The NXT image scanner
is a "stand alone" scanner that can scan and save
images as a BMP-file in the NXT's
flash memory. Images can be saved in either 24-bit true color,
8-bit color or gray scale. Gray scale images are scanned using
the light sensor whereas color images are scanned using the
HiTechnic color sensor. The scanner has a small LCD menu
that let's you set the properties for the scan. Obviously
the NXT image scanner is based on experience from my RCX
scanner. Where the RCX scanner would take forever to scan
even a small image the NXT scanner is much faster and the end
result is to my own surprise improved. At the bottom of the
page you will find building instructions and program for building
your own image scanner.
Pictures and video
Frontview
of the scanner
Topview
of the scanner
The
sledge uses a
worm gear for precise movement
A light sensor attached
on the sledge
View
of the light sensor
The scanner in action
A worm gear is also
used to feed the paper
Three
pairs of wheels
holds the paper in place
A touch sensor is
used to
reset the position of
the sledge
It is easy to replace
the
scanhead on the sledge
Use
Bricx CC to download
the image to a PC
Completed LDraw
model
Screenshot of the
set-up menu
Scanned images
Original
Normal resolution Gray Scale
Original
Normal resolution gray scale
Low resolution gray scale
Original
Low resolution gray scale
Low resolution 24-bit color
Original
Low resolution true-color
Normal resolution 8-bit color
How it works
Setting up a scan
There are eight options as illustrated
on the figure on the right. Although most options are
self explainable a few things has to be pointed out. With
the "File" option it is possible to enter a
name for the BMP-file. The "size" option is
not changeable but will display the size of the file as
well as the total available memory. When "free 3kb"
is displayed it simply means that you need to free 3 kb
to make the BMP-file fit in the flash memory. When the
options have been accepted the arrows are used to feed
the paper and adjust the scanhead. Once this is done the
scanning will begin.
Scanning an image
Each line of the image is created by
moving the sensor over the image at a constant speed while
recording the position and corresponding sensor values
with a fixed sample rate. The recorded data is then processed
to make up one line of the image. The line is saved to
the BMP-file and the scanner will feed the paper so the
next line can be scanned. This will be repeated until
the entire image has been scanned. The image processing
time between each line depends on the color depth and
of course the width of the image. The current status and
time of the scan is printed on the screen as shown on
the image - this is a screen shot from the mario scan.
Once the scanning is completed the image can be downloaded
to the PC using Bricx Command Center or a similar tool.
Resolution
From the set-up menu it is possible to
select between three different resolutions
High - there will be a pixel for
every 0.25 mm (100dpi)
Normal - there will be a pixel
for every 0.50 mm (50 dpi)
Low - there will be a pixel every
1 mm (25 dpi)
A higher resolution will of course produce
a larger file as more data is required. Since BMP images
are uncompressed the high resolution can only be used
to scan very small images due to limited flash memory.
Saving an image in a compressed format as JPG or PNG has
not been implemented (yet).
Gray scale
scan
To scan an image
in gray scale the standard light sensor is used
in raw mode. In a gray scale color the RGB values
are the same. This allows the BMP-file to be compressed
to an 8-bit BMP-file where the gray color can have
a value between 0 and 255. To convert the sensor
readings into a value between 0 and 255 the following
calculation is used:
gray scale
color =
255 ·(Sensor
reading - Black)
White
- Black
Where Black is
the sensor value that corresponds to the color black
and white is the sensor value that corresponds to
the color white. The light sensor is sensible to
ambient - but as long as you don't flash a light
near the light sensor the readings seems to be reliable.
When scanning a gray scale color spectrum with different
resolution the following results are achieved:
Original
Low resolution
(image has been scaled by 400%)
Normal resolution
(image has been scaled by 200%)
High resolution
Color scan
To scan
an image in color the HiTecnic color sensor is used.
The sensor allows you to read the raw RGB values.
From the set-up menu it is possible to select between
two different color depths. In the 24-bit mode the
raw RGB values are stored in the BMP-file. In the
8-bit color mode the RGB values are converted using
the netscape
color cube. Instead of saving the raw RGB values
a matching color in the color cube is found and
an 8-bit reference to this color is saved in the
BMP-file. This reduces the file size of the BMP-file
significantly but only allows for 216 different
colors. Because the color sensor is placed further
from the paper than the light sensor it is more
sensible to ambient light. This is also why a color
scan is more blurred compared to the gray scale
scan.When scanning a color spectrum with different
resolution and color depths it turns out that the
8-bit color scan doesn't perform that well - this
can also be seen on the scanned LEGO logo displayed
above. The results from the color spectrum scan
are shown below:
Original
Low resolution - true-color
(image has been scaled by 400%)
Normal resolution - true-color
(image has been scaled by 200%)
High resolution - true-color
Low resolution - 8-bit color
(image has been scaled by 400%)
Normal resolution - 8-bit color
(image has been scaled by 200%)
High resolution - 8-bit color
The images have
all been altered in brightness and contrast to make
them comparable with the original.
Scanner
program
The program for
the scanner is written in NXC
and runs on the enhanced NBC/NXC firmware (version
1.06a). The source code for the scanner can be viewed
by clicking on one of the four files below
You can download and use the code by clicking
here.
There are however a few things to note when using
the scanner.
Delete all files before downloading
the program to make room for images
Once the program has been
loaded - use BricxCC
to defrag the file system (thanks to robotmlg
at the Nxtasy.org
forum for pointing this out)
When entering a file name
simply double tap "enter" to end.
When entering a file name
the character '<' will delete a letter in
the file name
It is not possible to see
BMP-files from the NXT menu - they must be downloaded
to a PC using BrixcCC or another tool
Not all images are suited
to be scanned - chose an image without many
details to get the best result - a good idea
is to scan a few lines before scanning the entire
image
If you scan a large image
please be aware that it can take more than an
hour to complete. So scanning an image using 6
AA batteries won't be possible.
Building
Instructions
Below you
will find building instructions for building the
image scanner. Click on the image and a pop-up window
with the instructions will appear. Alternatively
you can
download the Ldraw-files or get a a quick run
through by clicking on the you tube video below.
It should be noticed that the sensor cable between
the NXT and the touch sensor is longer that the
standard retail cable. For instructions on how to
make your own cables please visit philos
homepage.
All in all the
results are quite good when you consider that the
scanner is in fact built entirely from LEGO bricks.
However there are few things that could/should be
improved:
Add the ability to save
the images in a compress image format.
Reduce the program size
to make room for more images
