[libcamera-devel] [PATCH 3/3] qcam: viewfinder_gl: Take color space into account for YUV rendering
Laurent Pinchart
laurent.pinchart at ideasonboard.com
Tue Aug 30 19:27:15 CEST 2022
Hi Umang,
On Tue, Aug 30, 2022 at 07:43:12PM +0530, Umang Jain wrote:
> On 8/29/22 3:34 PM, Laurent Pinchart via libcamera-devel wrote:
> > Update the YUV shaders and the viewfinder_gl to correctly take the
> > Y'CbCr encoding and the quantization range into account when rendering
> > YUV formats to RGB. Support for the primaries and transfer function will
> > be added in a subsequent step.
> >
> > Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
>
> Patch looks good and straight forward for most parts, however few
> specifics are still a bit unclear to me
>
> > ---
> > src/qcam/assets/shader/YUV_2_planes.frag | 27 ++++----
> > src/qcam/assets/shader/YUV_3_planes.frag | 23 ++++---
> > src/qcam/assets/shader/YUV_packed.frag | 17 ++---
> > src/qcam/viewfinder_gl.cpp | 79 +++++++++++++++++++++++-
> > src/qcam/viewfinder_gl.h | 2 +
> > 5 files changed, 115 insertions(+), 33 deletions(-)
> >
> > diff --git a/src/qcam/assets/shader/YUV_2_planes.frag b/src/qcam/assets/shader/YUV_2_planes.frag
> > index 254463c05cac..da8dbcc5f801 100644
> > --- a/src/qcam/assets/shader/YUV_2_planes.frag
> > +++ b/src/qcam/assets/shader/YUV_2_planes.frag
> > @@ -13,27 +13,30 @@ varying vec2 textureOut;
> > uniform sampler2D tex_y;
> > uniform sampler2D tex_u;
> >
> > +const mat3 yuv2rgb_matrix = mat3(
> > + YUV2RGB_MATRIX
> > +);
> > +
> > +const vec3 yuv2rgb_offset = vec3(
> > + YUV2RGB_Y_OFFSET / 255.0, 128.0 / 255.0, 128.0 / 255.0
>
> I understood the YUV2RGB_Y_OFFSET #define but don't understand where
> other values come from (or why they exist :D)
The quantization of the Cb and Cr values in all relevant color spaces
(ITU-R BT.601, BT.709, BT.2020, ...) add an offset of 128 (for 8-bit
values). For instance, in BT.709, we have
D'Cb = INT[(224*E'Cb + 128)*2^(n-8)]
where D'Cb is the Cb signal after quantization, E'Cb the Cb signal
before quantization (in the [-0.5, 0.5] range), and n the number of
bits). INT[] denotes rounding to the closest integer.
The 224 multiplier creates a limited quantization range, following the
above formula, -0.5 will be quantized to INT[224 * -0.5 + 128] = 16, and
0.5 to INT[224 * 0.5 + 128] = 240. The values are then stored as 8-bit
unsigned integers in memory.
For full range quantization, the same applies, with a multiplier equal
to 255 instead of 224. [-0.5, 0.5] is thus mapped to [0, 255].
We need to apply the reverse quantization on D'Y, D'Cb and D'Cr in order
to get the original E'Y, E'Cb and E'Cr values (in the [0.0, 1.0] and
[-0.5, 0.5] ranges respectively for E'Y and E'C[br]. Starting with full
range, given
D'Cb = INT[(255*E'Cb + 128)] (for 8-bit data)
the inverse is given by
E'Cb = (D'Cb - 128) / 255
or
E'Cb = D'Cb / 255 - 128 / 255
OpenGL, when reading texture data through a floating point texture
sampler (which we do in the shader by calling texture2D on a sampler2D
variable), normalizes the values stored in memory ([0, 255]) to the
[0.0, 1.0] range. This means that the D'Cb value is already divided by
255 by the GPU. We only need to subtract 128 / 255 to get the original
E'Cb value.
