Thresholding¶

The thresholding transforms an image by classifying the pixel values using these methods (detailed below):
- Automatic threshold
- AutoThreshold
- Manual threshold using one or two threshold values
- Histogram based (computed before using the thresholding function)
These functions also return the average gray levels of each pixel below and above the threshold.

Keys to Successful Thresholding¶

Object and background areas should be of uniform color and illumination. Image uniformization may be required prior to thresholding. See the page related to noise reduction.
The gray level range of the object and background must be sufficiently different (all background pixels should be darker than the darkest object pixel). You must decide if the threshold value should be:
- Constant: absolute threshold
- Adapted to ambient light intensity: relative or automatic threshold

Automatic Thresholding¶

The threshold is calculated automatically if you use one of these threshold modes with the EasyImage::Threshold ^.NET ^Python function.

EThresholdMode_MinResidue ^.NET ^Python: Minimizes the quadratic difference between the source and the resulting image (default behavior if the Threshold function is invoked without an argument).

Result of a threshold using minimal residue.
EThresholdMode_MaxEntropy ^.NET ^Python: Maximizes the entropy (that is, the amount of information) between object and background of the resulting image.

Result of a threshold using maximal entropy.
EThresholdMode_Isodata ^.NET ^Python: Iteratively calculates a threshold \(T\) as the midpoint between the mean gray level of pixels below and above \(T\):

\[T = \frac{\mu_{\text{below}} + \mu_{\text{above}}}{2}\]

where \(\mu_{\text{below}}\) is the mean gray level of pixels with values \(< T\), and \(\mu_{\text{above}}\) is the mean gray level of pixels with values \(\geq T\). The process repeats until \(T\) converges.

Result of a threshold using iso data.

Manual Thresholding¶

Manual thresholds require that the user supplies one or two threshold values:

one value to the EasyImage::Threshold ^.NET ^Python function to classify source image pixels into two classes and create a bi-level image. This can be:
- relativeThreshold is the percentage of pixels below the threshold. The Threshold function then computes the appropriate threshold value, or
- absoluteThreshold. This value must be within the range of pixel values in the source image.
two values to the EasyImage::DoubleThreshold ^.NET ^Python function to classify source image pixels into three classes and create a tri-level image.
- LowThreshold is the lower limit of the threshold
- HighThreshold is the upper limit of the threshold
two values to the EasyImage::Threshold ^.NET ^Python functions to classifiy color image pixels into two classes and create a bi-level image.
- Pixels in the given min/max range.
- Pixels outside the given min/max range.

Histogram Based¶

This is a two-step process: first compute the histogram with EasyImage::Histogram ^.NET ^Python (see Computing Image Statistics), then derive and apply the threshold from it using EasyImage::HistogramThreshold ^.NET ^Python or EasyImage::HistogramThresholdBW16 ^.NET ^Python.

The main advantage over AutoThreshold is that the histogram can be reused, useful if you need to threshold the same image multiple times or share the histogram with other operations.

These functions also return the average gray levels of each pixel below and above the threshold.

AutoThreshold¶

EasyImage::AutoThreshold ^.NET ^Python is a single-call alternative to the Histogram Based approach: it computes the histogram internally, derives the threshold automatically, and applies it, all in one step.

Use it for straightforward thresholding when you do not already have a histogram. If you do have one (for example, to reuse it across multiple operations and avoid redundant computation), use HistogramThreshold directly instead.

Supported threshold modes: EThresholdMode_Relative ^.NET ^Python, EThresholdMode_MinResidue ^.NET ^Python, EThresholdMode_MaxEntropy ^.NET ^Python and EThresholdMode_Isodata ^.NET ^Python.