How to get the calculate the RGB values of a pixel from the luminance? - c++

Question

I want to compute the RGB values from the luminance.
The data that I know are :
the new luminance (the value that I want to apply)
the old luminance
the old RGB values.
We can compute the luminance from the RGB values like this :
uint8_t luminance = R * 0.21 + G * 0.71 + B * 0.07;
My code is :
// We create a function to set the luminance of a pixel
void jpegImage::setLuminance(uint8_t newLuminance, unsigned int x, unsigned int y) {
// If the X or Y value is out of range, we throw an error
if(x >= width) {
throw std::runtime_error("Error : in jpegImage::setLuminance : The X value is out of range");
}
else if(y >= height) {
throw std::runtime_error("Error : in jpegImage::setLuminance : The Y value is out of range");
}
// If the image is monochrome
if(pixelSize == 1) {
// We set the pixel value to the luminance
pixels[y][x] = newLuminance;
}
// Else if the image is colored, we throw an error
else if(pixelSize == 3) {
// I don't know how to proceed
// My image is stored in a std::vector<std::vector<uint8_t>> pixels;
// This is a list that contain the lines of the image
// Each line contains the RGB values of the following pixels
// For example an image with 2 columns and 3 lines
// [[R, G, B, R, G, B], [R, G, B, R, G, B], [R, G, B, R, G, B]]
// For example, the R value with x = 23, y = 12 is:
// pixels[12][23 * pixelSize];
// For example, the B value with x = 23, y = 12 is:
// pixels[12][23 * pixelSize + 2];
// (If the image is colored, the pixelSize will be 3 (R, G and B)
// (If the image is monochrome the pixelSIze will be 1 (just the luminance value)
}
}
How can I proceed ?
Thanks !

Answer 1

You don't need the old luminance if you have the original RGB.
Referencing https://www.fourcc.org/fccyvrgb.php for YUV to RGB conversion.
Compute U and V from original RGB:
```
V = (0.439 * R) - (0.368 * G) - (0.071 * B) + 128
U = -(0.148 * R) - (0.291 * G) + (0.439 * B) + 128
```
Y is the new luminance normalized to a value between 0 and 255
Then just convert back to RGB:
B = 1.164(Y - 16) + 2.018(U - 128)
G = 1.164(Y - 16) - 0.813(V - 128) - 0.391(U - 128)
R = 1.164(Y - 16) + 1.596(V - 128)
Make sure you clamp your computed values of each equation to be in range of 0..255. Some of these formulas can convert a YUV or RGB value to something less than 0 or higher than 255.
There's also multiple formula for converting between YUV and RGB. (Different constants). I noticed the page listed above has a different computation for Y than you cited. They are all relatively close with different precisions and adjustments. For just changing the brightness of a pixel, almost any formula will do.
Updated
I originally deleted this answer after the OP suggested it wasn't working for him. I was too busy for the last few days to investigate, but I wrote some sample code to confirm my hypothesis. At the bottom of this answer is a snippet of GDI+ based code increases the luminance of an image by a variable amount. Along with the code is an image that I tested this out on and two conversions. One at 130% brightness. Another at 170% brightness.
Here's a sample conversion
Original Image
Updated Image (at 130% Y)
Updated Image (at 170% Y)
Source:
#define CLAMP(val) {val = (val > 255) ? 255 : ((val < 0) ? 0 : val);}
void Brighten(Gdiplus::BitmapData& dataIn, Gdiplus::BitmapData& dataOut, const double YMultiplier=1.3)
{
if ( ((dataIn.PixelFormat != PixelFormat24bppRGB) && (dataIn.PixelFormat != PixelFormat32bppARGB)) ||
((dataOut.PixelFormat != PixelFormat24bppRGB) && (dataOut.PixelFormat != PixelFormat32bppARGB)))
{
return;
}
if ((dataIn.Width != dataOut.Width) || (dataIn.Height != dataOut.Height))
{
// images sizes aren't the same
return;
}
const size_t incrementIn = dataIn.PixelFormat == PixelFormat24bppRGB ? 3 : 4;
const size_t incrementOut = dataOut.PixelFormat == PixelFormat24bppRGB ? 3 : 4;
const size_t width = dataIn.Width;
const size_t height = dataIn.Height;
for (size_t y = 0; y < height; y++)
{
auto ptrRowIn = (BYTE*)(dataIn.Scan0) + (y * dataIn.Stride);
auto ptrRowOut = (BYTE*)(dataOut.Scan0) + (y * dataOut.Stride);
for (size_t x = 0; x < width; x++)
{
uint8_t B = ptrRowIn[0];
uint8_t G = ptrRowIn[1];
uint8_t R = ptrRowIn[2];
uint8_t A = (incrementIn == 3) ? 0xFF : ptrRowIn[3];
auto Y = (0.257 * R) + (0.504 * G) + (0.098 * B) + 16;
auto V = (0.439 * R) - (0.368 * G) - (0.071 * B) + 128;
auto U = -(0.148 * R) - (0.291 * G) + (0.439 * B) + 128;
Y *= YMultiplier;
auto newB = 1.164*(Y - 16) + 2.018*(U - 128);
auto newG = 1.164*(Y - 16) - 0.813*(V - 128) - 0.391*(U - 128);
auto newR = 1.164*(Y - 16) + 1.596*(V - 128);
CLAMP(newR);
CLAMP(newG);
CLAMP(newB);
ptrRowOut[0] = newB;
ptrRowOut[1] = newG;
ptrRowOut[2] = newR;
if (incrementOut == 4)
{
ptrRowOut[3] = A; // keep original alpha
}
ptrRowIn += incrementIn;
ptrRowOut += incrementOut;
}
}
}