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Date: Fri, 08 Mar 2019 11:23:59 +0100
From: Philipp Zabel <p.zabel@...gutronix.de>
To: Steve Longerbeam <slongerbeam@...il.com>,
linux-media@...r.kernel.org
Cc: Tim Harvey <tharvey@...eworks.com>, stable@...r.kernel.org,
"open list:DRM DRIVERS FOR FREESCALE IMX"
<dri-devel@...ts.freedesktop.org>,
open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 2/7] gpu: ipu-v3: ipu-ic: Fix BT.601 coefficients
Hi Steve,
On Thu, 2019-03-07 at 15:33 -0800, Steve Longerbeam wrote:
> The ycbcr2rgb and inverse rgb2ycbcr tables define the BT.601 Y'CbCr
> encoding coefficients.
>
> The rgb2ycbcr table specifically describes the BT.601 encoding from
> full range RGB to full range YUV. Add table comments to make this more
> clear.
>
> The ycbcr2rgb inverse table describes encoding YUV limited range to RGB
> full range. To be consistent with the rgb2ycbcr table, convert this to
> YUV full range to RGB full range, and adjust/expand on the comments.
>
> The ic_csc_rgb2rgb table is just an identity matrix, so rename to
> ic_encode_identity.
>
> Fixes: 1aa8ea0d2bd5d ("gpu: ipu-v3: Add Image Converter unit")
>
> Suggested-by: Philipp Zabel <p.zabel@...gutronix.de>
> Signed-off-by: Steve Longerbeam <slongerbeam@...il.com>
> Cc: stable@...r.kernel.org
> ---
> drivers/gpu/ipu-v3/ipu-ic.c | 61 ++++++++++++++++++++++---------------
> 1 file changed, 37 insertions(+), 24 deletions(-)
>
> diff --git a/drivers/gpu/ipu-v3/ipu-ic.c b/drivers/gpu/ipu-v3/ipu-ic.c
> index 18816ccf600e..b63a2826b629 100644
> --- a/drivers/gpu/ipu-v3/ipu-ic.c
> +++ b/drivers/gpu/ipu-v3/ipu-ic.c
> @@ -175,7 +175,7 @@ static inline void ipu_ic_write(struct ipu_ic *ic, u32 value, unsigned offset)
> writel(value, ic->priv->base + offset);
> }
>
> -struct ic_csc_params {
> +struct ic_encode_coeff {
This less accurate. These are called IC (Task) Parameters in the
reference manual, the 64-bit aligned words are called CSC words. Beside
the coefficients, this structure also contains the coefficient scale,
the offsets, and the saturation mode flag.
> s16 coeff[3][3]; /* signed 9-bit integer coefficients */
> s16 offset[3]; /* signed 11+2-bit fixed point offset */
> u8 scale:2; /* scale coefficients * 2^(scale-1) */
> @@ -183,13 +183,15 @@ struct ic_csc_params {
> };
>
> /*
> - * Y = R * .299 + G * .587 + B * .114;
> - * U = R * -.169 + G * -.332 + B * .500 + 128.;
> - * V = R * .500 + G * -.419 + B * -.0813 + 128.;
> + * BT.601 encoding from RGB full range to YUV full range:
> + *
> + * Y = .2990 * R + .5870 * G + .1140 * B
> + * U = -.1687 * R - .3313 * G + .5000 * B + 128
> + * V = .5000 * R - .4187 * G - .0813 * B + 128
> */
> -static const struct ic_csc_params ic_csc_rgb2ycbcr = {
> +static const struct ic_encode_coeff ic_encode_rgb2ycbcr_601 = {
> .coeff = {
> - { 77, 150, 29 },
> + { 77, 150, 29 },
> { 469, 427, 128 },
> { 128, 405, 491 },
We could subtract 512 from the negative values, to improve readability.
> },
> @@ -197,8 +199,11 @@ static const struct ic_csc_params ic_csc_rgb2ycbcr = {
> .scale = 1,
> };
>
> -/* transparent RGB->RGB matrix for graphics combining */
> -static const struct ic_csc_params ic_csc_rgb2rgb = {
> +/*
> + * identity matrix, used for transparent RGB->RGB graphics
> + * combining.
