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ssocr(1) OCR for seven segment displays ssocr(1)

NAME

ssocr - optical recognition of seven segment displays

SYNOPSIS

ssocr [OPTION]... [COMMAND]... IMAGE

DESCRIPTION

ssocr reads an image file containing the picture of a seven segment display, recognizes the displayed digits and prints them to standard output. All image formats known by imlib2 are supported. Use - as file name to read the image from standard input. ssocr provides several image manipulation algorithms to enhance noisy images.

Options can be used to change ssocr behavior. Commands can be used to manipulate the input IMAGE before starting the recognition algorithm.

OPTIONS

-h, --help

Write a help message to standard output. The default settings are shown as well.

-V, --version

Write version information to standard output.

-v, --verbose

Print information about program execution to standard error.

-t, --threshold THRESHOLD

Specify a percentage value as luminance threshold to differentiate between black and white. This threshold is adjusted to the luminance values occurring in the image, unless option --absolute-threshold is used. The default threshold is 50.

-a, --absolute-threshold

Do not adjust the threshold to the luminance values occurring in the image. Consider this option when using the dynamic_threshold command.

-T, --iter-threshold

Use an iterative method (one-dimensional k-means clustering) to determine the threshold. The starting value can be specified with the --threshold option.

-n, --number-pixels NUMBER

Set the number of foreground pixels that have to be found in a scanline to recognize a segment. This does not apply to ratio based recognition. Can be used to ignore some noise in the picture. See the web page of ssocr(1) for a description of the algorithm.

-N, --min-segment SIZE

Set the minimum number of pixels required for width and height of an individual segment of a seven segment display. A set segment in the display must have both a width and height of at least SIZE pixels. This minimum is used for both scanline based and ratio based recognition. It is not applied to decimal separator detection, because those are not comprised of regular segments. This option can be used to ignore some noise in the picture. See the web page of ssocr(1) for a description of the algorithm.

-i, --ignore-pixels NUMBER

Set the number of foreground pixels that are ignored when deciding if a column or row consists only of background or foreground pixels. Can be used to ignore some noise in the picture. See the web page of ssocr(1) for a description of the algorithm.

-M, --min-char-dims WIDTHxHEIGHT

Specify the minimum dimensions of characters respectively digits. When the segmentation step finds potential digits, those with a width less than WIDTH or a height less than HEIGHT are ignored. Can be used to ignore some noise in the picture. See the web page of ssocr(1) for a description of the algorithm.

-d, --number-digits RANGE

Specifies the number of digits shown in the image. Default value is 6. Use -1 to automatically detect the number of digits. Use a single positive number to specify an exact number of digits. Use two positive numbers separated with a hyphen (-) to specify an inclusive range of acceptable values for the number of digits.

-r, --one-ratio RATIO

Set the height/width ratio threshold to recognize a digit as a one. A digit with a height/width ratio greater than RATIO is recognized as a one. RATIO takes integers only. See the web page of ssocr(1) for a description of the algorithm.

-m, --minus-ratio RATIO

Set the width/height ratio to recognize a minus sign. A digit with a width/height ratio greater than or equal to RATIO is recognized as a minus sign. RATIO takes integers only. This uses the same idea as recognizing the digit one.

-H, --dec-h-ratio RATIO

Set the max_digit_height/height ratio used for recognition of a decimal separator. RATIO takes integers only. This value is used in combination with the max_digit_width/width ratio. If the height of a digit is less than 1/RATIO of the maximum digit height in the image and the max_digit_width/width threshold is also reached, it is recognized as a decimal separator.

-W, --dec-w-ratio RATIO

Set the max_digit_width/width ratio used for recognition of a decimal separator. RATIO takes integers only. This value is used in combination with the max_digit_height/height ratio. If the width of a digit is less than 1/RATIO of the maximum digit width in the image and the max_digit_height/height threshold is also reached, it is recognized as a decimal separator.

