Reading Pixels from an image byte array

Question

I have a jpeg image. I save this bitmapdata to a byte array.

This jpeg has a width of 100 and a height of 100.

I want to extract an image of Rectanlge(10,10,20,20);

Obviously, I can interact through this byte array but I am unsure how to relate the x,y pixels of what I want to this byte array. I know that I have to use the stride and pixel size which is 4 as it is rgba.

I have this which was from this link cropping an area from BitmapData with C#.

Bitmap bmp = new Bitmap(_file);
Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
 BitmapData rawOriginal = bmp.LockBits(new Rectangle(0, 0, bmp.Width, bmp.Height), ImageLockMode.ReadWrite, PixelFormat.Format32bppRgb);

int origByteCount = rawOriginal.Stride * rawOriginal.Height;
byte[] origBytes = new Byte[origByteCount];
System.Runtime.InteropServices.Marshal.Copy(rawOriginal.Scan0, origBytes, 0, origByteCount);

int BPP = 4;  
int width = 20;
int height = 20;
int startX = 10;
int startY = 10;

for (int i = 0; i < height; i++)
{
for (int j = 0; j < width * BPP; j += BPP)
{
int origIndex = (startX * rawOriginal.Stride) + (i * rawOriginal.Stride) + (startY * BPP) + (j);
int croppedIndex = (i * width * BPP) + (j);
                        //copy data: once for each channel
for (int k = 0; k < BPP; k++)
{
croppedBytes[croppedIndex + k] = origBytes[origIndex + k];
}
}
}

But this:

int origIndex = (startX * rawOriginal.Stride) + (i * rawOriginal.Stride) + (startY * BPP) + (j);

I found is incorrect.

Does anyone know what value I should set here please?

thanks

Answer 1

When you're working with bitmap data, there are 2 important things to keep in mind:

1. the BPP (bytes per pixel): which here is 4
2. the stride (number of bytes on one line of the image), which here would be 4 * width

so if you want to get the offset of a pixel, just use this formula:

int offset = x * BPP + y * stride;

if you want to extract only a part of your bitmap you could just do this:

int i = 0;

for(int y = startY; y < startY + height; y++)
{
    for(int k = startX * bpp + y * stride; k < (startX + width) * bpp + y * stride; k++)
    {
        croppedBytes[i] = origBytes[k];
        i++;
    }
}

Answer 2

Stride is bytes per line (Y), you shouldn't multiply x at any point by Stride

int y = startY + i;
int x = startX;
int origIndex = y * rawOriginal.Stride + x * BPP;