> + */
> +static const struct ic_encode_coeff ic_encode_identity = {
> .coeff = {
> { 128, 0, 0 },
> { 0, 128, 0 },
> @@ -208,17 +213,25 @@ static const struct ic_csc_params ic_csc_rgb2rgb = {
> };
>
> /*
> - * R = (1.164 * (Y - 16)) + (1.596 * (Cr - 128));
> - * G = (1.164 * (Y - 16)) - (0.392 * (Cb - 128)) - (0.813 * (Cr - 128));
> - * B = (1.164 * (Y - 16)) + (2.017 * (Cb - 128);
> + * Inverse BT.601 encoding from YUV full range to RGB full range:
> + *
> + * R = 1. * Y + 0 * (Cb - 128) + 1.4020 * (Cr - 128)
> + * G = 1. * Y - .3442 * (Cb - 128) - 0.7142 * (Cr - 128)
Should that be ^^^^^ .3441 and ^^^^^ .7141 ?
The coefficients and offsets after rounding should end up the same.
Also, let's consistently either add the leading zero, or leave it out.
> + * B = 1. * Y + 1.7720 * (Cb - 128) + 0 * (Cr - 128)
> + *
> + * equivalently (factoring out the offsets):
> + *
> + * R = 1. * Y + 0 * Cb + 1.4020 * Cr - 179.456
> + * G = 1. * Y - .3442 * Cb - 0.7142 * Cr + 135.475
> + * B = 1. * Y + 1.7720 * Cb + 0 * Cr - 226.816
> */
> -static const struct ic_csc_params ic_csc_ycbcr2rgb = {
> +static const struct ic_encode_coeff ic_encode_ycbcr2rgb_601 = {
> .coeff = {
> - { 149, 0, 204 },
> - { 149, 462, 408 },
> - { 149, 255, 0 },
> + { 128, 0, 179 },
> + { 128, 468, 421 },
> + { 128, 227, 0 },
> },
> - .offset = { -446, 266, -554 },
> + .offset = { -359, 271, -454 },
These seem to be correct. Again, I think this would be easier to read if
the negative coefficients were written with a sign as well.
> .scale = 2,
> };
>
> @@ -228,7 +241,7 @@ static int init_csc(struct ipu_ic *ic,
> int csc_index)
> {
> struct ipu_ic_priv *priv = ic->priv;
> - const struct ic_csc_params *params;
> + const struct ic_encode_coeff *coeff;
> u32 __iomem *base;
> const u16 (*c)[3];
> const u16 *a;
> @@ -238,26 +251,26 @@ static int init_csc(struct ipu_ic *ic,
> (priv->tpmem_base + ic->reg->tpmem_csc[csc_index]);
>
> if (inf == IPUV3_COLORSPACE_YUV && outf == IPUV3_COLORSPACE_RGB)
> - params = &ic_csc_ycbcr2rgb;
> + coeff = &ic_encode_ycbcr2rgb_601;
> else if (inf == IPUV3_COLORSPACE_RGB && outf == IPUV3_COLORSPACE_YUV)
> - params = &ic_csc_rgb2ycbcr;
> + coeff = &ic_encode_rgb2ycbcr_601;
> else if (inf == IPUV3_COLORSPACE_RGB && outf == IPUV3_COLORSPACE_RGB)
> - params = &ic_csc_rgb2rgb;
> + coeff = &ic_encode_identity;
> else {
> dev_err(priv->ipu->dev, "Unsupported color space conversion\n");
> return -EINVAL;
> }
>
> /* Cast to unsigned */
> - c = (const u16 (*)[3])params->coeff;
> - a = (const u16 *)params->offset;
> + c = (const u16 (*)[3])coeff->coeff;
> + a = (const u16 *)coeff->offset;
This looks weird to me. I'd be in favor of not renaming the type.
regards
Philipp
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