On 2023-10-30 09:29, Pekka Paalanen wrote:
> On Thu, 19 Oct 2023 17:21:21 -0400
> Harry Wentland <[email protected]> wrote:
>
>> When the floor LUT index (drm_fixp2int(lut_index) is the last
>> index of the array the ceil LUT index will point to an entry
>> beyond the array. Make sure we guard against it and use the
>> value of the floot LUT index.
>>
>> Blurb about LUT creation and how first element should be 0x0 and
>> last one 0xffff.
>>
>> Hold on, is that even correct? What should the ends of a LUT be?
>> How does UNORM work and how does it apply to LUTs?
>
> Do you mean how should UNORM input value map to LUT entries for LUT
> indexing?
>
> I suppose UNORM 16-bit converts to nominal real values as:
> - 0x0: 0.0
> - 0xffff: 1.0
>
> And in LUT, you want 0.0 to map to the first LUT element exactly, and
> 1.0 to map to the last LUT element exactly, even if whatever
> interpolation may be in use, right?
>
> If so, it is important to make sure that, assuming linear interpolation
> for instance, there is no "dead zone" at either end. Given high
> interpolation precision, any step away from 0.0 or 1.0 needs to imply a
> change in the real-valued output, assuming e.g. identity LUT.
>
> If LUT has N elements, and 16-bit UNORM input value is I, then (in
> naive real-valued math, so no implicit truncation between operations)
>
> x = I / 0xffff * (N - 1)
> ia = floor(x)
> ib = min(ia + 1, N - 1)
>
> f = x - floor(x)
> y = (1 - f) * LUT[ia] + f * LUT[ib]
>
>
> Does that help?
>
Thanks. Yes, this is what the code is doing (with this commit).
The commit description was an oversight and only reflect my initial
thoughts when coding it, before I made sure this is the right way
to go about it. I'll update it.
Harry
> In my mind, I'm thinking of a uniformly distributed LUT as a 1-D
> texture, because that's how I have implemented them in GL. There you
> have to be careful so that input values 0.0 and 1.0 map to the *center*
> of the first and last texel, and not to the edges of the texture like
> texture coordinates do. Then you can use the GL linear texture
> interpolation as-is.
>
>
> Thanks,
> pq
>
>
>> Signed-off-by: Harry Wentland <[email protected]>
>> Cc: Ville Syrjala <[email protected]>
>> Cc: Pekka Paalanen <[email protected]>
>> Cc: Simon Ser <[email protected]>
>> Cc: Harry Wentland <[email protected]>
>> Cc: Melissa Wen <[email protected]>
>> Cc: Jonas Ådahl <[email protected]>
>> Cc: Sebastian Wick <[email protected]>
>> Cc: Shashank Sharma <[email protected]>
>> Cc: Alexander Goins <[email protected]>
>> Cc: Joshua Ashton <[email protected]>
>> Cc: Michel Dänzer <[email protected]>
>> Cc: Aleix Pol <[email protected]>
>> Cc: Xaver Hugl <[email protected]>
>> Cc: Victoria Brekenfeld <[email protected]>
>> Cc: Sima <[email protected]>
>> Cc: Uma Shankar <[email protected]>
>> Cc: Naseer Ahmed <[email protected]>
>> Cc: Christopher Braga <[email protected]>
>> Cc: Abhinav Kumar <[email protected]>
>> Cc: Arthur Grillo <[email protected]>
>> Cc: Hector Martin <[email protected]>
>> Cc: Liviu Dudau <[email protected]>
>> Cc: Sasha McIntosh <[email protected]>
>> ---
>> drivers/gpu/drm/vkms/vkms_composer.c | 14 ++++++++++----
>> 1 file changed, 10 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/vkms/vkms_composer.c
>> b/drivers/gpu/drm/vkms/vkms_composer.c
>> index a0a3a6fd2926..cf1dff162920 100644
>> --- a/drivers/gpu/drm/vkms/vkms_composer.c
>> +++ b/drivers/gpu/drm/vkms/vkms_composer.c
>> @@ -123,6 +123,8 @@ static u16 apply_lut_to_channel_value(const struct
>> vkms_color_lut *lut, u16 chan
>> enum lut_channel channel)
>> {
>> s64 lut_index = get_lut_index(lut, channel_value);
>> + u16 *floor_lut_value, *ceil_lut_value;
>> + u16 floor_channel_value, ceil_channel_value;
>>
>> /*
>> * This checks if `struct drm_color_lut` has any gap added by the
>> compiler
>> @@ -130,11 +132,15 @@ static u16 apply_lut_to_channel_value(const struct
>> vkms_color_lut *lut, u16 chan
>> */
>> static_assert(sizeof(struct drm_color_lut) == sizeof(__u16) * 4);
>>
>> - u16 *floor_lut_value = (__u16 *)&lut->base[drm_fixp2int(lut_index)];
>> - u16 *ceil_lut_value = (__u16 *)&lut->base[drm_fixp2int_ceil(lut_index)];
>> + floor_lut_value = (__u16 *)&lut->base[drm_fixp2int(lut_index)];
>> + if (drm_fixp2int(lut_index) == (lut->lut_length - 1))
>> + /* We're at the end of the LUT array, use same value for ceil
>> and floor */
>> + ceil_lut_value = floor_lut_value;
>> + else
>> + ceil_lut_value = (__u16
>> *)&lut->base[drm_fixp2int_ceil(lut_index)];
>>
>> - u16 floor_channel_value = floor_lut_value[channel];
>> - u16 ceil_channel_value = ceil_lut_value[channel];
>> + floor_channel_value = floor_lut_value[channel];
>> + ceil_channel_value = ceil_lut_value[channel];
>>
>> return lerp_u16(floor_channel_value, ceil_channel_value,
>> lut_index & DRM_FIXED_DECIMAL_MASK);
>
