如何正确地将指针[]或*（指针+索引）传递给C中的查找表？ [英] How do I correctly pass pointer[] or *(pointer+index) to lookup table in C ?
问题描述
我正在尝试使用查找表在openGL中使用输入图像进行像素计算，这是我的查找表：
I'm trying to use lookup table for pixel computation in openGL with input image, here's my lookup table :
float pix_scaling [256];
static void scaling(void)
{
for (int i = 0; i < 256; i++)
pix_scaling[i] = (i * 0.5) + 25;
}我在主循环中调用这两个函数，但是我无法得到结果输出，它只是空白的黑色，并且终端没有错误。
我尝试了什么：
我在其他功能中调用我的查找表：
I call both function in my main loop, but I can't get the result output, it's just blank black and somehow no error in terminal.
What I have tried:
I call my lookup table in other function :
unsigned char * image;
unsigned char * image1;
static void makerShift(){
unsigned char *p, *p1;
float r, g, b;
if(image1 == NULL) {
image1 = (unsigned char *)malloc(width*height*4);
if(image1 == NULL) exit(0);
p = image;
p1 = image1;
for(int i = 0; i < height*width; i++) {
r = pix_scaling[p[0]]; // here
g = pix_scaling[p[1]];
b = pix_scaling[p[2]];
//.. other computation process..//
//to set maximum and minimum pixel value
if (r < 0) *(p1+0) = 0;
else if(r > 255) *(p1+0) = 255;
else *(p1+0) = r;
if (g < 0) *(p1+1) = 0;
else if(g > 255) *(p1+1) = 255;
else *(p1+1) = g;
if (b < 0) *(p1+2) = 0;
else if(b > 255) *(p1+2) = 255;
else *(p1+2) = b;
p +=4;
p1 +=4;
}图片来自：
Image comes from this :
image = SOIL_load_image("Images/sample.jpg", &width, &height, 0, SOIL_LOAD_RGBA);
我试过指针
Either I tried with pointer
pix_scaling[p[0]];或指针和index
pix_scaling[*(p+0)];两者都不起作用。并且不使用 pix_scaling ，只需
both don't work. And without using pix_scaling , just
r = (*(p+0) * 0.5) + 25;
g = (*(p+1) * 0.5) + 25;
b = (*(p+2) * 0.5) + 25; 图像输出正常，所以我猜问题出在我的查找表函数中。我如何正确地做到这一点？
the image output came normally, so I guess the problem is in my lookup table function. How do I correctly do this ?
推荐答案
Rick是绝对正确的，你不会总是访问相同的指针，但你需要更仔细地编写代码并且使用调试器的。不要错过：
Rick is absolutly right that you arent accessing always the same pointers, but you need write code more carefully and use the debugger. Dont miss:
unsigned char * image = NULL;
unsigned char * image1 = NULL;
例如，使用不完整的路径加载图像是不合适的。总是检查是否成功，否则在控制台上写一些东西。
我也会定义一些 struct 用于颜色数据，以便您可以更好地使用。当你有这样的结构时，你可以使用C的力量：
For instance it isnt a good to load the image with a incomplete path. Always check for success and else write something on the console.
I also would define some struct for the color data so you can better work with. When you have such structs you can use the power of C:
pData[n];//access to n-th data element
pData[n].r;//access to the r-value of the n-th data element
提示：使用一些非常类似的图片进行调试小十字或圆
tip: debug with some very sample picture like a small cross or circle
所以我猜这个问题出在我的查找表函数中。我如何正确地执行此操作？
so I guess the problem is in my lookup table function. How do I correctly do this ?
我发现您的缩放表没有任何问题。当然你必须在调用 makeshift 之前初始化它（调用 scaling ）。
注意您要将 float 分配给 unsigned char 。我相信你的编辑警告过你。实际上你根本不需要为 pix_scaling 数组使用 float 。
[更新]
我想你没有展示的代码有问题。
以下程序
I find nothing wrong with your scaling table. Of course you have to initilialize it (calling scaling) before calling makeshift.
