**: Enhance subtle surface defects on dull materials
* **Surveillance footage**: Improve nighttime or backlit scene visibility
* **Document scanning**: Brighten poorly lit document photos
* **Microscopy**: Boost signal from weak fluorescence or phase-contrast images

## Input Parameters

**image** : Input image to enhance (color or grayscale)

* Can be single-channel, 3-channel (BGR), or 4-channel (BGRA)
* Each channel is normalized independently for color images

**clip\_limit** : Percentage of histogram range to skip at extremes (default: 0)

* Range: 0-50
* 0: No clipping, entire histogram from min to max is used
* 1-3: Skip dark and bright outliers (robust to noise)
* 5-10: Very aggressive outlier removal
* 20-50: Extreme outlier removal, may lose subtle details

**contrast\_multiplier** : Multiplier for contrast scaling after normalization (default: 1.0)

* Range: 0.1-5.0
* 1.0: No additional scaling, just normalization
* 0.5-0.9: Reduce contrast for smoother images
* 1.1-2.0: Increase contrast for more dramatic enhancement

**normalize\_brightness** : Apply brightness normalization using midtone equalization (default: False)

* False: Only histogram normalization and contrast scaling
* True: After histogram normalization, apply midtone adjustment for balanced brightness

## Outputs

**image** : Enhanced image with normalized contrast, same shape and type as input

## Notes

* **Sensitive to outliers**: Extreme min/max values (single dark/bright pixels) stretch most of the histogram into a narrow range. Use morphological opening (Morphological Transformation v2) as preprocessing to remove spurious dark/bright specks.
* **Color shift**: For color images, each channel stretches independently, which can shift hue if channels have very different dynamic ranges
* **Efficiency**: Very fast - linear scan for min/max, then linear transformation per pixel
* **Brightness normalization**: When enabled, applies midtone stretch (gamma ≈ 1.3) for more balanced perceived brightness

### Type identifier

Use the following identifier in step `"type"` field: `roboflow_core/contrast_enhancement@v1` to add the block as a step in your workflow.

### Properties

| **Name**               | **Type** | **Description**                                                                                                                                                                                                      | Refs |
| ---------------------- | -------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---- |
| `name`                 | `str`    | Enter a unique identifier for this step..                                                                                                                                                                            | ❌    |
| `clip_limit`           | `int`    | Percentage of histogram range to skip at dark and bright extremes. 0: use full range from min to max. 1-3: skip outliers (robust). 5-10: very aggressive outlier removal..                                           | ❌    |
| `contrast_multiplier`  | `float`  | Multiplier for contrast scaling after normalization. 1.0: no additional scaling (just histogram normalization). <1.0: reduce contrast. >1.0: increase contrast for more dramatic enhancement..                       | ❌    |
| `normalize_brightness` | `bool`   | Apply brightness normalization using midtone equalization. When False, only histogram normalization and contrast scaling are applied. When True, applies midtone adjustment for more balanced perceived brightness.. | ❌    |

The **Refs** column marks possibility to parametrise the property with dynamic values available in `workflow` runtime. See *Bindings* for more info.

### Input and Output Bindings

The available connections depend on its binding kinds. Check what binding kinds `Contrast Enhancement` in version `v1` has.

<details>

<summary>Input and output bindings</summary>

* input
  * `image` ([*`image`*](/workflows/developer-guide/developer-guide/kinds/image.md)): Image to enhance (color or grayscale). Each channel is normalized independently..
* output
  * `image` ([`image`](/workflows/developer-guide/developer-guide/kinds/image.md)): Image in workflows.

</details>

<details>

<summary>Example JSON definition</summary>

```json
{
	    "name": "<your_step_name_here>",
	    "type": "roboflow_core/contrast_enhancement@v1",
	    "image": "$inputs.image",
	    "clip_limit": "<block_does_not_provide_example>",
	    "contrast_multiplier": "<block_does_not_provide_example>",
	    "normalize_brightness": "<block_does_not_provide_example>"
	}
```

</details>