Purge calls of GL.GenerateMipmap from Texture constructors

[tomspilman]

In Texture2D in particular it calls GL.GenerateMipmap in some form to fill out the mip map chain.

This is wrong.

GL.GenerateMipmap literally means "generate mip maps from the level 0 data now". It reads each pixel of the level 0 data and filters it down to generate mips. Depending on the target platform on the CPU or the GPU.

It should be painfully obvious that doing this on a empty texture is a waste of time.

Also you cannot call GL.GenerateMipmap on compressed textures as GL will not auto generate mips for compressed formats. It would require GL to compress the texture at runtime which it does not do. So currently this fails for compressed formats.

Versions of MonoGame from just 6 months ago didn't call GL.GenerateMipmap at all:

MonoGame/MonoGame.Framework/Graphics/Texture2D.OpenGL.cs

Line 109 in 5e387fd

GL.CompressedTexImage2D(TextureTarget.Texture2D, 0, glInternalFormat,

So either A:

• We stop calling GL.GenerateMipmap and do nothing for mipmaps on construction (which I don't know is correct).

Or B:

• We correctly define mip levels on a texture by writing a loop and calling GL.CompressedTexImage2D or GL.TexImage2D for each level.

[tomspilman]

@Jjagg @KonajuGames

[tomspilman]

I'm actually fairly certain that B is the right path. We need to be calling GL.CompressedTexImage2D/GL.TexImage2D for each level on construction. This ensures a correct texture with the complete mipmap chain is created at construction like we have on all other platforms.

[KonajuGames]

Yes, that is correct. B is the right path.

[KakCAT]

I think the code to upload mipmaps as you say in 'B' is already working, and all that is needed is removing the Gl.GenerateMipmap call.

A few days ago when I was investigating the issue, removing the call made the mipmaps work, but I was out of time and I couldn't investigate further.

[tomspilman]

I think the code to upload mipmaps as you say in 'B' is already working

No it isn't. There is no loop in the PlatformConstructor code to generate all the mip levels.

https://github.com/MonoGame/MonoGame/blob/develop/MonoGame.Framework/Graphics/Texture2D.OpenGL.cs#L74

This is required to correctly construct a texture with mips.

[KakCAT]

I just checked it... the code to read the mipmaps is not in the PlatformConstructor but in the Texture2DReader.cs file.

https://github.com/MonoGame/MonoGame/blob/develop/MonoGame.Framework/Content/ContentReaders/Texture2DReader.cs#L72

https://github.com/MonoGame/MonoGame/blob/develop/MonoGame.Framework/Content/ContentReaders/Texture2DReader.cs#L172

I'm talking about the assets read with the Content object. If you're talking about creating a new Texture2D() then you may be right, I haven't explored that path.

[tomspilman]

the code to read the mipmaps is not in the PlatformConstructor but in the Texture2DReader.cs file.

Sure... I am not speaking of the code to read the data for a mip map from disk and copy it to the texture.

If you're talking about creating a new Texture2D() then you may be right,

Yes I am speaking of that. Specifically...

// On DirectX correctly returns a blank texture with mipmaps!
// On OpenGL incorrectly returns a blank texture without mipmaps!
var tex = new Texture2D(device, 1024, 1024, true, SurfaceFormat.Color);

[tomspilman]

Continuing a discussion with @Jjagg from #5557 (comment).

I don't know how to tell GL a texture needs mipmaps without generating them though.

From what I have found you need to call GL.CompressedTexImage2D or GL.TexImage2D for each level in a loop.

Although @KonajuGames has suggested that maybe he did something else to fix this?

[KonajuGames]

#5614 fixed this for Texture2D. Texture3D has no support for mipmaps at this stage. TextureCube still uses glGenerateMipmaps. These could be added as separate issues.