Order independent transparency with MSAA - opengl

Question

I have implemented OIT based on the demo in "OpenGL Programming Guide" 8th edition.(The red book).Now I need to add MSAA.Just enabling MSAA screws up the transparency as the layered pixels are resolved x times equal to the number of sample levels.I have read this article on how it is done with DirectX where they say the pixel shader should be run per sample and not per pixel.How id it done in OpenGL.
I won't put out here the whole implementation but the fragment shader chunk in which the final resolution of the layered pixels occurs:
vec4 final_color = vec4(0,0,0,0);
for (i = 0; i < fragment_count; i++)
{
/// Retrieving the next fragment from the stack:
vec4 modulator = unpackUnorm4x8(fragment_list[i].y) ;
/// Perform alpha blending:
final_color = mix(final_color, modulator, modulator.a);
}
color = final_color ;
Update:
I have tried the solution proposed here but it still doesn't work.Here are the full fragment shader for the list build and resolve passes:
List build pass :
#version 420 core
layout (early_fragment_tests) in;
layout (binding = 0, r32ui) uniform uimage2D head_pointer_image;
layout (binding = 1, rgba32ui) uniform writeonly uimageBuffer list_buffer;
layout (binding = 0, offset = 0) uniform atomic_uint list_counter;
layout (location = 0) out vec4 color;//dummy output
in vec3 frag_position;
in vec3 frag_normal;
in vec4 surface_color;
in int gl_SampleMaskIn[];
uniform vec3 light_position = vec3(40.0, 20.0, 100.0);
void main(void)
{
uint index;
uint old_head;
uvec4 item;
vec4 frag_color;
index = atomicCounterIncrement(list_counter);
old_head = imageAtomicExchange(head_pointer_image, ivec2(gl_FragCoord.xy), uint(index));
vec4 modulator =surface_color;
item.x = old_head;
item.y = packUnorm4x8(modulator);
item.z = floatBitsToUint(gl_FragCoord.z);
item.w = int(gl_SampleMaskIn[0]);
imageStore(list_buffer, int(index), item);
frag_color = modulator;
color = frag_color;
}
List resolve :
#version 420 core
// The per-pixel image containing the head pointers
layout (binding = 0, r32ui) uniform uimage2D head_pointer_image;
// Buffer containing linked lists of fragments
layout (binding = 1, rgba32ui) uniform uimageBuffer list_buffer;
// This is the output color
layout (location = 0) out vec4 color;
// This is the maximum number of overlapping fragments allowed
#define MAX_FRAGMENTS 40
// Temporary array used for sorting fragments
uvec4 fragment_list[MAX_FRAGMENTS];
void main(void)
{
uint current_index;
uint fragment_count = 0;
current_index = imageLoad(head_pointer_image, ivec2(gl_FragCoord).xy).x;
while (current_index != 0 && fragment_count < MAX_FRAGMENTS )
{
uvec4 fragment = imageLoad(list_buffer, int(current_index));
int coverage = int(fragment.w);
//if((coverage &(1 << gl_SampleID))!=0) {
fragment_list[fragment_count] = fragment;
current_index = fragment.x;
//}
fragment_count++;
}
uint i, j;
if (fragment_count > 1)
{
for (i = 0; i < fragment_count - 1; i++)
{
for (j = i + 1; j < fragment_count; j++)
{
uvec4 fragment1 = fragment_list[i];
uvec4 fragment2 = fragment_list[j];
float depth1 = uintBitsToFloat(fragment1.z);
float depth2 = uintBitsToFloat(fragment2.z);
if (depth1 < depth2)
{
fragment_list[i] = fragment2;
fragment_list[j] = fragment1;
}
}
}
}
vec4 final_color = vec4(0,0,0,0);
for (i = 0; i < fragment_count; i++)
{
vec4 modulator = unpackUnorm4x8(fragment_list[i].y);
final_color = mix(final_color, modulator, modulator.a);
}
color = final_color;
}

Answer 1

Without knowing how your code actually works, you can do it very much the same way that your linked DX11 demo does, since OpenGL provides the same features needed.
So in the first shader that just stores all the rendered fragments, you also store the sample coverage mask for each fragment (along with the color and depth, of course). This is given as fragment shader input variable int gl_SampleMaskIn[] and for each sample with id 32*i+j, bit j of glSampleMaskIn[i] is set if the fragment covers that sample (since you probably won't use >32xMSAA, you can usually just use glSampleMaskIn[0] and only need to store a single int as coverage mask).
...
fragment.color = inColor;
fragment.depth = gl_FragCoord.z;
fragment.coverage = gl_SampleMaskIn[0];
...
Then the final sort and render shader is run for each sample instead of just for each fragment. This is achieved implicitly by making use of the input variable int gl_SampleID, which gives us the ID of the current sample. So what we do in this shader (in addition to the non-MSAA version) is that the sorting step just accounts for the sample, by only adding a fragment to the final (to be sorted) fragment list if the current sample is actually covered by this fragment:
What was something like (beware, pseudocode extrapolated from your small snippet and the DX-link):
while(fragment.next != 0xFFFFFFFF)
{
fragment_list[count++] = vec2(fragment.depth, fragment.color);
fragment = fragments[fragment.next];
}
is now
while(fragment.next != 0xFFFFFFFF)
{
if(fragment.coverage & (1 << gl_SampleID))
fragment_list[count++] = vec2(fragment.depth, fragment.color);
fragment = fragments[fragment.next];
}
Or something along those lines.
EDIT: To your updated code, you have to increment fragment_count only inside the if(covered) block, since we don't want to add the fragment to the list if the sample is not covered. Incrementing it always will likely result in the artifacts you see at the edges, which are the regions where the MSAA (and thus the coverage) comes into play.
On the other hand the list pointer has to be forwarded (current_index = fragment.x) in each loop iteration and not only if the sample is covered, as otherwise it can result in an infinite loop, like in your case. So your code should look like:
while (current_index != 0 && fragment_count < MAX_FRAGMENTS )
{
uvec4 fragment = imageLoad(list_buffer, int(current_index));
uint coverage = fragment.w;
if((coverage &(1 << gl_SampleID))!=0)
fragment_list[fragment_count++] = fragment;
current_index = fragment.x;
}

Answer 2

The OpenGL 4.3 Spec states in 7.1 about the gl_SampleID builtin variable:
Any static use of this variable in a fragment shader causes the entire shader to be evaluated per-sample.
(This has already been the case in the ARB_sample_shading and is also the case for gl_SamplePosition or a custom variable declared with the sample qualifier)
Therefore it is quite automatic, because you will probably need the SampleID anyway.