Having trouble multiplying a four channel image by a matrix

Software G'MIC
1

I have a camera which has a four color sensor. I have debayered some raw images and stored as four channel tiff files using libraw (yes, I know it can convert to a 3 channel tiff if a color space is specified but I have reasons for wanting to do this manually. The reasons why are not important here).

I have been trying to use gmic to take this 4 channel tif and multiply it by the Camera2RGB matrix

[1.6374,-0.2528,-0.0035,-0.3811;
0.0672,0.8224,-0.5306,0.6410;
-0.0009,-0.3551,1.4153,-0.0593]

This matrix as the name implies takes the four color channel raw data and consolidates it into three channel raw RGB format. However I cannot seem to get the expected result using the mmul command, and mix_rgb only allows 3x3 matrices it seems. Is what I am wanting to do possible with gmic?

2

Yes, definitely, with command mix_channels.

mmul does a matrix product, but by considering that all your image is a single matrix (right-hand operand of the matrix multiplication), which is not your case here.

3

I am still getting errors with that command.

I also tried with the transpose of the matrix and tried a few things with the syntax since following the manual’s indicated syntax did not work.

gmic -input DSC00254-raw.tif +mix_channels (1.6374,0.0672,-0.0009,-0.2528;0.8224,-0.3551,-0.0035,-0.5306;1.4153,-0.3811,0.641,-0.0593)

gmic -input DSC00254-raw.tif +mix_channels (1.6374,-0.2528,-0.0035;-0.3811,0.0672,0.8224;-0.5306,0.641,-0.0009;-0.3551,1.4153,-0.0593)

Unexpected token ‘)’ in expression or statement.
+ CategoryInfo : ParserError: (: ) , ParentContainsErrorRecordException
+ FullyQualifiedErrorId : MissingEndParenthesisInExpression

gmic -input DSC00254-raw.tif +mix_channels ‘(1.6374,0.0672,-0.0009,-0.2528;0.8224,-0.3551,-0.0035,-0.5306;1.4153,-0.3811,0.641,-0.0593)’

gmic -input DSC00254-raw.tif +mix_channels ‘(1.6374,-0.2528,-0.0035;-0.3811,0.0672,0.8224;-0.5306,0.641,-0.0009;-0.3551,1.4153,-0.0593)’

[gmic] *** Error in ./mix_channels/*repeat/local/ *** Command ‘mmul’: [instance(3,4,1,1,00000000073441e0,non-shared)] gmic::operator(): Invalid multiplication of instance by specified
matrix (8083560,4,1,1,0000000012d5a040).

gmic -input DSC00254-raw.tif +mix_channels 1.6374,0.0672,-0.0009,-0.2528;0.8224,-0.3551,-0.0035,-0.5306;1.4153,-0.3811,0.641,-0.0593

gmic -input DSC00254-raw.tif +mix_channels 1.6374,-0.2528,-0.0035;-0.3811,0.0672,0.8224;-0.5306,0.641,-0.0009;-0.3551,1.4153,-0.0593

[gmic] *** Error in ./mix_channels/*if/ *** Command ‘input’: File ‘1.6374’, format does not take any input options (options ‘-0.2528,-0.0035’ specified).

4

Seemingly your command interpreter interferes with gmic script language!
Try gmic run “…”!

Besides that write your matrix with commas and semicolon. Look into
gmic h mix_channels

5

@bkv this is typical from the problems you’ll get when trying to mix bash and G’MIC syntax in the same line.
You should try writing this exact command in a G’MIC command file instead, otherwise, you’ll have to add some double-quotes or backslashes to escape the character that may be interpreted in bash (like ,, ;, ) and so on…).

6

I’m actually using powershell in windows, but I imagine the same risk can occur. If powershell and bash are prone to this kind of error, is there some other CLI that can be downloaded which lacks the issue?

7

It’s not really an error so much as respecting the rules for shell scripting. Shells are designed to process command lines with space-separated parameters, so surrounding parameters in quotes is how you keep them from interpreting your g’mic commands as separate parameters.

Putting the g’mic commands in a file and telling g’mic to execute that file is the best way to separate the strictures of the shell and g’mic parsers…

8

The advice given works. Thanks for your help everyone!

9

Make sure your matrix has the correct number of elements. If your image has 4 channels, the argument to mix_channels must have 4 in a row.

(a,b,c,d;e,f,g,h;..)

As for syntax, in this case, PS requires double quotes while CMD doesn’t. I typically use CMD because it is simpler and therefore would have less conflicts with G’MIC.

PS ..> gmic sp tiger r 100%,100%,100%,4 mix_channels "(1.6374,-0.2528,-0.0035,-0.3811;0.0672,0.8224,-0.5306,0.641;-0.0009,-0.3551,1.4153,-0.0593)"