CODEWITHSUNDEEP
openglglslshaderfragment-shader
mic
mic

Reputation: 851

GLSL: Are writes to Shader Storage Buffers discarded (or undone) by a later discard statement?

I have a fragment shader (GLSL 4.5), which is supposed to write non-transparent pixels to the framebuffer (by writing to the declared output variables) and transparent pixels into a shader storage buffer for later composition. To make that work I planned to write transparent pixels to the shader storage buffer and then call discard to prevent the write to the framebuffer.

Now I stumbled upon this section of the khronos.org's OpenGL Wiki (emphasis mine):

The discard command will also prevent any image store and atomic operations and Shader Storage Buffer Object writes (issued before the discard) from working.

I was very surprised to see this, because if you consider that different shader invocations are able to exchange information by writing and reading shader storage buffers, and therefor could have already behaved differently based on such a wirte before the discard statement is even reached.

I looked into the the GLSL 4.5 Specification, but I could not find anything, which supports the statement form the Wiki.

Also: If this is true, are maybe later writes to a shared storage buffer not discarded?

Upvotes: 2

Views: 691

Answers (1)

Rabbid76
Rabbid76

Reputation: 210948

The specification is clear and unequivocally. See OpenGL Shading Language 4.60 Specification - 6.4. Jumps:

[...] This keyword causes the fragment to be discarded and no updates to any buffers will occur. Any prior writes to other buffers such as shader storage buffers are unaffected. [...]

Thus it is possible to write to a SSBO and after that discard writing to the framebuffer, because prior writes to SSBOs are unaffected.

See further 4.60 revision changes and discard with prior image load/store or SSBO operations #118

[...] while a shader is running, it is updating externally visible buffers and it would be unworkable to say those updates were somehow no longer present after a later discard [...]

Upvotes: 4

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