机顶盒图像写入问题

Question

I am trying to write a simple image file to disk using stb_image_write. I use the simplest test case : a 128 by 128 pixels RGB image.

I found this example online, and the guy seems to say that it works fine, plus it looks exactly like what I have been writing for 2 hours :

void save_image(int w, int h, int channels_num)
{

    int data[w * h * channels_num];

     int index = 0;
     for (int j = h - 1; j >= 0; --j)
     {
      for (int i = 0; i < w; ++i)
      {
       float r = (float)i / (float)w;
       float g = (float)j / (float)h;
       float b = 0.2f;
       int ir = int(255.0 * r);
       int ig = int(255.0 * g);
       int ib = int(255.0 * b);

       data[index++] = ir;
       data[index++] = ig;
       data[index++] = ib;
      }
     }

    stbi_write_jpg("jpg_test_.jpg", w, h, channels_num, data, 100);
}

save_image(128, 128, 3);

The result should be a nice color gradient, but all I can get is a valid file with some vertical red, green, blue and black lines. Dimensions of the image are ok though. I really don't find the solution here. I am on linux Jessie. Could there be 'endianess' issue or something like that?

Answer 1

The question was answered in the comments. I had to replace : int data[] by : unsigned char data[];

Answer 2

=============== This part was not accurate. please see my solution part below ====

int data[w * h * channels_num]; is ok, that's not the reason you failed.

Change this line

stbi_write_jpg("jpg_test_.jpg", w, h, channels_num, data, 100);

to

stbi_write_jpg("jpg_test_.jpg", w, h, channels_num, data, w * sizeof(int));

You will get what you want. The last parameters, I believe it is stride. If so, stride means the offset between two lines. Based on your question, I assume you have 128 X 128 X 32bit color. Then stride will be w * sizeof(int).

If NOT, let me know.

Sorry I was in a hurry to rush to a meeting.

============ This part is the solution part ====

Yes,

int data[w * h * channels_num]; 

should be

unsigned char data[w * h * channels_num];

Plus,

stbi_write_jpg("jpg_test_.jpg", w, h, channels_num, data, 100);

should be changed to

stbi_write_jpg("jpg_test_.jpg", w, h, channels_num, data, w * channels_num);

Full solution

void save_image(int w, int h, int channels_num)
{
    unsigned char data[w * h * channels_num];

    int index = 0;
    for (int j = h - 1; j >= 0; --j)
    {
        for (int i = 0; i < w; ++i)
        {
            data[index++] = (unsigned char)(255.0 * i / w);
            data[index++] = (unsigned char)(255.0 * j / h);
            data[index++] = (unsigned char)(255.0 * 0.2);
        }
    }

    stbi_write_jpg("jpg_test_.jpg", w, h, channels_num, data, w * channels_num);
}