DCT-II and DCT-III implementation problem

[8macius7]

Hello guys

I'm implementing DCT and IDCT algorithms in mathematica, and i have following problem. DCT-II transformation looks good (the results are the same like in the book), but i cant get back to original data.

Results are very poor:

DCT input Data:

DCT[{12, 10, 8, 10, 12, 10, 8, 11}]

DCT result:

{28.6378,0.571202,0.46194,1.757,3.18198,-1.72956,0.191342,-0.308709}

IDCT input Data:

IDCT[{28.6, 0.6, 0.5, 1.8, 3.2, -1.8, 0.2, -0.3}];

IDCT result:

{11.465,14.2118,12.149,14.1194,16.2048,14.1817,12.132,15.1873}

As you can see the results of DCT-III are very, very poor. I attached my algorithm written in mathematica, based on D. Salomon Data Compression The Complete Reference book.

Thanks for any help!:wink:
Edit:
added attachment