When open an image using imread with no second argument, what is the color mode? BGR or RGB?

Question

The problem is when I need to convert it to HSV, CV_BGR2HSV and CV_RGB2HSV give me different results: So I really need to know what is the order of color when open by imread or how to force the imread to open an image in any particular order.

Answer 1

The OpenCV docs for imread state that by default for 3-channel color images the data is stored in BGR order, e.g. in your Mat, the data is stored as a 1D unsigned char pointer, such that any given color pixel at index px_idx is 3 elements in order, with [px_idx + 0]: blue channel, [px_idx + 1]: green channel, [px_idx + 2]: red channel

Note In the case of color images, the decoded images will have the channels stored in B G R order.

You have some (limited) control over the color type via the flag parameter you pass to imread, although you can't specify the channel ordering (you should assume all color images will be BGR)

CV_LOAD_IMAGE_ANYDEPTH - If set, return 16-bit/32-bit image when the input has the corresponding depth, otherwise convert it to 8-bit.

CV_LOAD_IMAGE_COLOR - If set, always convert image to the color one

CV_LOAD_IMAGE_GRAYSCALE - If set, always convert image to the grayscale one

Or more simply,

>0 Return a 3-channel color image. (same as CV_LOAD_IMAGE_COLOR)

=0 Return a grayscale image. (same as CV_LOAD_IMAGE_GRAYSCALE)

<0 Return the loaded image as is (with alpha channel). (same as CV_LOAD_IMAGE_ANYDEPTH)