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How To: Adjust image display brightness or contrast in ArcInfo Workstation

Summary

You can control image brightness and contrast, after conversion to BIL format, by editing the .stx file.

The .stx file stores the image display statistics within a line for each image band. Each line has the band number, minimum pixel value, maximum pixel value, mean, and standard deviation. Linear stretch minimum and maximum values can optionally be added.

Procedure

  1. Convert the image to BIL format using the CONVERTIMAGE command. If a .stx file is not created, convert the original image to a grid with IMAGEGRID and then to a BIL with GRIDIMAGE. Conversion to a grid will compile the statistics needed for creation of the .stx ASCII file.

  2. Open the statistics file in an ASCII text editor.

  3. Edit the mean, standard deviation, linear stretch minimum, or linear stretch maximum. The default display of the IMAGE command uses a linear stretch on values that fall within two standard deviations of the mean pixel value. Values beyond this range are displayed as 0 or 255 (black or white, respectively). You can adjust the image display by modifying the mean or standard deviation or adding stretch minimum and maximum values.
    [O] Image statistics adjustments
    Image brightness can be further adjusted beyond the limits set by the minimum and maximum pixel values by decreasing the linear contrast stretch minimum value below the minimum image pixel value.

    a. Convert image to grid using the IMAGEGRID command.
    b. Add a number to all the grid cells.

    Arc: IMAGEGRID <image_name> <grid_name>
    Arc: GRID
    Grid: <new_grid> = <grid_name> + 1000
    Grid: QUIT
    Arc: GRIDIMAGE <new_grid> # <new_image> BIL


    c. Set linear contrast stretch minimum between 0 and number added to grid.
    [O] Adjust brightness below min pixel value
  4. Save the edits to the .stx file and redraw the image. This will produce an image display that is adjusted according to the edits made to the .stx file.

    Note:
    For detailed information about the .stx file, see the online ArcDoc help section Contents > Cartography > Image Integration > BIL, BIP, and BSQ data > Image description files > The statistics file (.stx)