# How do I find the coordinates of points in this image?

I have never used image processing with Mathematica. I need to get the coordinates of the red points from this image I made in Illustrator. Is there a way to get Mathematica to read or detect the x-y coordinates?

• Of course, you need a coordinate system... where is the origin, for instance? Jun 4 '13 at 5:54
• Anywhere can be the origin. I will shift and scale the numbers as need be! Jun 4 '13 at 6:15
• Perhaps this question and its answers can be of help for you. I guess with a bit of work you can create a solution for your problem. Jun 4 '13 at 6:15
• After applying Binarize[] to the OP's image, the question is now equivalent to the one linked by @partial81; unless I see a reason why this is not a dupe, I'm leaning towards closing this. Jun 4 '13 at 6:36
• Wouldn't PixelValuePositions[i, Red, .2] be quicker though? Jun 4 '13 at 6:47

A solution for Mathematica version 9:

image = Import["http://i.stack.imgur.com/R0Dqo.png"]
pts = PixelValuePositions[image, Red, .2];
ListPlot[pts,
PlotStyle -> Darker@Orange,
PlotMarkers -> {Automatic, .05},
PlotRange -> {{0, 1500}, {0, 800}},
ImageSize -> 600]


• Very nice solution! Fortunately the question was not closed ;-) I did not know the PixelValuePositions so far (seems to be new in MMA 9). Jun 4 '13 at 8:11
• Are there really two thousand points in the original image (length of pts above)?
– BoLe
Jun 4 '13 at 9:35
• PixelValuePositions[Thinning@ColorNegate@ImageCrop@Binarize@image, 1] perhaps.
– BoLe
Jun 4 '13 at 9:37
• Or thinned = DeleteDuplicates[pts, EuclideanDistance[#1, #2] < 2 &] to keep it as data...? Jun 4 '13 at 10:33
• @cormullion I agree, should distance 2 not exclude certain original dot.
– BoLe
Jun 4 '13 at 12:03

The question leaves open what a "point" is, as opposed to a pixel.

## Attempt at points

If points overlap in the image, it is beyond my skill to separate them. Others here have far more experience in image processing and may be able to suggest things, within limits. If the points are separated, then here's a rough stab at finding them:

MorphologicalComponents[Binarize[img, {0.29, 0.6}], 0.68] // Colorize


(rules = ArrayRules[MorphologicalComponents[Binarize[img, {0.29, 0.6}], 0.68]];
points = Mean /@ Map[First, GatherBy[rules, Last], {2}];) // Timing
Length[points]

{0.072981, Null}
195


The points are image coordinates; we should convert them to graphics coordinates before plotting:

ListPlot[{#[[2]], Last@ImageDimensions[img] - #[[1]]} & /@ points,
PlotMarkers -> {Automatic, 2},
PlotRange -> Transpose[{{0, 0}, ImageDimensions[img]}],
PlotRangePadding -> 50, AxesOrigin -> {0, -50}]


## Pixels

The same method is an efficient way to get the pixels (especially if you do not have v9 and PixelValuePositions to use).

(rules = ArrayRules[MorphologicalComponents[Binarize[img, {0.29, 0.6}], 0.68]];
pixelCoords = SparseArray[rules]["NonzeroPositions"];) // Timing
Length@pixelCoords

{0.070562, Null}
2629

ListPlot[{#[[2]], Last@ImageDimensions[img] - #[[1]]} & /@ pixelCoords,
PlotMarkers -> {Automatic, 0.25},
PlotRange -> Transpose[{{0, 0}, ImageDimensions[img]}],
PlotRangePadding -> 50, AxesOrigin -> {0, -50}]


This way is nearly as fast and gives the same result as above:

pixelCoords = Position[ImageData@Binarize[img, {0.29, 0.6}], 1]; // Timing
Length@pixelCoords

{0.108690, Null}
2629

• Good work, +1. Although we're probably all just exploring for fun, since the OP's Illustrator file likely contains the actual coordinate pairs of every point Jun 4 '13 at 19:39
• @cormullion Yes, processing images is an attractive pastime, but it's not something I am very good at yet. Jun 4 '13 at 23:40

Using relatively simple functions :-

c = Import["http://i.stack.imgur.com/R0Dqo.png"] ;
a = Rasterize[c];
reds = Cases[Union[Flatten[a[[1, 1]], 1]], {r_ /; r > 200, g_ /; g < 50, b_ /; b < 50}];
Row[{First[Timing[b = Map[Position[a[[1, 1]], #] &, reds]]], " seconds"}]


13.073 seconds

ListPlot[Reverse /@ Flatten[b, 1], PlotStyle -> Red, AxesOrigin -> {0, 0}]


Another way without Mathematica version 9 PixelValuePositions

img = Import["http://i.stack.imgur.com/R0Dqo.png"];
pix = Round[ImageData[img, DataReversed -> True]];
ListPlot[Reverse /@ Position[pix, {1, 0, 0}], AxesOrigin -> {0, 0}]