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
Changes generated with clang-modernize.
Additionally, fixed some struct-vs-class usage to make clang happy.
Change-Id: Ic6ef2427401ff1e794d26f21f7b44868fc75fb72
Remove Clang cutout for unused parameters. Fix warnings.
Remove Clang cutout for deprecated Skia function usage. Has been
fixed in the L push.
Change-Id: I7ea073ff67127cc1e14e798b655e2c50615fe8e7
Quite a few things going on in this commit:
- Enable bitmap strikes by default in Paint objects.
The SkPaint parameter that enables bitmap strikes was not previously
included in DEFAULT_PAINT_FLAGS. This effectively disabled bitmap
fonts. Oops! It's for the best, though, as additional work was needed
in Skia to make bitmap fonts work anyway.
- Complain if TEXTURE_BORDER_SIZE is not 1.
Our glyph cache code does not currently handle any value other than 1
here, including zero. I've added a little C preprocessor check to
prevent future engineers (including especially future-me) from
thinking that they can change this value without updating the related
code.
- Add GL_RGBA support to hwui's FontRenderer and friends
This also happened to involve some refactoring for convenience and
cleanliness.
Bug: 9577689
Change-Id: I0abd1e5a0d6623106247fb6421787e2c2f2ea19c
PBOs (Pixel Buffer Objects) can be used on OpenGL ES 3.0 to perform
asynchronous texture uploads to free up the CPU. This change does not
enable the use of PBOs unless a specific property is set (Adreno drivers
have issues with PBOs at the moment, Mali drivers work just fine.)
This change also cleans up Font/FontRenderer a little bit and improves
performance of drop shadows generations by using memcpy() instead of
a manual byte-by-byte copy.
On GL ES 2.0 devices, or when PBOs are disabled, a PixelBuffer instance
behaves like a simple byte array. The extra APIs introduced for PBOs
(map/unmap and bind/unbind) are pretty much no-ops for CPU pixel
buffers and won't introduce any significant overhead.
This change also fixes a bug with text drop shadows: if the drop
shadow is larger than the max texture size, the renderer would leave
the GL context in a bad state and generate 0x501 errors. This change
simply skips drop shadows if they are too large.
Change-Id: I2700aadb0c6093431dc5dee3d587d689190c4e23
To select the gamma correction method, adb shell setprop hwui.text_gamma_correction
with one of the following values:
lookup3
lookup
shader3
shader
See Properties.h for more information about these different methods.
You can also control gamma correction using the following properties:
hwui.text_gamma
hwui.text_gamma.black_threshold
hwui.text_gamma.white_threshold
Change-Id: I47970b804d2c590c37d3da5008db094241579e25
To enable it, the system property ro.hwui.text_gamma_shader must be
set to true. For testing, DEBUG_FONT_RENDERER_FORCE_SHADER_GAMMA
can be set to 1 in libhwui/Debug.h.
Change-Id: If345c6b71b67ecf1ef2e8847b71f30f3ef251a27
Some GPU architectures could not handle the previous implementation
of our glyph cache. Frequent uploads would cause memory problems in the GPU
and eventually a crash due to these memory issues. The solution is to move to
a system of several, smaller caches instead of one monolythic cache for all
glyphs.
Change-Id: I0fc7a323360940d16d5a33eeb33abfab194c5920
Bug #5566149
Lazily initialize font renderers
Keep 60% of the texture cache when an app goes to the background
Delete least used font renderer when going to the background
Delete all font renderers on full memory trim
Change-Id: I3c2454d46dc1107ec0f0f72a9ce69cbbcc8825e7
This optimization is currently disabled until Launcher is
modified to take advantage of it. The optimization can be
enabled by turning on RENDER_LAYERS_AS_REGIONS in the
OpenGLRenderer.h file.
Change-Id: I2fdf59d0f4dc690a3d7f712173ab8db3848b27b1
