Romain Guy 253f2c213f Linear blending, step 1
NOTE: Linear blending is currently disabled in this CL as the
      feature is still a work in progress

Android currently performs all blending (any kind of linear math
on colors really) on gamma-encoded colors. Since Android assumes
that the default color space is sRGB, all bitmaps and colors
are encoded with the sRGB Opto-Electronic Conversion Function
(OECF, which can be approximated with a power function). Since
the power curve is not linear, our linear math is incorrect.
The result is that we generate colors that tend to be too dark;
this affects blending but also anti-aliasing, gradients, blurs,
etc.

The solution is to convert gamma-encoded colors back to linear
space before doing any math on them, using the sRGB Electo-Optical
Conversion Function (EOCF). This is achieved in different
ways in different parts of the pipeline:

- Using hardware conversions when sampling from OpenGL textures
  or writing into OpenGL frame buffers
- Using software conversion functions, to translate app-supplied
  colors to and from sRGB
- Using Skia's color spaces

Any type of processing on colors must roughly ollow these steps:

[sRGB input]->EOCF->[linear data]->[processing]->OECF->[sRGB output]

For the sRGB color space, the conversion functions are defined as
follows:

OECF(linear) :=
linear <= 0.0031308 ? linear * 12.92 : (pow(linear, 1/2.4) * 1.055) - 0.055

EOCF(srgb) :=
srgb <= 0.04045 ? srgb / 12.92 : pow((srgb + 0.055) / 1.055, 2.4)

The EOCF is simply the reciprocal of the OECF.
While it is highly recommended to use the exact sRGB conversion
functions everywhere possible, it is sometimes useful or beneficial
to rely on approximations:

- pow(x,2.2) and pow(x,1/2.2)
- x^2 and sqrt(x)

The latter is particularly useful in fragment shaders (for instance
to apply dithering in sRGB space), especially if the sqrt() can be
replaced with an inversesqrt().

Here is a fairly exhaustive list of modifications implemented
in this CL:

- Set TARGET_ENABLE_LINEAR_BLENDING := false in BoardConfig.mk
  to disable linear blending. This is only for GLES 2.0 GPUs
  with no hardware sRGB support. This flag is currently assumed
  to be false (see note above)
- sRGB writes are disabled when entering a functor (WebView).
  This will need to be fixed at some point
- Skia bitmaps are created with the sRGB color space
- Bitmaps using a 565 config are expanded to 888
- Linear blending is disabled when entering a functor
- External textures are not properly sampled (see below)
- Gradients are interpolated in linear space
- Texture-based dithering was replaced with analytical dithering
- Dithering is done in the quantization color space, which is
  why we must do EOCF(OECF(color)+dither)
- Text is now gamma corrected differently depending on the luminance
  of the source pixel. The asumption is that a bright pixel will be
  blended on a dark background and the other way around. The source
  alpha is gamma corrected to thicken dark on bright and thin
  bright on dark to match the intended design of fonts. This also
  matches the behavior of popular design/drawing applications
- Removed the asset atlas. It did not contain anything useful and
  could not be sampled in sRGB without a yet-to-be-defined GL
  extension
- The last column of color matrices is converted to linear space
  because its value are added to linear colors

Missing features:
- Resource qualifier?
- Regeneration of goldeng images for automated tests
- Handle alpha8/grey8 properly
- Disable sRGB write for layers with external textures

Test: Manual testing while work in progress
Bug: 29940137

Change-Id: I6a07b15ab49b554377cd33a36b6d9971a15e9a0b
2016-10-11 17:47:58 -07:00

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/*
* Copyright (C) 2010 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#ifndef ANDROID_HWUI_VERTEX_H
#define ANDROID_HWUI_VERTEX_H
#include "Vector.h"
#include "FloatColor.h"
#include "utils/Macros.h"
namespace android {
namespace uirenderer {
/**
* Simple structure to describe a vertex with a position and a texture.
*/
struct Vertex {
/**
* Fudge-factor used to disambiguate geometry pixel positioning.
*
* Used to offset lines and points to avoid ambiguous intersection with pixel centers (see
* Program::set()), and used to make geometry damage rect calculation conservative (see
* Rect::snapGeometryToPixelBoundaries())
*/
static float GeometryFudgeFactor() { return 0.0656f; }
float x, y;
static inline void set(Vertex* vertex, float x, float y) {
vertex->x = x;
vertex->y = y;
}
static inline void set(Vertex* vertex, Vector2 val) {
set(vertex, val.x, val.y);
}
static inline void copyWithOffset(Vertex* vertex, const Vertex& src, float x, float y) {
set(vertex, src.x + x, src.y + y);
}
}; // struct Vertex
REQUIRE_COMPATIBLE_LAYOUT(Vertex);
/**
* Simple structure to describe a vertex with a position and texture UV.
*/
struct TextureVertex {
float x, y;
float u, v;
static inline void set(TextureVertex* vertex, float x, float y, float u, float v) {
*vertex = { x, y, u, v };
}
static inline void setUV(TextureVertex* vertex, float u, float v) {
vertex[0].u = u;
vertex[0].v = v;
}
}; // struct TextureVertex
REQUIRE_COMPATIBLE_LAYOUT(TextureVertex);
/**
* Simple structure to describe a vertex with a position, texture UV and an
* sRGB color with alpha. The color is stored pre-multiplied in linear space.
*/
struct ColorTextureVertex {
float x, y;
float u, v;
float r, g, b, a; // pre-multiplied linear
static inline void set(ColorTextureVertex* vertex, float x, float y,
float u, float v, uint32_t color) {
FloatColor c;
c.set(color);
*vertex = { x, y, u, v, c.r, c.g, c.b, c.a };
}
}; // struct ColorTextureVertex
REQUIRE_COMPATIBLE_LAYOUT(ColorTextureVertex);
/**
* Simple structure to describe a vertex with a position and an alpha value.
*/
struct AlphaVertex {
float x, y;
float alpha;
static inline void set(AlphaVertex* vertex, float x, float y, float alpha) {
*vertex = { x, y, alpha };
}
static inline void copyWithOffset(AlphaVertex* vertex, const AlphaVertex& src,
float x, float y) {
AlphaVertex::set(vertex, src.x + x, src.y + y, src.alpha);
}
static inline void setColor(AlphaVertex* vertex, float alpha) {
vertex[0].alpha = alpha;
}
}; // struct AlphaVertex
REQUIRE_COMPATIBLE_LAYOUT(AlphaVertex);
}; // namespace uirenderer
}; // namespace android
#endif // ANDROID_HWUI_VERTEX_H