Module: DL_OCR

Performs optical character recognition using a pretrained deep learning model.

Name Type Range Description
Input value inImage Image Input image
Input value inRoi Rectangle2D* Limits the area where recognized characters are located
Input value inRoiAlignment CoordinateSystem2D*
Input value inModelId ReadCharactersModelId Identifier of a Read Characters model
Input value inCharHeight Integer 8 - Average height of characters in pixels
Input value inWidthScale Real 0.1 - 10.0 Scales image width by the given factor
Input value inCharSpacing Real -0.5 - Distance between characters denoted as fraction of inCharHeight
Input value inCharRange String* Limits the set of wanted characters
Input value inMinScore Real 0.0 - 1.0 Sets a minimum required score for a character to be returned
Input value inPolarization Polarity Sets a required polarity for a character to be returned
Input value inContrastThreshold Real -1.0 - 1.0 Sets a threshold for a contrast of found characters
Output value outCharacters OcrResultArray
Output value outMasks RegionArray Masks of found characters (in Extended model). Contains empty regions in case of using model not supporting masks (in Fast and Balanced model).
Output value outAlignedRoi Rectangle2D Input roi after transformation
Diagnostic input diagInputImage Image Analyzed area of the input image


For input inImage only pixel formats are supported: 1⨯uint8, 3⨯uint8.

Read more about pixel formats in Image documentation.


This tool locates and recognizes characters. Without additional training, it is suitable for reading characters:

  • horizontally oriented,
  • of height between 60% and 140% of inCharHeight (in pixels) - for the FastWide, Balanced, Extended or OcrA models
  • of height between 85% and 115% of inCharHeight (in pixels) - for the Fast model
  • of height between 45% and 225% of inCharHeight (in pixels) - for the Scalable model
  • being latin letters (upper- or lower-case), digits or one of: !#$%&()*+,-./:;<=>?@[]^_`{|}~"'\€£¥.

The pretrained models are described here.

This behavior can be configured with parameters described below.

The inRoi and inRoiAlignment inputs may be used to limit the analysed area, which, in most cases, leads to improved performance. Moreover, it may be used to adjust to text which is not horizontally oriented.

The inCharHeight should be set to the average height of characters (specifically, capital letters) in the analysed area. E.g. if image contains 2 kind of characters: one being 24 pixels high and the second being 40 pixels high, inCharHeight should be set to 32, irrespective of number of characters of each kind. Fast mode supports a more narrow range of character sizes, the character size should be between 30 and 40 pixels.

In case of fonts with exceptionally thin or wide symbols, inWidthScale may be used to reshape them to a more "typical" aspect ratio. The analysed area will be scaled by inWidthScale in the horizontal axis. It may improve quality of results. Furthermore, it may also help in reading a text with tight spaces between subsequent characters.

To limit the set of recognized characters, inCharRange may be used. This string has to be formatted according to the following rules:

  • allowed characters have to be separated with commas,
  • for ease of use, continuous range of letters or digits, may be written as starting_character-ending_character, e.g. A-Z or 1-6,
  • comma and backslash have to be prepended with backslash.

For example, inCharRange equal to A-F,g-o,0-9,X,Y,Z,-,\\,\, will result in recognizing only ABCDEFXYZghijklmno0123456789-\, characters.

The inMinScore parameter may be used to change minimum score of a character. By default, this threshold is set to 0.5.

The inContrastThreshold and inPolarization parameters sets a desired contrast interval of a character, which may be used to reduce amount of false positives:

  • For inPolarization = Polarity::Bright, only characters with a contrast greater than inContrastThreshold will be returned.
  • For inPolarization = Polarity::Dark, only characters with a contrast lower than -inContrastThreshold will be returned.
  • For inPolarization = Polarity::Any, only characters with a contrast lower than -inContrastThreshold or greater than inContrastThreshold will be returned.

Character which is darker than its background have a negative contrast. Opposite situation results in a positive contrast.


  • It is recommended that the deep learning model is deployed with DL_ReadCharacters_Deploy first and connected through the inModelId input.
  • If one decides not to use DL_ReadCharacters_Deploy, then the model will be loaded in the first iteration. It will take up to several seconds.
  • In case of characters having too much differing height, it is advised to separate the analysed area (e.g. with inRoi input) to smaller parts containing symbols with a more consistent height.
  • In case of poor quality results in Fast mode, check if inCharHeight is set correctly.


This filter should not be executed along with running Deep Learning Service as it may result in degraded performance or even out-of-memory errors.


This filter can throw an exception to report error. Read how to deal with errors in Error Handling.

List of possible exceptions:

Error type Description
DomainError Not supported inImage pixel format in AvsFilter_DL_ReadCharacters. Supported formats: 1xUInt8, 3xUInt8.

Complexity Level

This filter is available on Basic Complexity Level.

Disabled in Lite Edition

This filter is disabled in Lite Edition. It is available only in full, Aurora Vision Studio Professional version.

See Also