[shapes] Missing pixels on `DrawCircle()` with small radius and window `FLAG_MSAA_4X_HINT` active

[evanrinehart]

#include <raylib.h>

int main(int argc, char* argv[]){

    SetConfigFlags(FLAG_MSAA_4X_HINT);
    InitWindow(800, 600, "TEST CASE");

    while(!WindowShouldClose()){
        BeginDrawing();
        ClearBackground(WHITE);
        DrawCircle(400.5, 300.5, 8, BLACK);
        DrawCircle(528.0, 172.0, 8, BLACK);
        EndDrawing();
    }

    CloseWindow();

    return 0;
}

This is on latest Ubuntu with NVIDIA GeForce GTX 1080 Ti. It seems like the constituents of the disc don't line up exactly and MSAA_4X has found those spots not covered.

[evanrinehart]

I narrowed it down to the rshapes.c routine DrawCircleSector. Specifically the SUPPORT_QUADS_DRAW_MODE branch of that code. When I force the triangles version of DrawCircleSector to draw my discs, the visual errors go away.

Something is going wrong when trying to draw circles with quads

[raysan5]

@evanrinehart I'm aware of this issue. I don't know why it happens...

[krupitskas]

Can it be OpenGL bug?
However I've tried to compile raylib with 4.3 opengl, no difference.

I've briefly took a look:
When we are drawing to MSAA target samples look like this:
Sample 0:

Sample 1:

Other samples are having same issues, thats why resolve does look like this:

However I've checked shader and looks like issue with sampling of provided texture in fragment shader

    vec4 texelColor = texture(texture0, fragTexCoord);   
    finalColor = texelColor*colDiffuse*fragColor;        

I see that texture0 is R8G8_Unorm with RRRG swizzling, so I've tried to replicate same sampling in the shader

vec4 texelColor = texture(texture0, fragTexCoord).rrrg;   

And its gone...

With bigger circles it happens too

[raysan5]

@krupitskas thanks for the investigation and the detailed explanation. I'm not sure about the reasons for this issue but my guess is that could be related to the rasterization of the individual indexed quads that become 2 triangles of the circle each. It just approximates the mathematical defined borders to pixels and on some cases it choses not to add a pixel.

The solution to this issue is drawing using plain triangles or a triangle-strip but it's not possible to get it with quads...

It's a guess but I don't really understand the issue, probably I don't really understand what MSAAx4 really does internally (beside the theorical definition).

[raysan5]

I'm afraid this issue is out-of-scope of raylib. It seems it's more related to the driver implementation of OpenGL.

[evanrinehart]

I got the exact effect on my NVIDIA linux box and macbook air. I think a different way of drawing a circle with quads might help.

Currently I'm disabling SUPPORT_QUADS_DRAW_MODE in config.h which is enabled by default. It's a shame the default behavior is bugged.

I might be able to look at an alternative circle-with-quads drawing strategy at some point. I'll have to remember this closed issue.

[evanrinehart]

Well after a few minutes of messing around I noticed that disabling the 1x1 pixel texture that goes with each slice of DrawCircleSector, the erroneous results seem to go away. I'll have to do more testing.

In the mean time, what is the purpose of the dummy texture used in all the QUAD mode rshapes routines?

[raysan5]

I might be able to look at an alternative circle-with-quads drawing strategy at some point.

I tried this for a couple of hours and I couldn't find a solution.

what is the purpose of the dummy texture used in all the QUAD mode rshapes routines?

because raylib uses the same internal shader for shapes drawing and textures drawing, and that shader requires a texture. Also note that by default the font texture is used for shapes drawing, not the 1x1 pixel white texture.

[raysan5]

I'm reopening it to ask the X experts about it, maybe someone could provide some more info about the issue.

[mausimus]

I think it's texel bleeding, the only material difference between triangle and quad approaches are tex coords (constant w/ triangles, varied w/ quads). I think there's already some padding being added around the shape area on the font texture but it looks not enough for all MSAA scenarios (an MSAA sample of a small shape could resolve to more than 1 texel away). Setting texShapesRec to width 1 and height 1 (or 0 for that matter) fixes it for example.

[raysan5]

@mausimus you are right! thank you very much for finding it! Indeed there is a 1px white padding around the texture piece used for drawing but it seems it's not enough for MSAAx4! Reviewing it right now!