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Asked: 2025-02-06T12:14:11+05:30

Why does adding a visibility float to my vertex struct override UV data in my Monogame shader?

anonymous user

I’m really scratching my head over this issue in my trail renderer for Monogame. I’ve managed to create a procedurally generated trail mesh with properly applied UV coordinates, and I just want to add a visibility value per vertex to control the alpha blending for a nice fade effect. The problem is that the moment I try to include this visibility data in my Vertex struct, it feels like I hit a brick wall.

Here’s what I had working initially: I defined a `TrailVertex` struct containing position and UV data, and I used these in my shader to render everything perfectly. The UV texture coordinates showed up as expected, and the alpha was hardcoded at 0.5 just to get things looking good.

Then I thought, “Hey, why not make that alpha dynamic?” So, I added a `float visibility` member to my struct, hoping it would give me the fade effect I wanted. I updated the vertex declaration to account for this new data, calculating the offsets correctly. I was pretty confident that I was doing everything right; however, things went south as soon as I tried to use the visibility data in my shader.

When I wrote the new shader to incorporate the visibility value, it completely ruined my UV data. Instead of showing the proper texture coordinates, all the vertices seem to default to (1, 0), turning my trail into a solid red mess. It’s so confusing because I would expect the UV data not to be affected just by adding an extra float, right? I double-checked my offsets, and they seemed fine. It shouldn’t mess with the other values like that unless I miscalculated something major.

What perplexes me even more is that I’ve confirmed I’m using all the correct data types in the shader, so it should all compile correctly. Can someone explain why the addition of a `visibility` value makes my UVs go haywire? What am I missing here, or is there something fundamentally wrong in how shaders are interpreting these vertex declarations? Any insights would be greatly appreciated!

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    2 Answers

    1. anonymous user

      It sounds like you are running into issues with how your vertex structure is defined and how the vertex attributes are laid out in memory. When you add a new member like `visibility` to your `TrailVertex` struct, you need to ensure that the vertex declaration in your shader matches this new layout exactly. Specifically, you should confirm that the order and size of the members in the struct matches the order and types you are using in the vertex shader. The addition of the `visibility` member could potentially change the layout of your struct, leading to misalignment with how your UV coordinates are being accessed in the shader. If the shader expects the UV coordinates to be at a specific offset, but the compiler changes the layout by adding padding or rearranging the memory, this could cause the UVs to be read incorrectly, hence the solid color output.

      To resolve the issue, you should define the layout of your `TrailVertex` struct precisely and ensure that the corresponding vertex shader input is adjusted accordingly. Make sure to specify the input layout in the correct order, with types matching the declaration in your `TrailVertex`, such as `float2 position`, `float2 uv`, and `float visibility`. Additionally, ensure that in your shader, when you’re fetching the UVs, you’re using the correct offsets based on the new structure. If you’re using HLSL, you can utilize semantic modifiers to explicitly define the layout given in your struct and debug where things might be going wrong. Testing the shader with a simplified version of your struct, where only position and UV are defined, can also help you isolate the problem.

    2. anonymous user

      Troubleshooting Trail Renderer Issues in Monogame

      Sounds like a classic case of shader confusion! When you added that float visibility to your TrailVertex struct, you probably changed the way your vertex data is being laid out in memory. Here’s a breakdown of what might be happening:

      1. Structure Size and Alignment

      When you add a new field to your struct, the total size increases. Make sure your layout matches what your shader is expecting. If your struct is not aligned correctly, it might mess up the way data is interpreted.

      2. Vertex Declaration Update

      Check that you’ve updated your vertex declaration to match the new size of your struct. If your UV coordinates are defined at the wrong offsets, they will read incorrect data. Double-check that everything is defined correctly in your buffer and shader.

      3. Shader Input Layout

      Ensure that your shader’s input layout exactly matches the TrailVertex struct. If you’ve added that new visibility field, the shader must also be updated to take it into account. If it’s not reading from the right offset, it may just be picking up garbage data from the wrong part of the struct.

      4. Careful with Types

      Make sure the types you are using in both C# and GLSL/HLSL match as expected. If in doubt, double-check the shader inputs to ensure they match the data types of your struct.

      5. Debugging Outputs

      To narrow down the issue, try outputting the visibility value and the UV coordinates as colors from the shader, just for testing. This can help you see if they’re being passed correctly.

      6. Fancy Debugging Tools

      If you’re still stuck, consider using tools like RenderDoc or PIX, which can help you inspect what’s actually being drawn and see if the data going into your shader looks right.

      Debugging shaders can be tricky, but with a little patience, you’ll figure it out. Good luck!