1. A method for determining the intensity of a test strip comprising: obtaining, by a processor, an image comprising a sample line in a test strip and a plurality of reference lines in a test strip from a test cassette, wherein the reference lines have known intensities; identifying, by the processor, from the image a first quadrilateral having the sample line and a second quadrilateral having the reference lines; determining, by the processor, a grayscale value of the sample line from the first quadrilateral and grayscale values of the reference lines from the second quadrilateral by vertical projections of pixel values by: determining a first horizontal central line of the second quadrilateral, conducting vertical projections of pixel values of the second quadrilateral along the first horizontal central line, and selecting peak values as the grayscale values of the reference lines among the vertical projections of the pixel values of the second quadrilateral; constructing, by the processor, a standard curve based on the grayscale values of the reference lines versus the known intensities of the reference lines; and determining, by the processor, the intensity of the sample line by fitting the grayscale value of the sample line on the standard curve.