In the limited quantization range case, we have
D'Cb = INT[(225*E'Cb + 128)] (for 8-bit data)
the inverse is given by
E'Cb = (D'Cb - 128) / 224
Let's introduce the 255 factor:
E'Cb = (D'Cb - 128) / 255 * 255 / 224
which can also be written as
E'Cb = (D'Cb / 255 - 128 / 255) * 255 / 224
We thus have
E'Cb(lim) = E'Cb(full) * 255 / 224
The shader doesn't include the 255 / 224 multiplier directly, it gets
included by the C++ code in the yuv2rgb matrix, and there's no need for
a different offset between the limited and full range quantization.
I hope this helps clarifying the implementation.
> Maybe I should start learning shaders programming ;-)
>
> Reviewed-by: Umang Jain <umang.jain at ideasonboard.com>
>
> > +);
> > +
> > void main(void)
> > {
> > vec3 yuv;
> > - vec3 rgb;
> > - mat3 yuv2rgb_bt601_mat = mat3(
> > - vec3(1.164, 1.164, 1.164),
> > - vec3(0.000, -0.392, 2.017),
> > - vec3(1.596, -0.813, 0.000)
> > - );
> >
> > - yuv.x = texture2D(tex_y, textureOut).r - 0.063;
> > + yuv.x = texture2D(tex_y, textureOut).r;
> > #if defined(YUV_PATTERN_UV)
> > - yuv.y = texture2D(tex_u, textureOut).r - 0.500;
> > - yuv.z = texture2D(tex_u, textureOut).a - 0.500;
> > + yuv.y = texture2D(tex_u, textureOut).r;
> > + yuv.z = texture2D(tex_u, textureOut).a;
> > #elif defined(YUV_PATTERN_VU)
> > - yuv.y = texture2D(tex_u, textureOut).a - 0.500;
> > - yuv.z = texture2D(tex_u, textureOut).r - 0.500;
> > + yuv.y = texture2D(tex_u, textureOut).a;
> > + yuv.z = texture2D(tex_u, textureOut).r;
> > #else
> > #error Invalid pattern
> > #endif
> >
> > - rgb = yuv2rgb_bt601_mat * yuv;
> > + vec3 rgb = yuv2rgb_matrix * (vec3(y, uv) - yuv2rgb_offset);
> > +
> > gl_FragColor = vec4(rgb, 1.0);
> > }
> > diff --git a/src/qcam/assets/shader/YUV_3_planes.frag b/src/qcam/assets/shader/YUV_3_planes.frag
> > index 2be74b5d2a9d..e754129d74d1 100644
> > --- a/src/qcam/assets/shader/YUV_3_planes.frag
> > +++ b/src/qcam/assets/shader/YUV_3_planes.frag
> > @@ -14,20 +14,23 @@ uniform sampler2D tex_y;
> > uniform sampler2D tex_u;
> > uniform sampler2D tex_v;
> >
> > +const mat3 yuv2rgb_matrix = mat3(
> > + YUV2RGB_MATRIX
> > +);
> > +
> > +const vec3 yuv2rgb_offset = vec3(
> > + YUV2RGB_Y_OFFSET / 255.0, 128.0 / 255.0, 128.0 / 255.0
> > +);
> > +
> > void main(void)
> > {
> > vec3 yuv;
> > - vec3 rgb;
> > - mat3 yuv2rgb_bt601_mat = mat3(
> > - vec3(1.164, 1.164, 1.164),
> > - vec3(0.000, -0.392, 2.017),
> > - vec3(1.596, -0.813, 0.000)
> > - );
> >
> > - yuv.x = texture2D(tex_y, textureOut).r - 0.063;
> > - yuv.y = texture2D(tex_u, textureOut).r - 0.500;
> > - yuv.z = texture2D(tex_v, textureOut).r - 0.500;
> > + yuv.x = texture2D(tex_y, textureOut).r;
> > + yuv.y = texture2D(tex_u, textureOut).r;
