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JavaScript
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38 KiB
JavaScript
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Using it as a storage buffer proved less convenient. My previous code used a GLSL struct similar to my "),s("code",[t._v("ModelVertex")]),t._v(".")]),t._v(" "),s("div",{staticClass:"language-glsl extra-class"},[s("pre",{pre:!0,attrs:{class:"language-glsl"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("struct")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("ModelVertex")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("vec3")]),t._v(" position"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("vec2")]),t._v(" tex_coords"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("vec3")]),t._v(" normal"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("vec3")]),t._v(" tangent"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("vec3")]),t._v(" bitangent"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n")])])]),s("p",[t._v("At first glance, this seems just fine, but OpenGL experts would likely see a problem with the structure. Our fields aren't aligned properly to support the "),s("code",[t._v("std430")]),t._v(" alignment that storage buffers require. I won't get into detail but you can check out the "),s("a",{attrs:{href:"../alignment"}},[t._v("alignment showcase")]),t._v(" if you want to know more. To summarize, the "),s("code",[t._v("vec2")]),t._v(" for the "),s("code",[t._v("tex_coords")]),t._v(" was messing up the byte alignment, corrupting the vertex data resulting in the following:")]),t._v(" "),s("p",[s("img",{attrs:{src:a(383),alt:"./corruption.png"}})]),t._v(" "),s("p",[t._v("I could have fixed this by adding a padding field after "),s("code",[t._v("tex_coords")]),t._v(" on the Rust side, but that would require modifying the "),s("code",[t._v("VertexBufferLayout")]),t._v(". 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This is alignment matches what "),s("code",[t._v("ModelVertex")]),t._v(" uses in Rust. This was kind of a pain to work with, but it fixed the corruption issue.")]),t._v(" "),s("p",[t._v("The second problem required me to rethink how I was computing the tangent and bitangent. The previous algorithm I was using only computed the tangent and bitangent for each triangle and set all the vertices in that triangle to use the same tangent and bitangent. While this is fine in a single-threaded context, the code breaks down when trying to compute the triangles in parallel. The reason is that multiple triangles can share the same vertices. This means that when we go to save the resulting tangents, we inevitably end up trying to write to the same vertex from multiple different threads which is a big no no. You can see the issue with this method below:")]),t._v(" "),s("p",[s("img",{attrs:{src:a(384),alt:"./black_triangles.png"}})]),t._v(" "),s("p",[t._v("Those black triangles were the result of multiple GPU threads trying to modify the same vertices. Looking at the data in Render Doc I could see that the tangents and bitangents were garbage numbers such as "),s("code",[t._v("NaN")]),t._v(".")]),t._v(" "),s("p",[s("img",{attrs:{src:a(385),alt:"./render_doc_output.png"}})]),t._v(" "),s("p",[t._v("While on the CPU we could introduce a synchronization primitive such as a "),s("code",[t._v("Mutex")]),t._v(" to fix this issue, AFAIK there isn't really such a thing on the GPU. Instead, I decided to swap my code to work with each vertex individually. There are some hurdles with that, but those will be easier to explain in code. 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I opted to put the actual calculation into its own method. 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In a single-threaded context, this algorithm would end up being O(N*M). As we are utilizing the high number of threads available to our GPU, this is less of an issue, but it still means our GPU is burning more cycles than it needs to.")]),t._v(" "),s("p",[t._v("One way I came up with to possibly improve performance is to store the index of each triangle in a hash map like structure with the vertex index as keys. 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We'd also need a second array to index the first.")]),t._v(" "),s("div",{staticClass:"language-rust extra-class"},[s("pre",{pre:!0,attrs:{class:"language-rust"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("for")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("i"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("_v"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" t_list"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("in")]),t._v(" triangle_map"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("iter")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("enumerate")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n triangle_map_indices"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("push")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token class-name"}},[t._v("TriangleMapIndex")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v(" \n start"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" i"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v("\n len"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v(" t_list"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("len")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n flat_triangle_map"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("extend")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("t_list"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n")])])]),s("p",[t._v("I ultimately decided against this method as it was more complicated, and I haven't had time to benchmark it to see if it's faster than the simple method.")]),t._v(" "),s("h2",{attrs:{id:"results"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#results"}},[t._v("#")]),t._v(" Results")]),t._v(" "),s("p",[t._v("The tangents and bitangents are now getting calculated correctly and on the GPU!")]),t._v(" "),s("p",[s("img",{attrs:{src:a(386),alt:"./results.png"}})]),t._v(" "),s("AutoGithubLink")],1)}),[],!1,null,null,null);s.default=e.exports}}]); |