XYZ tristimulus bitmap

[ssh4net]

What white point is used for XYZ tristimulus values?
D50 or D65?

I need to import wider than sRGB gamut texture to Mitsuba and found that it support XYZ color space. But no clue what white point used. Color science usually use D50 but sRGB is D65.

Thank you.

[merlinND]

Hello @ssh4net,

I believe we are using D65, but maybe @wjakob can confirm.

[ssh4net]

@merlinND sorry, how should i define that this bitmap is XYZ values?

    'shape': {
        'type': 'rectangle',
        'to_world': mi.ScalarTransform4f.scale([1.0, 2.2*0.21, 2.2*0.29]) @
        mi.ScalarTransform4f(
            [[ 0, 0, 1, 0],
             [ 0, 1, 0, 0],
             [-1, 0, 0, 0],
             [ 0, 0, 0, 1]]
        ),
        'bsdf': {
            'type': 'diffuse',
            'reflectance': {
                'type': 'bitmap',
                'filename': r'x:\FTP\Vlad\SPECTRAL\ColorChartSG_XYZ_D65.exr'
            },
        }
    }

This one load image as RGB as I understand

[merlinND]

If the channels of your EXR image are called x, y and z the Bitmap's pixel format will be automatically detected as XYZ:

mitsuba3/src/core/bitmap.cpp

Lines 1077 to 1081 in d7fdd68

	} else if (found[X] && found[Y] && found[Z]) {
	if (m_struct->field_count() == 3)
	m_pixel_format = PixelFormat::XYZ;
	else if (found[A] && m_struct->field_count() == 4)
	m_pixel_format = PixelFormat::XYZA;

Otherwise, it looks to me like you'd have to build a new mi.Bitmap from the image you load, something like:

bmp_loaded = mi.Bitmap("my_file.exr")
bmp_xyz = mi.Bitmap(np.array(bmp_loaded), pixel_format=mi.PixelFormat.XYZ)

Then, you can pass bmp into your scene dict:

           'reflectance': {
                'type': 'bitmap',
                'bitmap': bmp_xyz,
            },

Not super convenient. I think it would be nicer to be able to:

1. Override pixel_format in the Bitmap(Stream *stream, ...) and Bitmap(const fs::path &path, ...) constructors.
2. Provide a pixel_format for override to the Bitmap texture props.

What do you think @njroussel ?

[njroussel]

Hi :)

Option 1 is nicer, I think. And your suggestion is the only current workaround I know of.

[ssh4net]

@merlinND

Using this way

bmp_loaded = mi.Bitmap("my_file.exr")
bmp_xyz = mi.Bitmap(np.array(bmp_loaded), pixel_format=mi.PixelFormat.XYZ)

Code give a warning:
2024-08-14 11:53:29 WARN wrk8 [BitmapTexture] BitmapTexture: texture named "" contains pixels that exceed the [0, 1] range!

Why can this warning be? The XYZ data in file is in 0~1 range. And the
np.array(bmp_xyz ).tofile('d:/text.txt,sep=' ') do not have any values higher than 1.0 or less than 0.0