> > + yuv.z = texture2D(tex_v, textureOut).r;
> > +
> > + vec3 rgb = yuv2rgb_matrix * (vec3(y, uv) - yuv2rgb_offset);
> >
> > - rgb = yuv2rgb_bt601_mat * yuv;
> > gl_FragColor = vec4(rgb, 1.0);
> > }
> > diff --git a/src/qcam/assets/shader/YUV_packed.frag b/src/qcam/assets/shader/YUV_packed.frag
> > index d6efd4ce92a9..b9ef9d41beae 100644
> > --- a/src/qcam/assets/shader/YUV_packed.frag
> > +++ b/src/qcam/assets/shader/YUV_packed.frag
> > @@ -14,15 +14,16 @@ varying vec2 textureOut;
> > uniform sampler2D tex_y;
> > uniform vec2 tex_step;
> >
> > +const mat3 yuv2rgb_matrix = mat3(
> > + YUV2RGB_MATRIX
> > +);
> > +
> > +const vec3 yuv2rgb_offset = vec3(
> > + YUV2RGB_Y_OFFSET / 255.0, 128.0 / 255.0, 128.0 / 255.0
> > +);
> > +
> > void main(void)
> > {
> > - mat3 yuv2rgb_bt601_mat = mat3(
> > - vec3(1.164, 1.164, 1.164),
> > - vec3(0.000, -0.392, 2.017),
> > - vec3(1.596, -0.813, 0.000)
> > - );
> > - vec3 yuv2rgb_bt601_offset = vec3(0.063, 0.500, 0.500);
> > -
> > /*
> > * The sampler won't interpolate the texture correctly along the X axis,
> > * as each RGBA pixel effectively stores two pixels. We thus need to
> > @@ -76,7 +77,7 @@ void main(void)
> >
> > float y = mix(y_left, y_right, step(0.5, f_x));
> >
> > - vec3 rgb = yuv2rgb_bt601_mat * (vec3(y, uv) - yuv2rgb_bt601_offset);
> > + vec3 rgb = yuv2rgb_matrix * (vec3(y, uv) - yuv2rgb_offset);
> >
> > gl_FragColor = vec4(rgb, 1.0);
> > }
> > diff --git a/src/qcam/viewfinder_gl.cpp b/src/qcam/viewfinder_gl.cpp
> > index ec295b6de0dd..e2aa24703ff0 100644
> > --- a/src/qcam/viewfinder_gl.cpp
> > +++ b/src/qcam/viewfinder_gl.cpp
> > @@ -7,9 +7,12 @@
> >
> > #include "viewfinder_gl.h"
> >
> > +#include <array>
> > +
> > #include <QByteArray>
> > #include <QFile>
> > #include <QImage>
> > +#include <QStringList>
> >
> > #include <libcamera/formats.h>
> >
> > @@ -56,7 +59,8 @@ static const QList<libcamera::PixelFormat> supportedFormats{
> > };
> >
> > ViewFinderGL::ViewFinderGL(QWidget *parent)
> > - : QOpenGLWidget(parent), buffer_(nullptr), image_(nullptr),
> > + : QOpenGLWidget(parent), buffer_(nullptr),
> > + colorSpace_(libcamera::ColorSpace::Raw), image_(nullptr),
> > vertexBuffer_(QOpenGLBuffer::VertexBuffer)
> > {
> > }
> > @@ -72,10 +76,10 @@ const QList<libcamera::PixelFormat> &ViewFinderGL::nativeFormats() const
> > }
> >
> > int ViewFinderGL::setFormat(const libcamera::PixelFormat &format, const QSize &size,
> > - [[maybe_unused]] const libcamera::ColorSpace &colorSpace,
> > + const libcamera::ColorSpace &colorSpace,
> > unsigned int stride)
> > {
> > - if (format != format_) {
> > + if (format != format_ || colorSpace != colorSpace_) {
> > /*
> > * If the fragment already exists, remove it and create a new
> > * one for the new format.
