I want to create color mapping, define few color names and boundaries in range of which those colors should fall. For example (BGR format),
colors = {
'red': ((0, 0, 255), (125, 125, 255)),
'blue': ((255, 0, 0), (255, 125, 125)),
'yellow' ....
}
So if I receive color, let's say (255, 50, 119) I can call it blue. I want to make such mapping for at least colors of rainbow plus gray, black, white. Using Python and openCV.
The problem is that I don't really understand where to get those values for boundaries, is there kind of lowest / highest value for blue, red and so on?
I would suggest using HSV colourspace for comparing colours because it is less sensitive to variable lighting than RGB, where green in the sunlight might be rgb(20,255,10), but green in a shadow might be rgb(3,45,2), whereas both will have a very similar Hue in HSV colourspace.
So, to get started...
Create a little 10x1 numpy array and make the first pixel red, the second orange, then yellow, green, blue, indigo, violet then black, mid-grey and white. There's a table here.
Then convert to HSV colourspace and note the Hue values.
I have started some code...
#!/usr/local/bin/python3
import numpy as np
import imageio
import cv2
# Create black image 10x1
im = np.zeros([1,10,3], dtype=np.uint8)
# Fill with colours of rainbow and greys
im[0,0,:]=[255,0,0] # red
im[0,1,:]=[255,165,0] # orange
im[0,2,:]=[255,255,0] # yellow
im[0,3,:]=[0,255,0] # green
im[0,4,:]=[0,0,255] # blue
im[0,5,:]=[75,0,130] # indigo
im[0,6,:]=[238,130,238] # violet
im[0,7,:]=[0,0,0] # black
im[0,8,:]=[127,127,127] # grey
im[0,9,:]=[255,255,255] # white
imageio.imwrite("result.png",im)
hsv=cv2.cvtColor(im,cv2.COLOR_RGB2HSV)
print(hsv)
Check image:
Check colours with Imagemagick too:
convert result.png txt:
# ImageMagick pixel enumeration: 10,1,65535,srgb
0,0: (65535,0,0) #FF0000 red
1,0: (65535,42405,0) #FFA500 orange
2,0: (65535,65535,0) #FFFF00 yellow
3,0: (0,65535,0) #00FF00 lime
4,0: (0,0,65535) #0000FF blue
5,0: (19275,0,33410) #4B0082 indigo
6,0: (61166,33410,61166) #EE82EE violet
7,0: (0,0,0) #000000 black
8,0: (32639,32639,32639) #7F7F7F grey50
9,0: (65535,65535,65535) #FFFFFF white
Now look at the HSV array below - specifically the first column (Hue). You can see Red has a Hue=0, Orange is 19, Yellow is 30 and so on. Note too that the Black, Grey and White all have zero Saturation and Black has a low Value, Grey has a medium Value and White has a high Value.
[[[ 0 255 255]
[ 19 255 255]
[ 30 255 255]
[ 60 255 255]
[120 255 255]
[137 255 130]
[150 116 238]
[ 0 0 0]
[ 0 0 127]
[ 0 0 255]]]
Now you can make a data-structure in Python that stores, for each colour:
Lowest included Hue
Highest included Hue
Name
So, you might use:
... see note at bottom for Red
14,23,"Orange"
25,35,"Yellow"
55,65,"Green"
115,125,"Blue"
...
and so on - omit Black, Grey and White from the table.
So, how do you use this?
Well, When you get a colour to check, first convert the R, G and B values to HSV and look at the resulting Saturation - which is a measure of vividness of the colour. Garish colours will have high saturation, whereas lacklustre, greyish colours will have low saturation.
So, see if the Saturation is more than say 10% of the max possible, e.g. more than 25 on a scale of 0-255.
If the Saturation is below the limit, check the Value and assign Black if Value low, Grey if middling and White if Value is high.
If the Saturation is above the limit, check if it is within the lower and upper limits of one of your recorded Hues and name it accordingly.
So the code is something like this:
def ColorNameFromRGB(R,G,B)
# Calculate HSV from R,G,B - something like this
# Make a single pixel from the parameters
onepx=np.reshape(np.array([R,G,B],dtype=np.uint8),(1,1,3))
# Convert it to HSV
onepxHSV=cv2.cvtColor(onepx,cv2.COLOR_RGB2HSV)
...
...
if S<25:
if V<85:
return "black"
elsif V<170:
return "grey"
return "white"
# This is a saturated colour
Iterate through colour names table and return name of entry with matching Hue
There are 2 things to be aware of:
There is a discontinuity in the Hue values for Red, because the HSV colour wheel is a circular wheel and the Hue value for Red is at an angle of 0, so values above 350 and below 10 are all Reds. It so happens that OpenCV scales the 0-360 range by dividing by 2, meaning it comes out as 0-180... which neatly fits in a single unsigned byte. So, for Red, you need to check for Hue greater than 175 and less than 5, say.
Be careful to always generate an 8-bit image when looking up colours, as the Hue values are scaled differently on 16-bit and float images.
Define a distance between two colors. Then find the "closest" color name for the given color. Which definition of distance you will choose has to be guided by your requirements, because there is no "best" definition, as far as I know.
