Performance metrics for image steganography
Various methods are used to evaluate the quality of image steganography. Each of these methods assesses a different aspect of the result obtained after steganography. Some of the well-known methods are Mean Square Error(MSE), Peak Signal to Noise Ratio(PSNR), Structured Similarity Index Measure(SSIM), Payload Capacity.
Payload capacity :
Payload capacity refers to the measure of the volume of information present within the cover image. This measure is important in a steganographic system as the communication overhead depends on the maximum payload capacity. It is measured in Bits Per Pixel(BPP).
BPP = NUMBER OF SECRET BITS EMBEDDED/ TOTAL NUMBER OF PIXELS
Mean Square Error(MSE):
Mean Square Error is the averaged value of the square of the pixel-by-pixel difference between the original image and stego-image. It gives us a measure of the error produced in the cover image due to the data embedding process.
A lower value of MSE indicates a good quality embedding.
m,n = Dimensions of the image I = Original Image K = stego-image
Peak Signal to Noise Ratio(PSNR) :
PSNR is another popular way to measure the degree of distortion in the cover image due to embedding. It is the ratio between the maximum possible value of a signal and the power of distortion noise(MSE). It is measured in dB’s. A higher value of PSNR indicates a better quality embedding.
PSNR = 10xlog(MAX2/MSE)
MAX = 255 for a 8-bit grayscale image
Structured Similarity Index Measurement(SSIM):
SSIM is a metric of comparison to check the similarity between the cover image and stego-image. It measures the perceptual difference between the two images.
SSIM=(2μxμy + c1)(2σxy +c2)/((μx)2+(μy)2 +c1)((σx)2 +(σy)2 + c2)
c1 = (k1l)2 c2 = (k2l)2 μx and μy are the mean intensity values of images x and y. (σx)2 is the variance of x, (σy)2 is the variance of y (σxy)2 is the covariance of x and y. c₁ and c₂ are the two stabilizing parameters, L is the dynamic range of pixel values (2#bits per pixel - 1) the contents k1=0.01 and k2=0.03. SSIM value close to 1 indicates good quality.