> > @@ -89,7 +93,10 @@ int ViewFinderGL::setFormat(const libcamera::PixelFormat &format, const QSize &s
> > if (!selectFormat(format))
> > return -1;
> >
> > + selectColorSpace(colorSpace);
> > +
> > format_ = format;
> > + colorSpace_ = colorSpace;
> > }
> >
> > size_ = size;
> > @@ -318,6 +325,72 @@ bool ViewFinderGL::selectFormat(const libcamera::PixelFormat &format)
> > return ret;
> > }
> >
> > +void ViewFinderGL::selectColorSpace(const libcamera::ColorSpace &colorSpace)
> > +{
> > + std::array<double, 9> yuv2rgb;
> > +
> > + /* OpenGL stores arrays in column-major order. */
> > + switch (colorSpace.ycbcrEncoding) {
> > + case libcamera::ColorSpace::YcbcrEncoding::None:
> > + yuv2rgb = {
> > + 1.0000, 0.0000, 0.0000,
> > + 0.0000, 1.0000, 0.0000,
> > + 0.0000, 0.0000, 1.0000,
> > + };
> > + break;
> > +
> > + case libcamera::ColorSpace::YcbcrEncoding::Rec601:
> > + yuv2rgb = {
> > + 1.0000, 1.0000, 1.0000,
> > + 0.0000, -0.3441, 1.7720,
> > + 1.4020, -0.7141, 0.0000,
> > + };
> > + break;
> > +
> > + case libcamera::ColorSpace::YcbcrEncoding::Rec709:
> > + yuv2rgb = {
> > + 1.0000, 1.0000, 1.0000,
> > + 0.0000, -0.1873, 1.8856,
> > + 1.5748, -0.4681, 0.0000,
> > + };
> > + break;
> > +
> > + case libcamera::ColorSpace::YcbcrEncoding::Rec2020:
> > + yuv2rgb = {
> > + 1.0000, 1.0000, 1.0000,
> > + 0.0000, -0.1646, 1.8814,
> > + 1.4746, -0.5714, 0.0000,
> > + };
> > + break;
> > + }
> > +
> > + double offset;
> > +
> > + switch (colorSpace.range) {
> > + case libcamera::ColorSpace::Range::Full:
> > + offset = 0.0;
> > + break;
> > +
> > + case libcamera::ColorSpace::Range::Limited:
> > + offset = 16.0;
> > +
> > + for (unsigned int i = 0; i < 3; ++i)
> > + yuv2rgb[i] *= 255.0 / 219.0;
> > + for (unsigned int i = 4; i < 9; ++i)
> > + yuv2rgb[i] *= 255.0 / 224.0;
> > + break;
> > + }
> > +
> > + QStringList matrix;
> > +
> > + for (double coeff : yuv2rgb)
> > + matrix.append(QString::number(coeff, 'f'));
> > +
> > + fragmentShaderDefines_.append("#define YUV2RGB_MATRIX " + matrix.join(", "));
> > + fragmentShaderDefines_.append(QString("#define YUV2RGB_Y_OFFSET %1")
> > + .arg(offset, 0, 'f', 1));
> > +}
> > +
> > bool ViewFinderGL::createVertexShader()
> > {
> > /* Create Vertex Shader */
> > diff --git a/src/qcam/viewfinder_gl.h b/src/qcam/viewfinder_gl.h
> > index 798830a31cd2..68c2912df12f 100644
> > --- a/src/qcam/viewfinder_gl.h
> > +++ b/src/qcam/viewfinder_gl.h
> > @@ -57,6 +57,7 @@ protected:
> >
> > private:
> > bool selectFormat(const libcamera::PixelFormat &format);
> > + void selectColorSpace(const libcamera::ColorSpace &colorSpace);
> >
> > void configureTexture(QOpenGLTexture &texture);
> > bool createFragmentShader();
> > @@ -67,6 +68,7 @@ private:
> > /* Captured image size, format and buffer */
> > libcamera::FrameBuffer *buffer_;
> > libcamera::PixelFormat format_;
> > + libcamera::ColorSpace colorSpace_;
> > QSize size_;
> > unsigned int stride_;
> > Image *image_;
--
Regards,
Laurent Pinchart
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