-o, --output-image FILE

Write the processed image to FILE. Use - to write to standard output. Unless this option is used no image is written to disk. If a standard filename extension is used it is interpreted as the image format to use. Can be useful together with the --process-only option.

-O, --output-format FORMAT

Specify the image format to use with --output-image. This format must be recognized by imlib2. Standard filename extensions are used to describe the format. Overwrites the image file format automatically determined via the filename. If no format is specified via this option or the filename, png is used.

-p, --process-only

Use ssocr(1) as an image manipulation program. No image recognition is performed. Should be used together with the -B --output-image option.

-D, --debug-image[=FILE]

Write a debug image showing the results of thresholding, segmentation and character recognition to disk. The image is written to the file testbild.png unless a filename FILE is given. This debug image often helps in understanding why ssocr(1) does not recognize the number from a given image.

-P, --debug-output

Print information helpful for debugging to standard error.

-f, --foreground COLOR

Specify the foreground color (either black or white). This automatically sets the background color as well. Default is black.

-b, --background COLOR

Specify the background color (either black or white). This automatically sets the foreground color as well. Default is white.

-I, --print-info

Prints image dimensions and range of used luminance values to standard error.

-g, --adjust-gray

Interpret the values T1 and T2 given to the command gray_stretch as percentages instead of absolute luminance values.

-l, --luminance KEYWORD

Choose the type of luminace computation. Using help as KEYWORD prints the list of implemented luminance keywords with a short description of the used formula. The default of Rec709 should work well in most cases.

-s, --print-spaces

Print space characters between digits (characters) that are farther apart than a factor times the minimum (default) or average distance between digits (characters).

-A, --space-factor FACTOR

Use the given FACTOR instead of the default value to determine white space between digits (characters).

-G, --space-average

Use the average distance between digits (characters) instead of the minimum distance to determine white space between digits.

-S, --ascii-art-segments

Prints the recognized segments, i.e. the display as seen by ssocr, as ASCII art to standard error.

-X, --print-as-hex

Prints the recognized segments as a string of hexadecimal numbers separated by a colon instead of digits and characters. Each number comprises two hexadecimal digits (one byte). 0x01 represents the upper horizontal segment, 0x02 represents the upper left vertical segment, 0x04 represents the upper right vertical segment, 0x08 represents the middle horizontal segment, 0x10 represents the lower left vertical segment, 0x20 represents the lower right vertical segment, 0x40 represents the lower horizontal segment, 0x80 represents a decimal point (or comma or thousands separator). Each hexadecimal number printed is the logical or of the set segments.

-C, --omit-decimal-point

Omit decimal points from output. Decimal points are still recognized and counted against the number of digits. This can be used together with automatically detecting the number of digits to ignore isolated groups of pixels in an image.

-c, --charset KEYWORD

Select the set of characters that ssocr shall recognize. This affects, e.g., if a display showing a six with missing top segment is recognized as 6 (with digits and decimal) or b (with hexadecimal and full). Using help as KEYWORD prints the list of implemented character set keywords with a short description of the included characters. The default is full (recognizing all characters known to ssocr in the image).

COMMANDS

Most commands do not change the image dimensions. The crop command is a notable exception to this rule.

dilation [N]

Filter image using dilation algorithm. Any pixel with at least one neighbour pixel set in the source image will be set in the filtered image. If a number N > 1 is specified, the dilation algorithm is executed N times.

erosion [N]

Filter image using erosion algorithm. Any pixel with every neighbour pixel set in the source image will be set in the filtered image. If a number N > 1 is specified, the erosion algorithm is executed N times.

closing [N]

Filter image using closing algorithm, i.e. erosion and then dilation. If a number N > 1 is specified, N times dilation and then N times erosion is executed.

opening [N]

Filter image using opening algorithm, i.e. dilation and then erosion. If a number N > 1 is specified, N times dilation and then N times erosion is executed.

remove_isolated

Remove any foreground pixels without neighbouring foreground pixels.

