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Asked: 2025-06-10T08:14:22+05:30

Why does geometry from the front of my Vulkan model peek through when rendering the back with depth testing?

I’m running into a puzzling issue while trying to render my `.obj` model using Vulkan 1.4. I’ve got this model of a character (let’s call it Suzanne) and I’m dynamically rotating it using quaternions. So far, everything’s working fine when I’m looking at the front of the model; it displays just as I expect. However, the problem arises when I rotate the model to view the back side.

Despite having depth testing enabled, I’m noticing that some parts of the front geometry are intruding into the view of the back. For example, when I looked at the back of Suzanne, the insides of the eyes and parts of the ears were peeking through! It’s quite bizarre since I thought the depth test would handle this correctly.

I’ve studied the pipeline setup and tried to follow the depth buffering code from the Vulkan tutorial on the Khronos site closely. Here’s a brief overview of my pipeline configuration: I’m using a depth stencil state where depth testing and writing are both enabled, and I have the depth comparison set to `VK_COMPARE_OP_LESS`, which should be appropriate for back faces.

I’ve experimented with various `VkCullModeFlagBits` settings, trying both culling front faces and back faces, but with no success. Transparency seems to be a problematic factor too, especially with the materials applied to my model. I also rendered the front of Suzanne using `VK_POLYGON_MODE_LINE`, which showcased the geometry nicely, so I know the vertices are in the right place.

I’m stuck trying to figure out why this peek-through is happening, particularly given that depth testing is enabled. Is there a specific trick or detail that I’m missing? Has anyone else encountered this when rendering 3D models in Vulkan? Any advice or insight would be greatly appreciated!

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    2 Answers

    1. It sounds like you’re experiencing a common issue when rendering 3D models, especially with transparency and backface rendering in Vulkan. Here are a few things you might want to check:

      • Check the winding order: Ensure that your vertices are defined with the correct winding order. Vulkan uses clockwise winding for back faces by default. If your model’s faces are defined counter-clockwise, they might not be culled properly.
      • Depth buffer precision: Make sure your depth buffer has enough precision. If your depth buffer isn’t precise enough, it can cause depth fighting where geometry incorrectly appears in front of other geometry.
      • Culling mode: You mentioned trying both front and back face culling, but remember that sometimes turning off culling entirely can give you insight into what’s going wrong. This way, you can see if the issue is in the geometry or the culling setup.
      • Transparency issues: If you’re dealing with transparent materials, the order in which you render objects can affect depth testing. For transparent materials, it’s usually best to draw opaque geometry first and then handle transparency after. This can sometimes require sorting your draw calls.
      • Depth comparison function: While using `VK_COMPARE_OP_LESS` should be fine, ensure that you’re not making assumptions about which face should show. Sometimes switching to `VK_COMPARE_OP_LESS_OR_EQUAL` helps in edge cases.
      • Debugging: Utilize Vulkan’s validation layers. They can provide insight into what might be misconfigured in your pipeline. Enabling these layers can surface warnings or errors related to rendering issues.

      Finally, consider isolating the issue by testing with simpler shapes (like cubes or planes) to see if the problem persists. This may help you identify if the issue lies with the model itself or how it’s being processed in your Vulkan pipeline. Good luck!

    2. From your description, it sounds like you’re encountering typical symptoms of depth-buffer issues, most commonly related to an incorrectly configured depth buffer or projection matrix. Even if depth testing and writing are enabled with VK_COMPARE_OP_LESS, verify that your depth framebuffer and depth image are using a suitable depth buffer format like VK_FORMAT_D32_SFLOAT or VK_FORMAT_D24_UNORM_S8_UINT. Ensure your pipeline explicitly includes the depth attachment properly, and remember that an incorrect image layout or incomplete framebuffer setup can silently cause strange depth artifacts. Double-check the projection matrices (particularly near and far-plane settings) as well—having a near plane value too close to zero or far plane excessively distant may drastically impact depth precision, causing rendering artifacts in certain angles.

      Moreover, certain transparent or semi-transparent surfaces could exacerbate depth-ordering problems, as standard depth tests struggle with rendering transparent objects correctly without explicitly handling alpha-blending operations and sorting objects from back-to-front. If your model materials utilize any transparency states or blending, temporarily disabling these can confirm if transparency contributes to the problem. Additionally, consider explicitly checking vertex winding order to confirm that your geometry faces outward appropriately; flipping vertex winding or adjusting culling temporarily (VK_CULL_MODE_NONE) can help rule this out. If none of these approaches resolve the issue, performing a Vulkan validation layer check and inspecting your rendered model in a GPU-specific debugging tool (like RenderDoc) may shed more light on subtle problems related to depth-buffer configuration.