One possibility is distance in RGB space. The distance between two colors can be defined, for example, as the euclidean (L2) distance between the colors as represented by vectors in three dimensional space - distance(a,b) = (a-b).length() Alternatively, try the Manhattan (L1) metric if the result makes sense, because the euclidean distance in RGB space is more of a heuristic than a valid measurement.
Another possibility is to first convert to HSV space. Then the closest color will be the one that has the closest hue to the given color. Unless the given color has insufficient saturation, then the color is either white, gray or black, depending on the color's lightness.
Sorry for the oddly worded title. I'd like to know how the ncurses init_color function maps it's input to colors. Essentially, most developers are used to colors being represented by red, green, and blue on a 0 - 255 scale, but init_color takes an int on a 0 - 1000 scale.
for example:
If I wanted to get the color (75, 0, 130) in ncurses, would I call init_color(COLOR_NAME, 300, 0, 520)?
short:
(n) * 1000 / 256
which is a little different from your numbers:
293 0 508
long: That of course assumes that the terminal description is written to match ncurses' documentation. But the assumption is from X/Open Curses:
The init_color() function redefines colour number color, on terminals that support the redefinition of colours, to have the red, green, and blue intensity components specified by red, green, and blue, respectively. Calling init_color() also changes all occurrences of the specified colour on the screen to the new definition.
The color_content() function identifies the intensity components of colour number color. It stores the red, green, and blue intensity components of this colour in the addresses pointed to by red, green, and blue, respectively.
For both functions, the color argument must be in the range from 0 to and including COLORS-1. Valid intensity values range from 0 (no intensity component) up to and including 1000 (maximum intensity in that component).
everyone. How would I generate the next color in the color spectrum? Like, a function that takes a red value, a green value, and a blue value for input and output. I could input solid red (RGB 255, 0, 0) and it would output an orangish-red.
EDIT: Some more background info: I'm assuming the H, S, and V values have numeric ranges from 0-255. The C program I'm writing increments the hue value if it is less than 256, resets it to 0 if it's not, converts the HSV to RGB, displays the color on the screen, and loops. I've tried a couple HSV-to-RGB functions, but they're not working.
Instead of the RGB domain for colors, you should work with HSV values. This way, you can modify the H value to move around the spectrum.
Do you have to work with RGB values? If you don't, then use HSL as #sukru suggested, otherwise, try to convert it into HSL by following the instructions here, then increment the H value by 1/12, and convert to RGB.
If I have four colours (A, B, C & D) on four corners of a square and I want to fill that square with a gradient that blends nicely between the four colours how would I calculate the colour of the point E?
The closer E is to any of the other points, the strong that colour should affect the result.
Any idea how to do that? Speed and simplicity is preferred to accuracy.
colours http://rabien.com/image/colours.png
The best solution when a gradient is required between two colors, is to use the HSV representation (Hue Saturation Value).
If you have the HSV values for your two colors, you just make linear interpolation for H, S and V, and you have nice colors (interpolation in RGB space always lead to "bad" results).
You also find here the formulae to go from RGB to HSV and from HSV to RGB, respectively.
Now, for your problem with the four corner, you can make a linear combination of the four H/S/V values, weighted by the distance from E to that four points A,B,C and D.
EDIT: same method than tekBlues, but in HSV space (it is quite easy to test it in RGB and in HSV spaces. And you will see the differences. In HSV, you just turn around the chromatic cylinder, and this is why it gives nice result)
EDIT2: if you prefer "speed and simplicity", you may use a L1-norm, instead of a L2-norm (euclidian norm)
So, if a is the size of your square and the coordinate of your points are A(0,0), B(0,a), C(a,0), D(a,a), then the Hue of a point E(x,y) can be computed with:
Hue(E) = ( Hue(B)*y/a + Hue(A)*(1-y/a) ) * (x/a) + ( Hue(D)*y/a + Hue(C)*(1-y/a) ) * (1-x/a)
where Hue(A) is the Hue of point A, Hue(B) the Hue of B, etc...
You apply the same formulae for the Saturation and Value.
Once you have the Hue/Saturation/Value for your point E, you can transform it in RGB space.
Check out this site, which gives a visual demo of #ThibThib's comment that "gradients in HSV will be more satifying":
http://www.perbang.dk/rgbgradient/
It is a gradient creator that will create and show BOTH an RGB gradient and an HSV gradient.
If you try 9 steps from FFAAAA to AAFFAA (light red to green), you’ll get a nice transition through light yellow, and the HSV and RGB ones look similar.
But try 9 steps from FF0000 to 00FF00 (bold red to green), and you’ll see the RGB one transition through a yucky greenish brown. The HSV gradient, however, transitions through bold yellow.
Determine the distance of point E to each point A,B,C,D
The color for point E will be the combination of Red / Green / Blue. Calculate each color axis as the average of the same color axis for A,B,C,D, ponderating by distance.
distance_a = sqrt((xa-xe)^2+(ya-ye)^2)
distance_b = ....
sum_distances = distance_a + distance_b ...
red = (red_adistance_a + red_bdistance_b ... ) / sum_distances
color_E = ColorFromARgb(red,green,blue)