OpenCV image conversion from RGB to Grayscale using imread giving poor results

Question

I\'m loading a 24 Bit RGB image from a PNG file into my OpenCV application.

However loading the image as grayscale directly using imread gives a very po

Answer 1

I was having the same issue today. Ultimately, I compared three methods:

//method 1
cv::Mat gs = cv::imread(filename, CV_LOAD_IMAGE_GRAYSCALE);

//method 2
cv::Mat color = cv::imread(filename, 1); //loads color if it is available
cv::Mat gs_rgb(color.size(), CV_8UC1);
cv::cvtColor(color, gs_rgb, CV_RGB2GRAY);

//method 3
cv::Mat gs_bgr(color.size(), CV_8UC1);
cv::cvtColor(color, gs_bgr, CV_BGR2GRAY);

Methods 1 (loading grayscale) and 3 (CV_BGR2GRAY) produce identical results, while method 2 produces a different result. For my own ends, I've started using CV_BGR2GRAY.

My input files are jpgs, so there might be issues related to your particular image format.