Image

Raster images can be represented in the local database of sa.engine and also appear in data streams. The OSQL datatype to represent raster images is named Image. An object :im of type Image represents meta-data properties of an image along with its pixel array:im[x,y] of pixels at positions x/y, where :im[1,1] represents the lowest leftmost corner of the image. Each 24-bits pixel in the pixel array is represented as a pixel vector [R,G,B] of red-green-blue 8-bits color intensities; thus the image depth is always three.

A raster image stored in an external .bmp file is viewed as an image object by the function bmp_image(Charstring file)->Inage. For example, the following OSQL call accesses a small raster image file stored in the system model data:

set :im = bmp_image(system_models:folder()+'data/domherre.bmp')

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Exercise

Use the system call signature(methods(typenamed('Image'))) to find out how to how to obtain the number of pixels in the image :im.

We are now ready to do some simple analyses of our image. For example, the following function finds the coordinates of the pixels in the image dominated by red with a factor f over blue and green:

create function red(Image img, Real f)
                  -> Bag of (Integer x, Integer y)
    as select x, y
         from Integer r, Integer g, Integer b
         where [r,g,b] = img[x,y]
           and r > f * g
           and r > f * b

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Exercise

Use the function red to find the coordinates of the pixels in :im dominated by red.

Note

You can access the pixel vector of :im at position x/y with :im[x,y].

Exercise

Use red() combined with pixel vector access to find the pixel vectors dominated by red.

The physical representation of raster images is a sequence of pixel triplets using the 24 bits pixel array representation of the Device Independent Bitmap format (DIB) used in .bmp files. The physical representation on an Image object is called the DIB array. It is represented as an unsigned byte sequence represented of type Memory and accessed by the function dib_array(Image)->Memory. The order of the bytes in a DIB array is row-by-row bottom-up in blue - green - red order. Since the depth of the images is three, each pixel of the image is represented by three consecutive bytes in the DIB array.

Exercise

Use the function size(Memory)->Integer to see the size of :im in bytes. How would the same query be expressed without explicitly accessing the DIB vector?

Functions