How can I use Histogram and Histogram3D on an image?

Question

I have a .jpg image named r. I am using the following code to plot a histogram:

r // ImageData // Flatten // Histogram

Is it correct? Another task is to apply Histogram3D. However, I am unable to use Histogram3D. I know I need to convert my data into a 2D list but I am having difficulties figuring out how.

Answer 1

ImageHistogram

If you have a colour image, such as

img = ImageResize[ExampleData[{"TestImage", "Apples"}], 100]

As mentioned by JM, there is ImageHistogram

ImageHistogram[img]

Dimensions

But for some reason you have NOT explained, you want to use Histogram3D, which takes arguments in the form of list of pairs of values.

Then the number of colour channels is 3

ImageChannels[img]

That implies that the dimensions of the data array, is the dimensions of the pixel array and depth 3.

Dimensions[ImageData[img]]
(* {67, 100, 3} *)

So the data is incompatible.

Re-arrange

Of course you could rearrange the data.

Partition[Flatten[ImageData[img]], 2]

or

ArrayReshape[
 ImageData[img]
 , {Times @@ Dimensions[ImageData[img]]/2, 2}
 ]

but then you are mixing the RGB values and

{{r[1], g[1], b[1]}, {r[2], g[2], b[2]}, {r[3], g[3], b[3]}, {r[4], 
  g[4], b[4]}, {r[5], g[5], b[5]}, ... , {r[n], g[n], b[n]}}

becomes

{{r[1], g[1]}, {b[1], r[2]}, {g[2], b[2]}, {r[3], g[3]}, {b[3], 
  r[4]}, ... , {g[n], b[n]}}

Manipulate

you can process or manipulate the data to create some other set, for example averaging two channels

Map[{First[#], Mean[Rest[#]]} &,
 Flatten[ImageData[img], 1]
 ]

you will get something like

{{r[1], (b[1] + g[1])/2}, {r[2], (b[2] + g[2])/2},..., {r[n], (b[n] + g[n])/2}

Histogram3D

Whatever you did, then you can apply the Histogram3D to the array woth dimensions {n,2}

Dimensions@Map[{First[#], Mean[Rest[#]]} &,
  Flatten[ImageData[img], 1]
  ]
(* {6700, 2} *)

Histogram3D[
 Map[{First[#], Mean[Rest[#]]} &,
  Flatten[ImageData[img], 1]
  ]]