Can't change bit depth of image in magick++

[rohitlal]

I'm generating an image from the Pixel data array in magick++ :

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    image.read(4096,4096, "R", CharPixel, pixelArrayPointer);
    image.read(4096,4096, "G", CharPixel, pixelArrayPointer);
    image.read(4096,4096, "B", CharPixel, pixelArrayPointer);

Then, converting it to grayscale :

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    image.quantizeColorSpace(GRAYColorspace);
    image.quantizeColors(256);
    image.quantize();

Then, writing the image :

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    image.write("my_image.bmp");

On measuring the image, it shows bit-depth of 4 Pixel.

I tried,

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image.channelDepth(ChannelType:GrayChannel, 8); 

after the quantization steps, but still Bit-depth remains 4 bit.

Can anyone please tell me, where I'm going wrong.

[snibgo]

What version of IM?

Perhaps your quantized image has only 16 shades of gray.

[rohitlal]

What version of IM?

Perhaps your quantized image has only 16 shades of gray.

IM version : 7.0.8-23 Q16 x64 2019-01-02

My image in pixel data is 8-bit grayscale image. But, magick++ is writing it as 4-bit image from the above code. Please tell me where I'm going wrong.