Partial difference derivative functions (
ddx
and ddy
in HLSL[a], dFdx
and dFdy
in GLSL[b]) (in the rest of this article I will use both terms according to the code examples I will provide) are fragment shader instructions wich can be used to compute the rate of variation of any value with respect to the screen-space coordinates.Jun 21, 2018 Populating shader cache. I bought the game yesterday, have spent almost 10 hours trying to get it working right now - my PC meets the requirements to run this game, and plays other games that are more demanding than this one. My issue is that when I load the game, it updates fine and then attempts to populate the shader cache (which then stays.
快速shader迭代和做原型的技巧 1.创建Global functions UE4 会把材质编辑器里的customnode节点里的内容翻译成对应的HLSL,因此如果我们有以下的custom代码:. Oct 11, 2019 Failing that, type shadercache into search (if using Windows) and see if that throws up any matches. #6 Showing 1-6 of 6 comments. Per page: 15 30 50. Hi the shader cache location seems to have changed in the latest drivers and I can't find it, I use it for a few games copying the shader cache files and then putting them back in when I next want to launch the games making them stutter less. Therefore, implementations employ a cache on the results of vertex shaders. If an index/instance pair comes up again, and the result is still in the cache, then the vertex shader is not executed again. Thus, there can be fewer vertex shader invocations than there are vertices specified.
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Derivatives computation
During triangles rasterization, GPUs run many instances of a fragment shader at a time organizing them in blocks of 2×2 pixels. Derivatives are calculated by taking differences between the pixel values in a block;
dFdx
subtracts the values of the pixels on the left side of the block from the values on the right side, and dFdy
subtracts the values of the bottom pixels from the top ones. See the image below where the grid represents the rendered screen pixels and dFdx
, dFdy
expressions are provided for the generic value p evaluated by the fragment shader instance at (x, y) screen coordinates and belonging to the 2×2 block highlighted in red.Derivatives can be evaluated for every variable in a fragment shader. For vector and matrix types, derivatives are computed element-wise.
Derivatives functions are fundamental for texture mipmaps implementation and are very useful in a series of algorithms and effects, in particular when there is some kind of dependence on screen space coordinates (for example when rendering wireframe edges with uniform screen pixel thickness).
![What is shader cache on amd settings What is shader cache on amd settings](https://forums.flightsimlabs.com/uploads/monthly_2017_01/5883de2d85c87_ShaderCache.jpg.8f1c24250cca9f2c48ce6f846b1ce277.jpg)
Derivatives and mipmaps
Mipmaps[d] are pre-computed sequences of images obtained by filtering down a texture into smaller sizes (each mipmap level is two times smaller than the previous). They are used to avoid aliasing artifacts when minifying a texture.
Mipmapping is also important for texture cache coherence, since it enforces a near-one texel to pixel ratio: when traversing a triangle, each new pixel represents a step in texture space of one texel at most. Mipmapping is one of the few cases in rendering where a technique
improves both visuals and performance.
improves both visuals and performance.
Derivatives are used during texture sampling to select the best mipmap level. The rate of variation of the texture coordinates with respect to the screen coordinates is used to choose a mipmap; the larger the derivatives, the greater the mipmap level (and the lesser the mipmap size).
Face normal computation (flat shader)
Derivatives can be used to compute the current triangle’s face normal in a fragment shader. The horizontal and vertical derivatives of the current fragment’s world-position are two vectors laying in the triangle’s surface. Their cross product is a vector orthogonal to the surface and its norm is the triangle’s normal vector (see the 3d model below). Particular attention must be paid to the ordering of the cross product: being the OpenGL coordinate system left-handed (at least when working in window space which is the context where the fragment shader works[e]) and being the horizontal derivative vector always oriented right and the vertical down, the ordering of the cross product to obtain a normal vector oriented toward the camera is horizontal x vertical (more about cross products and basis orientations in this article). The interactive model below shows the link between screen pixels and fragmets over a triangle surface being rasterized, the derivative vectors on the surface (in red and green), and the normal vector (in blue) obtained by the cross product of the twos.
Face normal as the cross product of position derivatives
Here is a GLSL code line to compute a flat normal given the fragment position
pos
in camera space:And below there is a complete pocket.gl demo with a vertex and fragment shader at work on an Utah Teapot[f]. You can toggle the flat shader using the Flat shaded checkbox.
Flat shader example
Derivatives and branches
Derivatives computation is based on the parallel execution on the GPU’s hardware of multiple instances of a shader. Scalar operations are executed with a SIMD (Single Instruction Multiple Data) architecture on registers containing a vector of 4 values for a block of 2×2 pixels. This means that at every step of execution, the shader instances belonging to each 2×2 block are synchronized making derivative computation fast and easy to implement in hardware, being a simple subtraction of values contained in the same register.
But what happens in the case of a conditional branch? In this case, if not all of the threads in a core take the same branch, there is a divergence in the code execution. In the image below an example of divergence is shown: a conditional branch execution in a GPU core with 8 shader instances. Three instances take the first branch (yellow). During the yellow branch execution the other 5 instances are inactive (an execution bitmask is used to activate/deactivate execution). After the yellow branch, the execution mask is inverted and the blue branch is executed by the remaining 5 instances.
In addition to the efficiency and performance loss of the branch, the divergence is breaking the synchronization between the pixels in a block making derivatives operations undefined. This is a problem for texture sampling which needs derivatives for mipmap level selection, anisotropic filtering, etc. When facing such a problem, a shader compiler could flatten the branch (thus avoiding it) or try to rearrange the code moving texture reads outside of the branch control flow. This problem can be avoided by using explicit derivatives or mipmap level when sampling a texture.
Below you can see a HLSL branching experiment written in UE4 using a custom expression node.
Here is the shader code I’m using in the previous example: