[Mesa-dev] [PATCH 09/11] nir: add lowering stage for user-clip-planes / clipdist

[Erik Faye-Lund]

On Mon, Sep 14, 2015 at 11:53 AM, Erik Faye-Lund <kusmabite at gmail.com> wrote:
> On Sun, Sep 13, 2015 at 5:51 PM, Rob Clark <robdclark at gmail.com> wrote:
>> From: Rob Clark <robclark at freedesktop.org>
>>
>> The vertex shader lowering adds calculation for CLIPDIST, if needed
>> (ie. user-clip-planes), and the frag shader lowering adds conditional
>> kills based on CLIPDIST value (which should be treated as a normal
>> interpolated varying by the driver).
>
> <snip>
>
>> +
>> +/*
>> + * FS lowering
>> + */
>> +
>> +static void
>> +lower_clip_fs(nir_function_impl *impl, unsigned ucp_enables,
>> +              nir_variable **in)
>> +{
>> +   nir_ssa_def *clipdist[MAX_CLIP_PLANES];
>> +   nir_builder b;
>> +
>> +   nir_builder_init(&b, impl);
>> +   b.cursor = nir_before_cf_list(&impl->body);
>> +
>> +   if (ucp_enables & 0x0f)
>> +      load_clipdist_input(&b, in[0], &clipdist[0]);
>> +   if (ucp_enables & 0xf0)
>> +      load_clipdist_input(&b, in[1], &clipdist[4]);
>> +
>> +   for (int plane = 0; plane < MAX_CLIP_PLANES; plane++) {
>> +      if (ucp_enables & (1 << plane)) {
>> +         nir_intrinsic_instr *discard;
>> +         nir_ssa_def *cond;
>> +
>> +         cond = nir_flt(&b, clipdist[plane], nir_imm_float(&b, 0.0));
>> +
>> +         discard = nir_intrinsic_instr_create(b.shader,
>> +                                              nir_intrinsic_discard_if);
>> +         discard->src[0] = nir_src_for_ssa(cond);
>> +         nir_builder_instr_insert(&b, &discard->instr);
>
> I think it's worth noting that this isn't *strictly* correct for
> multi-sampling, unless the shader is s run with per-sample shading
> (ala GL_ARB_sample_shading). Otherwise, all samples for a pixel will
> get the same discard-condition, leading to aliasing along the
> resulting edge.
>
> That being said, per-fragment clipping is better than no clipping, so
> it's clearly an improvement.

So, in order to do this correctly for MSAA, I guess you'd need to use
SYSTEM_VALUE_SAMPLE_POS and FRAG_RESULT_SAMPLE_MASK, to perform
alpha-testing for each sample-point.