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雖然標題名為色彩反轉  (Invert Color)，在這裡我要展示兩個函式如何使用，也就是cvLUT和cvSetData。

什麼是LUT？

其實是指顯示查找表（Look-Up-Table)，LUT實際上就是一張像素灰度值的映射表，它將實際採樣到的像素灰度值經過一定的變換如閾值、反轉、二值化、對比度調整、線性變換等，變成了另外一個與之對應的灰度值，這樣可以起到突出圖像的有用信息，增強圖像的光對比度的作用。

簡單來說，就是一個值丟進一個函數而變成另一個值，好比f(x)=y。

那為什麼要使用LUT呢？

這是因為要節省計算的成本，畢竟我們一張圖RGB每個通道像素的值介於[0, 255]，我們只要事先計算這256個值出來，整張影像中每個像素就可以直接查表得出相對應的值。

譬如512×512的影像，以一個通道(R、G或B)來說，若不使用LUT，你就要計算512×512這麼多次，而大部份的結果值是重複運算得來的，可說是浪費相當多的計算資源。

測試用程式碼：

/**
	Theme: Invert Color
	Compiler: Dev C++ 4.9.9.2
	Library: OpenCV 2.0
	Date: 101/10/09
	Author: HappyMan
	Blog: https://cg2010studio.wordpress.com/
*/
#include <cv.h>
#include <highgui.h>
using namespace std;

int main(){
    IplImage *src_image = 0 , *dst_image = 0;
    CvMat *lut_mat;
    lut_mat = cvCreateMatHeader(1, 256, CV_8UC1);
    uchar lut[256];
    for (int i = 0; i < 256; i++){
        lut[i] = 255 - i;
    }
    src_image = cvLoadImage("night.jpg");
    dst_image = cvCloneImage(src_image);
    cvSetData(lut_mat, lut, 0);
    cvLUT(src_image, dst_image, lut_mat);
    cvNamedWindow("src_image", 1);
    cvNamedWindow("dst_image", 1);
    cvShowImage("src_image", src_image);
    cvShowImage("dst_image", dst_image);
    cvWaitKey(0);
    cvReleaseImage(&src_image);
    cvReleaseImage(&dst_image);
    return 0;
}

原始圖：

結果圖：

void cvLUT(const CvArr* src, CvArr* dst, const CvArr* lut)

Performs a look-up table transform of an array.

Parameters:

src – Source array of 8-bit elements dst – Destination array of a given depth and of the same number of channels as the source array lut – Look-up table of 256 elements; should have the same depth as the destination array. In the case of multi-channel source and destination arrays, the table should either have a single-channel (in this case the same table is used for all channels) or the same number of channels as the source/destination array.

The function fills the destination array with values from the look-up table. Indices of the entries are taken from the source array. That is, the function processes each element of src.

void cvSetData(CvArr* arr, void* data, int step)

Assigns user data to the array header.

Parameters:	arr – Array header data – User data step – Full row length in bytes

The function assigns user data to the array header. Header should be initialized before using cvCreate*Header , cvInit*Header or Mat (in the case of matrix) function.

CvMat* cvCreateMatHeader(int rows, int cols, int type)

Creates a matrix header but does not allocate the matrix data.

Parameters:	rows – Number of rows in the matrix cols – Number of columns in the matrix type – Type of the matrix elements, see CreateMat

The function allocates a new matrix header and returns a pointer to it. The matrix data can then be allocated using CreateData or set explicitly to user-allocated data via SetData .

參考：OpenCV函數學習之cvLUT、OpenCV – cvLUT、OpenCV – cvSetData、OpenCV – cvCreateMatHeader。