OpenCV: How to draw a line with colors that are inversed relatively to the surface it should be drawn on?

Question

So we have an image. We want to draw a line that must definitely be seen. So how to draw a lines with colors that are inverted relatively to the surface it should be drawn on in each point?

Answer 1

Use a LineIterator and XOR the colour values of each pixel manually.

Answer 2

The XOR trick is trivially different. It's not visually the most distinct, though, if only because it entirely ignores how human eyes work. For instance, on light greys, a saturated red color is visually quite distinct.

You might want to convert the color to HSV and check the saturation S. If low (greyscale), draw a red pixel. If the saturation is high, the hue is quite obvious, and a white or black pixel will stand out. Use black (V=0) if the the original pixel had a high V; use white if the original pixel had a low V (dark saturated color)

You can use the LineIterator method as suggested earlier.

(BTW, the XOR trick has quite bad cases too. 0x7F ^ 0xFF = 0x80. That's bloody hard to see)

Answer 3

This is from the top of my head and I'm not a c++ dev, but it should be possible to draw the line into a separate image and then mimic an invert blend mode...basically you need to get the 'negative'/inverted colour behind a pixel, which you get by subtracting the color bellow your line from the maximum colour value.

Something like:

uint invert(uint topPixel, uint bottomPixel) {
            return (255 - bottomPixel);
}

Not sure how if colours are from 0 to 255 or from 0.0 to 1.0, but hopefully this illustrates the idea.