DL_ReadCharacters


Header: AVLDL.h
Namespace: avl
Module: DeepLearning

Performs optical character recognition using a pretrained deep learning model.

Syntax

void avl::DL_ReadCharacters
(
	const avl::Image& inImage,
	atl::Optional<const avl::Rectangle2D&> inRoi,
	atl::Optional<const avl::CoordinateSystem2D&> inRoiAlignment,
	const avl::ReadCharactersModelId& inModelId,
	const int inCharHeight,
	const float inWidthScale,
	atl::Optional<const atl::String&> inCharRange,
	const float inMinScore,
	avl::Polarity::Type inPolarization,
	const float inContrastThreshold,
	atl::Array<avl::OcrResult>& outCharacters,
	atl::Array<avl::Image>& outMasks,
	atl::Optional<avl::Rectangle2D&> outAlignedRoi = atl::NIL,
	avl::Image& diagInputImage
)

Parameters

Name Type Range Default Description
Input value
inImage const Image& Input image
Input value
inRoi Optional<const Rectangle2D&> NIL Limits the area where recognized characters are located
Input value
inRoiAlignment Optional<const CoordinateSystem2D&> NIL
Input value
inModelId const ReadCharactersModelId& Identifier of a Read Characters model
Input value
inCharHeight const int 8 - 35 Average height of characters in pixels
Input value
inWidthScale const float 0.1 - 10.0 1.0f Scales image width by the given factor
Input value
inCharRange Optional<const String&> \"A-Z,a-z,0-9,\\\\\\\\,/,-\" Limits the set of wanted characters
Input value
inMinScore const float 0.0 - 1.0 0.5f Sets a minimum required score for a character to be returned
Input value
inPolarization Polarity::Type Any Sets a required polarity for a character to be returned
Input value
inContrastThreshold const float -1.0 - 1.0 0.0f Sets a threshold for a contrast of found characters
Output value
outCharacters Array<OcrResult>&
Output value
outMasks Array<Image>& Masks of found characters. Contains empty images in case of using model not supporting masks.
Output value
outAlignedRoi Optional<Rectangle2D&> NIL Input roi after transformation
Diagnostic input
diagInputImage Image& Analysed area of the input image

Requirements

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

Read more about pixel formats in Image documentation.

Optional Outputs

The computation of following outputs can be switched off by passing value atl::NIL to these parameters: outAlignedRoi.

Read more about Optional Outputs.

Description

This tool locates and recognizes characters. By default, it is suitable for reading characters:

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

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.

Hints

  • 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.

Remarks

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.

Errors

List of possible exceptions:

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

See Also