我试图用C中的libpng和Python中的NumPy计算RGB三刺激空间中的平均向量，但我得到的结果各不相同。我非常有信心Python使用[ 127.5 127.5 0. ] [ 38.406494 38.433670 38.459641 ]给出了正确的结果。但是，使用以下C块，我得到了/* See if our average vector matches that of Python's */
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <png.h>
// For getting the PNG data and header/information back
typedef struct
{
uint32_t width; // width of image
uint32_t height; // height of image
int bit_depth; // bits/pixel component (should be 8 in RGB)
png_bytep datap; // data
} rTuple;
#define PNG_BYTES_TO_CHECK 8
#define CHANNELS 3
int
check_PNG_signature(unsigned char *buffer)
{
unsigned i;
const unsigned char signature[8] = { 0x89, 0x50, 0x4e, 0x47,
0x0d, 0x0a, 0x1a, 0x0a };
for (i = 0; i < PNG_BYTES_TO_CHECK; ++i)
{
if (buffer[i] != signature[i])
{
fprintf(stderr, "** File sig does not match PNG, received ");
for (i = 0; i < PNG_BYTES_TO_CHECK; ++i)
fprintf(stderr, "%.2X ", buffer[i]);
fprintf(stderr, "\n");
abort();
}
}
return 1;
}
rTuple
read_png_file(char *file_name)
{
/* Get PNG data - I've pieced this together by reading `example.c` from
beginning to end */
printf("** Reading data from %s\n", file_name);
png_uint_32 width, height; // holds width and height of image
uint32_t row; // for iteration later
int bit_depth, color_type, interlace_type;
unsigned char *buff = malloc(PNG_BYTES_TO_CHECK * sizeof(char));
memset(buff, 0, PNG_BYTES_TO_CHECK * sizeof(char));
FILE *fp = fopen(file_name, "rb");
if (fp == NULL) abort();
if (fread(buff, 1, PNG_BYTES_TO_CHECK, fp) != PNG_BYTES_TO_CHECK) {
fprintf(stderr, "** Could not read %d bytes\n", PNG_BYTES_TO_CHECK);
abort();
}
check_PNG_signature(buff);
rewind(fp);
// create and initialize the png_struct, which will be destroyed later
png_structp png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING
, NULL /* Following 3 mean use stderr & longjump method */
, NULL
, NULL
);
if (!png_ptr) abort();
png_infop info_ptr = png_create_info_struct(png_ptr);
if (!info_ptr) abort();
// following I/O initialization method is required
png_init_io(png_ptr, fp);
png_set_sig_bytes(png_ptr, 0); // libpng has this built in too
// call to png_read_info() gives us all of the information from the
// PNG file before the first IDAT (image data chunk)
png_read_info(png_ptr, info_ptr);
// Get header metadata now
png_get_IHDR(png_ptr, info_ptr, &width, &height, &bit_depth, &color_type,
&interlace_type, NULL, NULL);
// Scale 16-bit images to 8-bits as accurately as possible (shouldn't be an
// issue though, since we're working with RGB data)
#ifdef PNG_READ_SCALE_16_TO_8_SUPPORTED
png_set_scale_16(png_ptr);
#else
png_set_strip_16(png_ptr);
#endif
png_set_packing(png_ptr);
// PNGs we're working with should have a color_type RGB
if (color_type == PNG_COLOR_TYPE_PALETTE)
png_set_palette_to_rgb(png_ptr);
// Required since we selected the RGB palette
png_read_update_info(png_ptr, info_ptr);
// Allocate memory to _hold_ the image data now (lines 547-)
png_bytep row_pointers[height];
for (row = 0; row < height; ++row)
row_pointers[row] = NULL;
for (row = 0; row < height; ++row)
row_pointers[row] = png_malloc(png_ptr,\
png_get_rowbytes(png_ptr, info_ptr)
);
png_read_image(png_ptr, row_pointers);
png_read_end(png_ptr, info_ptr);
// Now clean up - the image data is in memory
png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
fclose(fp);
rTuple t = { width, height, bit_depth, *row_pointers };
return t;
}
int
main(int argc, char *argv[])
{
if (argc != 2) {
printf("** Provide filename\n");
abort();
}
char *fileName = argv[1];
// get data read
rTuple data = read_png_file(fileName);
/* let's try computing the absolute average vector */
uint32_t i, j, k;
double *avV = malloc(CHANNELS * sizeof(double));
memset(avV, 0, sizeof(double) * CHANNELS);
double new_px[CHANNELS];
png_bytep row, px;
for (i = 0; i < data.height; ++i)
{
row = &data.datap[i];
for (j = 0; j < data.width; ++j)
{
px = &(row[j * sizeof(int)]);
for (k = 0; k < CHANNELS; ++k) {
new_px[k] = (double)px[k];
avV[k] += new_px[k];
}
}
}
double size = (double)data.width * (double)data.height;
for (k = 0; k < CHANNELS; ++k) {
avV[k] /= size;
printf("channel %d: %lf\n", k + 1, avV[k]);
}
printf("\n");
return 0;
}
的荒谬结果。几个星期以来，我一直盯着我的代码而没有给予任何帮助，所以我想我能看出其他人是否有想法。
此外，我已经使用其他图片测试了此代码，并且它提供了类似的荒谬结果。这很奇怪，因为所有三个数字通常都匹配前4个左右的数字。我不确定是什么导致了这一点。
np.mean(image_data, axis=(0, 1))现在使用Python我只需使用简单的上下文管理器打开图像并计算var shuffle = function (items) {
var item, randomIndex;
for(var i = 0; i < items.length; i++){
randomIndex= (Math.random() * items.length) | 0;
item = items[i];
items[i] = items[randomIndex];
items[randomIndex] = item;
}
}
describe('Suite', function() {
it("should a", function () {
console.log("execute a");
});
it("should b", function () {
console.log("execute b");
});
it("should c", function () {
console.log("execute c");
});
shuffle(this.children); // shuffle the 'it' blocks
});
，这会产生上面的结果。
答案 0 :(得分：1)
基本上，你有一些错误（libpng方面和指针算术），我试图通过比较你的代码Github gist来找到它们。以下是我为制作与Python NumPy相同的图像均值所做的更改列表。
rTuple结构中，您需要使用png_bytep datap将png_byte更改为png_bytep *datap;类型的指针。在read_png_file中，使用png_set_filler在读取图像后添加填充字节。有关详细信息，请参阅here。
if(color_type == PNG_COLOR_TYPE_RGB ||
color_type == PNG_COLOR_TYPE_GRAY ||
color_type == PNG_COLOR_TYPE_PALETTE)
png_set_filler(png_ptr, 0xFF, PNG_FILLER_AFTER);
在read_png_file中，在使用row_pointers
png_read_update_info(png_ptr, info_ptr);之前更新更改
再次，在read_png_file中，使用以下方法更改为图像像素设置内存的方式：
png_bytep *row_pointers = (png_bytep*)malloc(sizeof(png_bytep) * height);
for(row = 0; row < height; row++)
{
row_pointers[row] = malloc(png_get_rowbytes(png_ptr,info_ptr));
}
在main中，将row = &data.datap[i];更改为row = data.datap[i];，以便在此处访问指针。
我不想用与问题几乎相同的代码填充答案，因此如果您只想复制并粘贴答案，这就是指向complete code的链接。