120 lines
6.6 KiB
Markdown
120 lines
6.6 KiB
Markdown
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# ANGLE Development Update - July 4, 2012
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We haven't posted an update on the development status of ANGLE in quite some
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time and we'd like to provide an update on some of the new features and
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improvements that we've been working on.
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## Conformance
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As announced in the [Chromium Blog]
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(http://blog.chromium.org/2011/11/opengl-es-20-certification-for-angle.html),
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ANGLE v1.0 has passed the Khronos OpenGL ES 2.0 certification process and is now
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a [conformant](http://www.khronos.org/conformance/adopters/conformant-products/)
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OpenGL ES 2.0 implementation.
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## Extensions
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We have recently completed the implementation of depth texture support
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([ANGLE\_depth\_texture]
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(https://code.google.com/p/angleproject/source/browse/extensions/ANGLE_depth_texture.txt?name=master))
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and earlier in the year we added support for instancing via attribute array
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divisors ([ANGLE\_instanced\_arrays]
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(https://code.google.com/p/angleproject/source/browse/extensions/ANGLE_instanced_arrays.txt?name=master)).
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See ExtensionSupport for a complete list of extensions that are supported by
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ANGLE.
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## Shader Compiler
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We have also made a number of improvements in the shader compiler.
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* We addressed a number of defects related to scoping differences between HLSL and
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GLSL and improved the scoping support in ANGLE's compiler front-end. We also
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worked with The Khronos Group to get an ESSL spec bug fixed and several items
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clarified.
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* We addressed a number of correctness issues in the GLSL to HLSL
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translation process. We fixed some bugs related to constant propagation and
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comma conditional assignments. More importantly, we fully implemented support
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for short-circuiting boolean logic operations. In GLSL, Boolean expressions do
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short-circuit evaluation as in C, but HLSL evaluates them entirely. This only
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has an observable effect if a short-circuited operation has side effects, such
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as a function call that modifies global variables.
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* We implemented detection
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for discontinuous gradient or derivative computations inside loops and replace
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them with explicitly defined continuous behaviour. HLSL and GLSL differ in their
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specified behaviour for operations which compute gradients or derivatives.
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Gradients are computed by texture sampling functions which don't specify a
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specific mipmap LOD level, and by the OES\_standard\_derivatives built-in
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functions. To determine the gradient, the corresponding values in neighbouring
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pixels are differentiated. If neighbouring pixels execute different paths
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through the shader this can cause a discontinuity in the gradient. GLSL
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specifies that in these cases the gradient is undefined. HLSL tries to avoid the
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discontinuity in the compiler by unrolling loops so that every pixel executes
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all iterations. This can make the D3D HLSL compiler spend a long time generating
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code permutations, and possibly even fail compilation due to running out of
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instruction slots or registers. Because the GLSL specification allows undefined
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behaviour, we can define such texture sampling functions to use mipmap LOD level
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0, and have the derivatives functions return 0.0. To do this we examine the GLSL
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code's abstract syntax tree and detect whether the shader contains any loops
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with discontinuities and gradient operations. Within such loops, we generate
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HLSL code that uses explicitly defined texture LODs and derivative information.
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One additional consideration is that within these loops there can be calls to
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user-defined functions which may contain gradient operations. In this case, we
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generate variants of user-defined functions where these operations are
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explicitly defined. We use these new functions instead of the original ones in
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loops with discontinuities.
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These fixes result in ANGLE being able successfully compile a number of the more
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complex shaders. Unfortunately there are still some complex shaders which we
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have not yet been able to obtain solutions for. Ultimately Direct3D 9 SM3
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shaders are more restricted than what can be expressed in GLSL. Most of the
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problematic shaders we've encountered will also not compile successfully on
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current ES 2.0 implementations. We would only be able to achieve parity with
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Desktop GL implementations by using Direct3D 10 or above.
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## Texture Origin Changes
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We have also made a major change to ANGLE in the way the origin difference
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between D3D and OpenGL is handled. This difference is normally observable when
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using render-to-texture techniques, and if not accounted for, it would appear
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that images rendered to textures are upside down. In recent versions of ANGLE
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(r536 (on Google Code)-r1161 (on Google Code)), we have been storing surfaces
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following the D3D Y convention where (0, 0) is the top-left, rather than GL's
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bottom-left convention. This was done by vertically flipping textures on load
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and then adjusting the texture coordinates in the shaders to compensate. This
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approach worked well, but it did leave the orientation of pbuffers inverted when
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compared to native GL implementations. As of ANGLE r1162 (on Google Code), we
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have changed this back to the original way it was implemented - textures are
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loaded and stored in the GL orientation, and the final rendered scene is flipped
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when it is displayed to a window by eglSwapBuffers. This should be essentially
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transparent to applications except that orientation of pbuffers will change. In
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addition to fixing the pbuffer orientation, this change:
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* eliminates
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dependent-texture look-ups in the shaders, caused by flipping the texture
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y-coordinates
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* rounding of texture coordinates (while previously within spec)
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will be more consistent with other implementations, and
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* allows potential
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faster paths for loading texture data to be implemented. The only potential
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downside to this approach is that window-based rendering may be a bit slower for
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simple scenes. The good news is that this path is not used by browser
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implementations on most versions of Windows.
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## Preprocessor
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Finally, Alok P. from Google has been working on implementing a new shader
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preprocessor for the last number of months and this effort is nearly complete.
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This new preprocessor should be more robust and much more maintainable. It also
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includes many (~5000) unit tests and passes all WebGL conformance tests. If you
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wish to try this out before it is enabled by default, define
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ANGLE\_USE\_NEW\_PREPROCESSOR=1 in your project settings for the
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translator\_common project.
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## Contributions
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As always we welcome contributions either in the bug reports (preferably with an
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isolated test-case) or in the form of code contributions. We have added a
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[ContributingCode](ContributingCode.md) wiki page documenting the preferred
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process for contributing code. We do need to ask that you sign a Contributor
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License Agreement before we can integrate your patches.
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