Note you are assigning a float to an unsigned char. I believe your compiled warned you about. You actually don't need at all to use floats for the pix_scaling array.
[update]
I suppose there is something wrong in the code you didn't show.
The following program
#include <stdlib.h>
#include <stdio.h>
float pix_scaling [256];
static void scaling(void)
{
for (int i = 0; i < 256; i++)
pix_scaling[i] = (i * 0.5) + 25;
}
#define H 4
#define W 4
int main()
{
unsigned char image[H*W*4];
unsigned char image_scaled[H*W*4];
for (int y = 0; y < H; ++y)
{
for (int x = 0; x < W; ++x)
{
for (int c = 0; c < 3; ++c)
{
int i = y* W * 4 + x * 4 + c;
image[i] = (unsigned char)rand();
}
}
}
scaling();
unsigned char *p = image;
unsigned char *p1 = image_scaled;
float r,g,b;
for(int i = 0; i < H*W; i++)
{
r = pix_scaling[p[0]]; // here
g = pix_scaling[p[1]];
b = pix_scaling[p[2]];
//to set maximum and minimum pixel value
if (r < 0) *(p1+0) = 0;
else if(r > 255) *(p1+0) = 255;
else *(p1+0) = r;
if (g < 0) *(p1+1) = 0;
else if(g > 255) *(p1+1) = 255;
else *(p1+1) = g;
if (b < 0) *(p1+2) = 0;
else if(b > 255) *(p1+2) = 255;
else *(p1+2) = b;
p +=4;
p1 +=4;
}
for (int y = 0; y < H; ++y)
{
for (int x = 0; x < W; ++x)
{
int i = y* W * 4 + x * 4 ;
printf("(%d,%d), image { %d, %d, %d }, image_scaled {%d, %d, %d}\n", x, y, image[i], image[i+1], image[i+2], image_scaled[i], image_scaled[i+1], image_scaled[i+2]);
}
}
return 0;
}产生：
produces:
(0,0), image { 103, 198, 105 }, image_scaled {76, 124, 77}
(1,0), image { 115, 81, 255 }, image_scaled {82, 65, 152}
(2,0), image { 74, 236, 41 }, image_scaled {62, 143, 45}
(3,0), image { 205, 186, 171 }, image_scaled {127, 118, 110}
(0,1), image { 242, 251, 227 }, image_scaled {146, 150, 138}
(1,1), image { 70, 124, 194 }, image_scaled {60, 87, 122}
(2,1), image { 84, 248, 27 }, image_scaled {67, 149, 38}
(3,1), image { 232, 231, 141 }, image_scaled {141, 140, 95}
(0,2), image { 118, 90, 46 }, image_scaled {84, 70, 48}
(1,2), image { 99, 51, 159 }, image_scaled {74, 50, 104}
(2,2), image { 201, 154, 102 }, image_scaled {125, 102, 76}
(3,2), image { 50, 13, 183 }, image_scaled {50, 31, 116}
(0,3), image { 49, 88, 163 }, image_scaled {49, 69, 106}
(1,3), image { 90, 37, 93 }, image_scaled {70, 43, 71}
(2,3), image { 5, 23, 88 }, image_scaled {27, 36, 69}
(3,3), image { 233, 94, 212 }, image_scaled {141, 72, 131}
[/ update]
在我看来问题出在这里：
It appears to me the problem is here :
r = pix_scaling[p[0]];
g = pix_scaling[p[1]];
b = pix_scaling[p[2]];
r，g，b将始终具有相同的值，因为p [1]永远不会改变。我认为你必须增加p指向循环每次迭代的下一个像素。
r, g, b will ALWAYS have the same values because p[1] is a never changed. I think you have to increment p to point to the next pixel at every iteration of the loop.
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