CODEWITHSUNDEEP
c++cqtimage-processing
Andy
Andy

Reputation: 3839

How can I store each pixel in an image as a 16 bit index into a colortable?

I have a 2D array of float values:

float values[1024][1024];

that I want to store as an image.

The values are in the range: [-range,+range].

I want to use a colortable that goes from red(-range) to white(0) to black(+range).

So far I have been storing each pixel as a 32 bit RGBA using the BMP file format. The total memory for storing my array is then 1024*1024*4 bytes = 4MB.

This seems very vasteful knowing that my colortable is "1 dimensional" whereas the 32 RGBA is "4 dimensional".

To see what I mean; lets assume that my colortable went from black(-range) to blue(+range). In this case the only component that varies is clearly the B, all the others are fixed. So I am only getting 8bits of precision whereas I am "paying" for 32 :-(.

I am therefore looking for a "palette" based file format. Ideally I would like each pixel to be a 16 bit index (unsigned short int) into a "palette" consisting of 2^16 RGBA values. The total memory used for storing my array in this case would be: 1024*1024*2 bytes + 2^16*4bytes = 2.25 MB. So I would get twice as good precision for almost half the "price"!

Which image formats support this?

At the moment I am using Qt's QImage to write the array to file as an image. QImage has an internal 8 bit indexed ("palette") format. I would like a 16 bit one. Also I did not understand from Qt's documentation which file formats support the 8 bit indexed internal format.

Upvotes: 2

Views: 960

Answers (3)

Darren Stone
Darren Stone

Reputation: 2068

"I want to use a colortable that goes from red(-range) to white(0) to black(+range)."

Okay, so you've got FF,00,00 (red) to FF,FF,FF (white) to 00,00,00 (black). In 24 bit RGB, that looks to me like 256 values from red to white and then another 256 from white to black. So you don't need a palette size of 2^16 (16384); you need 2^9 (512).

If you're willing to compromise and use a palette size of 2^8 then the GIF format could work. That's still relatively fine resolution: 128 shades of red on the negative size, plus 128 shades of grey on the positive. Each of a GIF's 256 palette entries can be an RGB value.

PNG is another candidate for palette-based color. You have more flexibility with PNG, including RGBA if you need an alpha channel.

You mentioned RGBA in your question but the use of the alpha channel was not explained.

So independent of file format, if you can use a 256 entry palette then you will have a very well compressed image. Back to your mapping requirement (i.e. mapping floats [-range -> 0.0 -> +range] to [red -> white -> black], here is a 256 entry palette that covers the range red-white-black you wanted:

float    entry#  color  rgb
------  -------  -----  --------
-range    0  00  red    FF,00,00
          1  01         FF,02,02
          2  02         FF,04,04
             ...          ...
             ...          ...
        127  7F         FF,FD,FD
  0.0   128  80  white  FF,FF,FF
        129  81         FD,FD,FD
             ...          ....
             ...          ...
        253  FD         04,04,04
        254  FE         02,02,02
+range  255  FF  black  00,00,00

If you double the size of the color table to be 9 bits (512 values) then you can make the increments between RGB entries more fine: increments of 1 instead of 2. Such a 9-bit palette would give you full single-channel resolution in RGB on both the negative and positive sides of the range. It's not clear that allocating 16 bits of palette would really be able to store any more visual information given the mapping you want to do. I hope I understand your question and maybe this is helpful.

Upvotes: 1

koan
koan

Reputation: 3686

Store it as a 16 bit greyscale PNG and do the colour table manually yourself.

You don't say why your image can be decomposed in 2^16 colours but using your knowledge of this special image you could make an algorithm so that indices that are near each other have similar colours and are therefore easier to compress.

Upvotes: 2

user694733
user694733

Reputation: 16047

PNG format supports paletted format up to 8-bits, but should also support grayscale images up to 16-bits. However, 16-bit modes are less used, and software support may be lacking. You should test your tools first.

But you could also test with plain 24-bit RGB truecolor PNG images. They are compressed and should produce better result than BMP in any case.

Upvotes: 1

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