GlShaderStorageBlockBinding() allows to dynamically connect the shader storage block and the SSBO. GlShaderStorageBlockBinding(program, block_index, ssbo_binding_point_index) Īctually this last step is not required: the binding point can be hard coded directly in the GLSL shader in the buffer layout: In our case, the SSBO is bound on the point 2: To be able to read from or write to a SSBO, the following steps are required:īlock_index = glGetProgramResourceIndex(program, GL_SHADER_STORAGE_BLOCK, 'shader_data') Ģ – connect the shader storage block to the SSBO: we tell the shader on which binding point it will find the SSBO. With a GeForce GTX 660, each type of shader (vertex, fragment, geometry, tessellation and compute) can have up to 16 storage blocks. For a GeForce GTX 660, this table is available with 96 entries: This table stores a kind a reference on each SSBO.

Like with UBOs, OpenGL holds a binding point table in each rendering context. The uniform keyword of uniform block is replaced by the buffer keyword that shows the read-write feature of the buffer.

Layout (std430, binding=2) buffer shader_data The storage block describes the data structure a shader can read from or write to: For SSBOs, we have a storage block in the shader. Like for UBOs, there is an equivalent to uniform blocks in GLSL shaders. Memcpy(p, &shader_data, sizeof(shader_data)) GLvoid* p = glMapBuffer(GL_SHADER_STORAGE_BUFFER, GL_WRITE_ONLY) SSBO update: we get the pointer on the GPU memory and we copy our data: GlBindBuffer(GL_SHADER_STORAGE_BUFFER, 0) GlBufferData(GL_SHADER_STORAGE_BUFFER, sizeof(shader_data), &shader_data, GL_DYNAMIC_COPY)

GlBindBuffer(GL_SHADER_STORAGE_BUFFER, ssbo) Let’s take again our C/C++ data structure, used in the article about UBOs: The management of SSBOs is very similar to the management of UBOs. The SSBO bible can be found here: GL_ARB_shader_storage_buffer_object. On Windows and Linux (with NVIDIA or AMD closed-source drivers), SSBOs are available for all OpenGL 4 capable GPUs. Then no SSBOs on OS X before at least a decade 😉 Mavericks, the last version of OS X, supports OpenGL 4.1 only. On a GeForce GTX 660, it’s possible to allocate a 2GB of VRAM for a SSBO. Shader Storage Buffer Objects (or SSBO) can be seen as unlocked UBOs: they are accessible in reading AND writing in a GLSL shader and their size seems to be limited by the amount of GPU memory available. Limited size, read-only mode, humm… With all modern graphics cards and their tons of gigabytes of dedicated vram, we can do better than 64KB for a GPU buffer. The size of an UBO is somewhat limited: 64KB for AMD and NVIDIA GPUs and 16KB for Intel ones.

To sum up a little bit, UBOs are read-only GPU-accessible memory zones for a GLSL shader. In this tutorial, we meet Uniform Buffer Objects (or UBO).

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