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Broadly speaking there are 4 steps involved in embedding a message using DCT:

  1. Divide the image into 8x8 blocks
  2. Transform each block using DCT mathematical operations
  3. Quantitize each DCT block (lossy compression)
  4. Embed the message bits from the quantitized coefficients (avoid 0, 1, -1, and the AC)

Thats ok, but I am not sure how to extract the data back out. Are you suppose to go through steps 1-3 to quantitize each block in order to extract the LSB of each coefficient?

If that's the case, would you not be losing (possible the embedded data) data because it is a lossy technique?

Also when I tried to quantitize each block, I am left with the same coefficients from the previous image (as in the LSB change had no effect) ??

Could someone enlighten me on this problem?

Thanks

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2 回答 2

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当您将 DCT 块乘以量化矩阵时，您很可能会在大空间频率下得到非常大的数字 - 图像会变得嘈杂。但是随后您需要对系数进行归一化，以便任何像素的任何值不超过 [0..1] 范围（ [0..255] ）归一化后您可能会丢失一些信息。

于 2014-03-13T22:11:37.787 回答
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消息的嵌入发生在有损压缩之后——不可能丢失消息，因为已经执行了丢失数据的步骤（除了实际嵌入，它只丢失图像数据，用您的消息替换它）。理想情况下，您可以直接从系数本身中提取信息——也就是说，提取过程中不涉及解压缩。

于 2014-03-13T22:07:25.810 回答