mt_lut problem / How luma is calculated in YV12?

zee944 · 1st February 2009, 18:14

I have a script in which I make the pixels black below luma 40 and over luma 230 with mt_lut:

Code:

LoadPlugin("E:\mt_masktools[2.0a30].dll")

ImageReader("gradient-h.jpg")
ConvertToYV12()
unprocessed=last

mt_lut(last,expr="x 0 >= x 40 <= & 0 x ?",U=2,V=2)  # between luma 0 and 40, make it black
mt_lut(last,expr="x 230 >= 0 x ?",U=2,V=2)  # between luma  230 and 255, make it black
# mt_lut(last,expr="x 40 >= x 90 <= & x 40 - 90 40 - / 90 * x ?",U=2,V=2)
# mt_lut(last,expr="x 180 >= x 230 <= & 180 x 180 - 230 180 - / 180 * - x ?",U=2,V=2)

StackVertical(unprocessed,last)

When I check the result, the cut off point is at luma 29 and 248... instead of the desired 40 and 230. The original image contains only grey shades from [0, 0, 0] to [255, 255, 255]. In the resulting image the darkest pixel is [29, 29, 29], the brightest is [248, 248, 248] in RGB colorspace:

Why? Shouldn't be the cutoff point at [40, 40, 40] and [230, 230, 230]?

How can I fix it?

How luma is converted from RGB to YV12? How an [29, 29, 29] RGB pixel becomes [40, 40, 40] in YV12? Is there a formula for that?

1st February 2009, 18:14	#1 \| Link
zee944 Registered User Join Date: Apr 2007 Posts: 240	mt_lut problem / How luma is calculated in YV12? I have a script in which I make the pixels black below luma 40 and over luma 230 with mt_lut: Code: LoadPlugin("E:\mt_masktools[2.0a30].dll") ImageReader("gradient-h.jpg") ConvertToYV12() unprocessed=last mt_lut(last,expr="x 0 >= x 40 <= & 0 x ?",U=2,V=2) # between luma 0 and 40, make it black mt_lut(last,expr="x 230 >= 0 x ?",U=2,V=2) # between luma 230 and 255, make it black # mt_lut(last,expr="x 40 >= x 90 <= & x 40 - 90 40 - / 90 * x ?",U=2,V=2) # mt_lut(last,expr="x 180 >= x 230 <= & 180 x 180 - 230 180 - / 180 * - x ?",U=2,V=2) StackVertical(unprocessed,last) When I check the result, the cut off point is at luma 29 and 248... instead of the desired 40 and 230. The original image contains only grey shades from [0, 0, 0] to [255, 255, 255]. In the resulting image the darkest pixel is [29, 29, 29], the brightest is [248, 248, 248] in RGB colorspace: Why? Shouldn't be the cutoff point at [40, 40, 40] and [230, 230, 230]? How can I fix it? How luma is converted from RGB to YV12? How an [29, 29, 29] RGB pixel becomes [40, 40, 40] in YV12? Is there a formula for that?