Why does Unity Cull Transparent Mesh skip masked UI?

Cull Transparent Mesh uses the alpha channel to skip transparent triangles. Masks affect rendering but not vertex alpha; culled triangles can include those a mask would have shown. Disable Cull Transparent Mesh on masked content or replace transparency with the mask shape.

What is Cull Transparent Mesh used for?

It is a CanvasRenderer optimization that drops triangles whose vertex alpha is fully transparent, reducing fill cost. Useful for partial-opacity UI; problematic when transparency is achieved via a mask rather than per-vertex alpha.

Should I keep Cull Transparent Mesh on globally?

On for purely opaque or per-vertex-alpha UI; off for masked or shader-clipped UI. Per-graphic toggle on the CanvasRenderer lets you mix.

Fix: Unity Canvas Cull Transparent Mesh Not Respecting Mask

Quick answer: Disable Cull Transparent Mesh on Image components inside masks. The flag drops fully-transparent triangles by alpha; masks change visibility post-rasterization, so the engine cannot know what the mask reveals.

Here is how to fix Unity UI elements that disappear unexpectedly inside Mask or RectMask2D when Cull Transparent Mesh is on. The optimization assumes vertex alpha indicates final visibility; masks invalidate that assumption.

The Symptom

Image with transparent areas inside a Mask. Some parts inside the mask vanish even though they should be visible.

What Causes This

Vertex alpha-based culling. The renderer drops triangles whose corner alpha is 0; mask reveals do not change vertex alpha.

Sprite alpha edges. Sprites with feathered edges have low-alpha triangles; the cull skips them even when masked region would include them.

The Fix

Step 1: Disable Cull Transparent Mesh on masked images. On the Image component, expand Maskable section. Uncheck Cull Transparent Mesh.

Step 2: Bulk update via script.

[UnityEditor.MenuItem("Tools/Disable Cull On Masked Images")]
static void Disable()
{
    foreach (var img in Object.FindObjectsByType<UnityEngine.UI.Image>(FindObjectsSortMode.None))
    {
        if (img.GetComponentInParent<UnityEngine.UI.Mask>() != null)
        {
            var cr = img.GetComponent<CanvasRenderer>();
            cr.cullTransparentMesh = false;
        }
    }
}

Step 3: For RectMask2D, the same applies. RectMask2D uses shader-side rect clipping. Vertex alpha culling is independent and produces the same artifact.

Step 4: Verify with Frame Debugger. Find the masked image draw call. Check vertex count vs expected; if drastically fewer than expected, culling is over-aggressive.

Step 5: Performance check. Cull Transparent Mesh saves fill rate. Re-enabling on every image adds overhead. Disable only on masked images, leave on for opaque ones.

“Mask reveals what culling already dropped. Disable cull on masked images. Both behaviors stay correct.”

Related Issues

For UI Mask issues, see UI Mask Children. For CanvasGroup alpha, see CanvasGroup Alpha.

Cull off on masked Images. Mask reveal works as expected.