This transform operates on 4×4 blocks of residual data after motion-compensated prediction or Intra prediction. The transform is based on the DCT but with some fundamental differences: 

1.          It is an integer transform, all operations can be carried out with integer arithmetic, without loss of accuracy. 
2.          The inverse transform is fully specified in the H.264 standard and if this specification is followed correctly, mismatch between encoders and decoders should not occur. 
3.          The core part of the transform is multiply-free, i.e. it only requires additions and shifts. 
4.          A scaling multiplication (part of the complete transform) is integrated into the quantizer (reducing the total number of multiplications). 

The entire process of transform and quantization can be carried out using 16-bit integer arithmetic and only a single multiply per coefficient, without any loss of accuracy.