[英]Loading image with stb_image library as float outputs wrong values
When I load an image as float using STB_Image the values seem to be off.当我使用STB_Image将图像加载为浮点数时,值似乎已关闭。 I created an image to test it
我创建了一个图像来测试它
.
. (The used RGB code here is [127, 255, 32])
(这里使用的 RGB 代码是 [127, 255, 32])
When I load this image as unsigned char
using stbi_load()
I get the correct values.当我使用
stbi_load()
将此图像加载为unsigned char
我得到了正确的值。 But when I load it as float
using stbi_loadf()
I get wrong values which don't really make sense to me.但是当我使用
stbi_loadf()
它加载为float
我得到了错误的值,这对我来说没有意义。
This is the code I used for testing:这是我用于测试的代码:
#define STB_IMAGE_IMPLEMENTATION
#include "stb_image.h"
#include <sstream>
#include <iomanip>
#include <iostream>
#include <string>
struct ColorF {
float r;
float g;
float b;
float a;
std::string toString() {
std::stringstream stream;
stream << std::fixed << std::setprecision(2) << "[" << this->r << ", " << this->g << ", " << this->b << ", " << this->a << "]";
return stream.str();
}
};
struct ColorUC {
unsigned char r;
unsigned char g;
unsigned char b;
unsigned char a;
std::string toString() {
std::stringstream stream;
stream << "[" << (float) this->r / 255.0f << ", " << (float) this->g / 255.0f << ", " << (float) this->b / 255.0f << ", " << (float) this->a / 255.0f << "]";
return stream.str();
}
};
int main() {
int width, height, channels;
float* image = stbi_loadf("test.png", &width, &height, &channels, STBI_rgb_alpha);
// print content of first pixel of the image
std::cout << ((ColorF*) image)->toString() << std::endl;
unsigned char* jpeg = stbi_load("test.png", &width, &height, &channels, STBI_rgb_alpha);
// print content of first pixel of the image
std::cout << ((ColorUC*) jpeg)->toString() << std::endl;
stbi_image_free(image);
stbi_image_free(jpeg);
return 0;
}
The test output I get is this:我得到的测试输出是这样的:
[0.22, 1.00, 0.01, 1.00]
[0.22, 1.00, 0.01, 1.00]
[0.50, 1.00, 0.13, 1.00][0.50, 1.00, 0.13, 1.00]
In theory this should print out the same values, the bottom one being the correct one, on both lines but for some reason it doesn't.从理论上讲,这应该打印出相同的值,底部的值是正确的值,在两行上,但由于某种原因它没有。
Now i could of course use the unsigned char values and write myself a function that converts everything to the proper float values but I feel like there should be a way to do this just using STB_Image itself.现在我当然可以使用 unsigned char 值并为自己编写一个函数,将所有内容转换为正确的浮点值,但我觉得应该有一种方法可以仅使用 STB_Image 本身来做到这一点。
From the stb_image.h
header comments:从
stb_image.h
标题注释:
// If you load LDR images through this interface, those images will
// be promoted to floating point values, run through the inverse of
// constants corresponding to the above:
//
// stbi_ldr_to_hdr_scale(1.0f);
// stbi_ldr_to_hdr_gamma(2.2f);
That means the stbi_loadf()
function uses built-in gamma correction to return its float values.这意味着
stbi_loadf()
函数使用内置伽马校正来返回其浮点值。
The gamma correction formula looks like this:伽马校正公式如下所示:
Vout = (Vin / 255)^γ; Where γ = 2.2
When you put [127, 255, 32]
through the gamma correction formula, you get the result you are seeing:当你把
[127, 255, 32]
通过伽马校正公式时,你会得到你看到的结果:
R: 0.22 = (127 / 255)^2.2
G: 1.00 = (255 / 255)^2.2
B: 0.01 = ( 32 / 255)^2.2
You can set the stb_image gamma factor with stbi_ldr_to_hdr_gamma(1.0f)
as a quick fix.您可以使用
stbi_ldr_to_hdr_gamma(1.0f)
设置 stb_image 伽马系数作为快速修复。
However, stb_image defines the stbi_loadf()
specifically to support non-linear encoded images (HDR or gamma corrected).但是,stb_image 专门定义了
stbi_loadf()
以支持非线性编码图像(HDR 或伽马校正)。 If you were to load an actual HDR image with that function, setting the gamma to 1.0
would negatively impact the accuracy of the image when drawn;如果要使用该功能加载实际的 HDR 图像,将 gamma 设置为
1.0
会对绘制图像的准确性产生负面影响; So it is best to only use stbi_loadf()
if you actually need the image to be loaded with gamma correction.因此,如果您确实需要使用伽马校正加载图像,最好只使用
stbi_loadf()
。
Now i could of course use the unsigned char values and write myself a function that converts everying to the proper float values but I feel like there should be a way to do this just using STB_Image itself.
现在我当然可以使用 unsigned char 值并为自己编写一个函数,将每个值转换为正确的浮点值,但我觉得应该有一种方法可以仅使用 STB_Image 本身来做到这一点。
Manually converting the 8-bit image data to a float using Vout = (float)Vin / 255f
is an obvious tell to readers of your code that the image data is not gamma corrected.使用
Vout = (float)Vin / 255f
手动将 8 位图像数据转换为浮点数明显告诉代码的读者,图像数据没有经过伽马校正。 Using stbi_loadf()
with stbi_ldr_to_hdr_gamma(1.0f)
can obscure this important detail.将
stbi_loadf()
与stbi_ldr_to_hdr_gamma(1.0f)
可能会掩盖这一重要细节。
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