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There are several problems in your approach that the other answers didn't adress: - in OpenCV HSV space the Hue angle is between 0-180! (not 360) - to get the HSV color map, the saturation/lightness should be 255, not 1! - probably it's easier to use color mapping than HSV conversion.

So the second part of strann's answer would be:

// Normalize NDVI between 0 and 255 and convert to byte type (CV_8UC1)
normalize(ratio, ratio, 0, 255, NORM_MINMAX, CV_8UC1);
//or: normalize(ratio, ratio, 127,127,NORM_L2,CV_8UC1);
// Apply a color map 
applyColorMap(ratio, cm_img0, COLORMAP_HSV);
// Show the result:
imshow("NDVI color map", cm_img0);

Some more information about color maps: http://docs.opencv.org/3.1.0/d3/d50/group__imgproc__colormap.html#gsc.tab=0

BTW, it seems that you try to use Public lab's method of calculating NDVI using a camera without IR filter. According to my experiences it's results aren't very accurate, but it's a good start for experimenting.

There are several problems in your approach that the other answers didn't adress: - adress:

• in OpenCV HSV space the Hue angle is between 0-180! (not 360) - 360)
• to get the HSV color map, the saturation/lightness should be 255, not 1! - 1!
• probably it's easier to use color mapping than HSV conversion.

So the second part of strann's answer would be:

// Normalize NDVI between 0 and 255 and convert to byte type (CV_8UC1)
normalize(ratio, ratio, 0, 255, NORM_MINMAX, CV_8UC1);
//or: normalize(ratio, ratio, 127,127,NORM_L2,CV_8UC1);
// Apply a color map 
applyColorMap(ratio, cm_img0, COLORMAP_HSV);
// Show the result:
imshow("NDVI color map", cm_img0);

Some more information about color maps: http://docs.opencv.org/3.1.0/d3/d50/group__imgproc__colormap.html#gsc.tab=0

BTW, it seems that you try to use Public lab's method of calculating NDVI using a camera without IR filter. According to my experiences it's results aren't very accurate, but it's a good start for experimenting.

There are several problems in your approach that the other answers didn't adress:

• in OpenCV HSV space the Hue angle is between 0-180! (not 360)
• to get the HSV color map, the saturation/lightness should be 255, not 1!
• they all forgot to convert back the hsv image to RGB: cvtColor(hsvimage,color,COLOR_HSV2BGR);
• probably it's easier to use color mapping mapping than HSV conversion.

So the second part of strann's answer using color mapping would be:

// Normalize NDVI between 0 and 255 and convert to byte type (CV_8UC1)
normalize(ratio, ratio, 0, 255, NORM_MINMAX, CV_8UC1);
//or: normalize(ratio, ratio, 127,127,NORM_L2,CV_8UC1);
// Apply a color map 
applyColorMap(ratio, cm_img0, COLORMAP_HSV);
// Show the result:
imshow("NDVI color map", cm_img0);

Some more information about color maps: http://docs.opencv.org/3.1.0/d3/d50/group__imgproc__colormap.html#gsc.tab=0

BTW, it seems that you try to use Public lab's method of calculating NDVI using a camera without IR filter. According to my experiences it's results aren't very accurate, but it's a good start for experimenting.