How to clear unsigned int texture?

[HansLjy]

I tried using glClearBuffer and glClearTexImage, and both got the same error: Integer fast clear not enabled for ISL_FORMAT_R32_UINT. The texture data consists of a single unsigned int per vertex.

Here is how I declare and attach the buffer:

glGenTextures(1, &_vertex_backbuffer_color);
glBindTexture(GL_TEXTURE_2D, _vertex_backbuffer_color);
glTexImage2D(
    GL_TEXTURE_2D,
    0,
    GL_R32UI,
    _width,
    _height,
    0,
    GL_RED_INTEGER,
    GL_UNSIGNED_INT,
    nullptr
);
glTexParameteri(
    GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST
);
glTexParameteri(
    GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST
);
glBindTexture(GL_TEXTURE_2D, 0);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, _vertex_backbuffer_color, 0);

And here is how I try to clear the texture with glClearTexImage

GLuint val = 0;
glClearTexImage(_vertex_backbuffer_color, 0, GL_RED_INTEGER, GL_UNSIGNED_INT, &val);

and with glClearBuffer:

glClearBufferuiv(GL_COLOR, 0, &val);

Any help would be appreciated.

[TooGood]

I am not 100% sure but I believe Integer fast clear not enabled for ISL_FORMAT_R32_UINT is not an error this is a warning.

However if you believe that the texture is not clearing and that warning is the cause of the issue have you tried actually looking at the data of the texture in a graphics debugger such as nsight or renderdoc? Also if you set the texture as a draw buffer to your framebuffer you can simply bind the framebuffer and call glClearColor and it will do it for you.

Some more tips is to use glDebugMessageCallback and set breakpoints on warning and high level errors. Consider this glDebugMessageCallback - OpenGL 4 - docs.gl and enable it after you initialize opengl.

[Dark_Photon]

@HansLjy

I tried using glClearBuffer and glClearTexImage, and both got the same error: Integer fast clear not enabled for ISL_FORMAT_R32_UINT. The texture data consists of a single unsigned int per vertex.

@TooGood

I am not 100% sure but I believe Integer fast clear not enabled for ISL_FORMAT_R32_UINT is not an error this is a warning.

Right.

As for what a “fast clear” is, see:

• Auxiliary surface compression — The Mesa 3D Graphics Library latest documentation

The simplest example of this is single-sample fast clears … For this scheme, the auxiliary surface stores a single bit for each cache-line-pair in the main surface.

• If that bit is set, then the entire cache line pair contains only the clear color as provided in the RENDER_SURFACE_STATE for the image.
• If the bit is unset, then it’s not clear and you should look at the main surface.

Since a cache line is 64B, this yields a scale-down factor of 1:1024.

Even the simple fast-clear scheme saves us bandwidth in two places.

1. The first is when we go to clear the surface. If we’re doing a full-surface clear or clearing to the same color that was used to clear before, we don’t have to touch the main surface at all. All we have to do is record the clear color and smash the aux data to 0xff. The hardware then knows to ignore whatever is in the main surface and look at the clear color instead.
2. The second is when we go to render. Say we’re doing some color blending. Instead of the blend unit having to read back actual surface contents to blend with, it looks at the clear bit and blends with the clear color recorded with the surface state instead. Depending on the geometry and cache utilization, this can save as much as one whole read of the surface worth of bandwidth.

So basically when you see:

Integer fast clear not enabled for ISL_FORMAT_R32_UINT 

You know that the Mesa3D Intel driver couldn’t take the fast route of storing “this thing is clear” bits and had to go write a bunch of clear color values to the physical color buffer. This works, but when you’re using slow CPU DRAM, it’s much slower.