What is the use of "maxidx = max(A(:))+1" in the below code ? How does it work? Any alternate syntax for the below function?

The question and code is given below :-

input: spine.tif from MATLAB

Read the indexed image with the associated colormap, show the colormap functions of all color channels with the corresponding color;

[A,map]=imread("spine.tif");
maxidx = max(A(:))+1;
figure()
hold on;
plot(map(1:maxidx,1),'r')
plot(map(1:maxidx,2),'g')
plot(map(1:maxidx,3),'b')
hold off

Can someone explain the use of "maxidx = max(A(:))+1;" in this code .

Like what did the above code do?

Is there any syntax to do the above function?

[Stephen23]

https://web.archive.org/web/20211108191648/https://www.mathworks.com/matlabcentral/answers/1581824-what-is-the-use-of-maxidx-max-a-1-in-the-below-code-how-does-it-work-any-alternate-syntax

Answer 1

[Stephen23]

"Can someone explain the use of "maxidx = max(A(:))+1;" in this code"

The answer can be derived from https://www.mathworks.com/help/matlab/creating_plots/image-types.html

The image file contains an indexed image, stored using integer indices:

class(imread("spine.tif"))

ans = 'uint8'

imfinfo("spine.tif")

ans = struct with fields:
                     Filename: '/MATLAB/toolbox/images/imdata/spine.tif'
                  FileModDate: '14-Apr-2015 15:06:00'
                     FileSize: 73166
                       Format: 'tif'
                FormatVersion: []
                        Width: 490
                       Height: 367
                     BitDepth: 8
                    ColorType: 'indexed'
              FormatSignature: [73 73 42 0]
                    ByteOrder: 'little-endian'
               NewSubFileType: 0
                BitsPerSample: 8
                  Compression: 'PackBits'
    PhotometricInterpretation: 'RGB Palette'
                 StripOffsets: [8 289 4019 8116 12408 16005 19646 23350 26632 29577 31936 34376 36508 38024 39717 41397 43125 46361 50216 54920 60170 65751 67261]
              SamplesPerPixel: 1
                 RowsPerStrip: 16
              StripByteCounts: [281 3730 4097 4292 3597 3641 3704 3282 2945 2359 2440 2132 1516 1693 1680 1728 3236 3855 4704 5250 5581 1510 120]
                  XResolution: 72
                  YResolution: 72
               ResolutionUnit: 'Inch'
                     Colormap: [256×3 double]
          PlanarConfiguration: 'Chunky'
                    TileWidth: []
                   TileLength: []
                  TileOffsets: []
               TileByteCounts: []
                  Orientation: 1
                    FillOrder: 1
             GrayResponseUnit: 0.0100
               MaxSampleValue: 255
               MinSampleValue: 0
                 Thresholding: 1
                       Offset: 71244
             ImageDescription: ''

"Like what did the above code do?"

Converts the maximum index of a integer indexed image (i.e. zero-based) into a MATLAB index (i.e. one-based).

"Is there any syntax to do the above function?"

What function?

[Samson David Puthenpeedika]

Thankyou for the info. Is there any other way or alternate solution to show the colormap functions of all color channels with the corresponding color

[Stephen23]

" Is there any other way or alternate solution to show the colormap functions of all color channels with the corresponding color"

[A,map] = imread("spine.tif");
mx = 1+max(A(:));
rgbplot(map(1:mx,:))

Compared against the original approach:

figure()
hold on;
plot(map(1:mx,1),'r')
plot(map(1:mx,2),'g')
plot(map(1:mx,3),'b')

[Samson David Puthenpeedika]

Thankyou so much .