make_mono

Convert the image to monochrome using thresholding. The threshold can be specified with option --threshold and is adjusted to the used luminance interval of the image unless option --absolute-threshold is used.

grayscale

Transform image to gray values using luminance. The formula to compute luminance can be specified using option --luminance.

invert

Set every foreground pixel to background color and vice versa.

gray_stretch T1 T2

Transform image so that the luminance interval [ T1,T2 ] is projected to [ 0,255 ] with any value below T1 set to 0 and any value above T2 set to 255. Together with the option --adjust-gray, the values T1 and T2 are interpreted as percentages.

dynamic_threshold W H

Convert the image to monochrome using dynamic thresholding a.k.a local adaptive thresholding. A window of width W and height H around the current pixel is used to determine the (local) thresholding value. Consider using the --absolute-threshold option together with a manually adjusted --threshold for predictable results.

rgb_threshold

Convert the image to monochrome using simple thresholding for every color channel. This is the same as --luminance=minimum make_mono. You should use --luminance=minimum and make_mono or dynamic_threshold instead.

r_threshold

Convert the image to monochrome using simple thresholding. Only the red color channel is used. This is the same as --luminance=red make_mono. You should use --luminance red and make_mono or dynamic_threshold instead.

g_threshold

Convert the image to monochrome using simple thresholding. Only the green color channel is used. This is the same as --luminance=green make_mono. You should use --luminance green and make_mono or dynamic_threshold instead.

b_threshold

Convert the image to monochrome using simple thresholding. Only the blue color channel is used. This is the same as --luminance=blue make_mono. You should use --luminance blue and make_mono or dynamic_threshold instead.

white_border [WIDTH]

The border of the image is set to the foreground color. This border is one pixel wide unless a WIDTH > 1 is specified.

shear OFFSET

Shear the image OFFSET pixels to the right. The OFFSET is used at the bottom. Image dimensions do not change, pixels in background color are used for pixels that are outside the image and shifted inside. Pixels shifted out of the image are dropped. Many seven segment displays use slightly skewed digits, this command can be used to compensate this. Sometimes ssocr(1) cannot separate a decimal point from the preceding digit without shearing the image.

rotate THETA

Rotate the image THETA degrees clockwise around the center of the image. Image dimensions do not change, pixels rotated out of the image area are dropped, pixels from outside the image rotated into the new image are set to the background color.

mirror { horiz | vert }

Mirror the image horizontally or vertically.

crop X Y W H

Use only the subpicture with upper left corner ( X,Y ), width W and height H. This command changes the image dimensions.

set_pixels_filter MASK

Set every pixel in the filtered image that has at least MASK neighbour pixels set in the source image.

keep_pixels_filter MASK

Keep only those foreground pixels in the filtered image that have at least MASK neighbour pixels set in the source image (not counting the checked pixel itself).

LUMINANCE KEYWORDS

  • rec601
  • rec709
  • linear
  • minimum
  • maximum
  • red
  • green
  • blue

CHARACTER SET KEYWORDS

  • full
  • digits
  • decimal
  • hex
  • tt_robot

EXIT STATUS

  • 0, if the correct number of digits have been recognized
  • 1, if an incorrect number of digits have been found
  • 2, if not all digits have been recognized
  • 3, if only image processing was requested and successful
  • 42, if help or version info was requested
  • 99, if some other error occurred

ENVIRONMENT

TMP can be used to specify a different directory for temporary files than /tmp.

BUGS

Imlib2 (and therefore ssocr(1)) does not work well with Netpbm(1) images.

AUTHOR

ssocr was written by Erik Auerswald <auerswal@unix-ag.uni-kl.de>.

COPYRIGHT

Copyright © 2004-2023 Erik Auerswald. License GPLv3+: GNU GPL version 3 or later https://gnu.org/licenses/gpl.html.
This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.

SEE ALSO

netpbm(1), ImageMagick(1),

https://www.unix-ag.uni-kl.de/~auerswal/ssocr/

2023-05-07 2.23.0