WebGL Black Screen on Android? A Root Cause Deep Dive

// In your fragment shader
#ifdef GL_ES
  // Most mobile devices will fall into this block
  precision mediump float;
#else
  // Desktops can usually handle high precision
  precision highp float;
#endif

varying vec2 vTextureCoord;
uniform sampler2D uSampler;

void main(void) {
   gl_FragColor = texture2D(uSampler, vTextureCoord);
}

Texture Troubles: Power-of-Two, Formats, and More

Desktop GPUs are very forgiving about texture dimensions. Mobile GPUs? Not so much. Many older or lower-end Android devices have a strict requirement for textures: their dimensions must be a Power-of-Two (POT), e.g., 64x64, 128x256, 512x512.

If you try to use a Non-Power-of-Two (NPOT) texture (like 300x400) on one of these devices with mipmapping enabled or with texture wrapping set to REPEAT or MIRRORED_REPEAT, it will likely fail and result in a black or white texture.

Solutions:

• Best Practice: Always use POT textures in your asset pipeline.
• Workaround: If you must use an NPOT texture, ensure you're using WebGL 2 or, in WebGL 1, set wrapping to CLAMP_TO_EDGE and disable mipmaps for that texture:

gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE);
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.CLAMP_TO_EDGE);
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);

Shader Compilation Errors (The Silent Killers)

Sometimes your GLSL code has a subtle syntax error that a desktop driver forgives but a strict mobile driver rejects. The problem is that WebGL won't throw a JavaScript exception for this. It fails silently. You must explicitly ask for the error log.

Always use a helper function to compile your shaders that checks the compilation status and retrieves the info log on failure.

function compileShader(gl, source, type) {
  const shader = gl.createShader(type);
  gl.shaderSource(shader, source);
  gl.compileShader(shader);

  if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) {
    console.error(`An error occurred compiling the shaders: ${gl.getShaderInfoLog(shader)}`);
    gl.deleteShader(shader);
    return null;
  }

  return shader;
}

Call this for both your vertex and fragment shaders. Do the same for linking the program using gl.getProgramParameter and gl.getProgramInfoLog. These logs are your best friends.

Context Loss & Restoration

On a mobile device, resources are scarce. If a user switches to another app or a phone call comes in, the OS might decide to reclaim the GPU memory used by your WebGL context. When the user returns, your canvas is black because the context, along with all your buffers, textures, and shaders, is gone.

You need to handle this gracefully. WebGL provides events for this scenario:

• webglcontextlost: Fires when the context is lost. You should stop your render loop here.
• webglcontextrestored: Fires when the context is available again. Here, you need to re-create all your WebGL resources—re-upload textures, re-compile shaders, re-create buffers, etc.

canvas.addEventListener('webglcontextlost', (event) => {
  event.preventDefault();
  // Stop rendering, maybe show a "paused" overlay
  console.log('WebGL context lost!');
}, false);

canvas.addEventListener('webglcontextrestored', () => {
  // Re-initialize all WebGL resources (textures, shaders, buffers)
  console.log('WebGL context restored!');
  initializeApp(gl);
}, false);

Failing to handle context restoration is a guaranteed way to get a permanent black screen after the app is backgrounded.

The Alpha Channel & Premultiplied Alpha

This is a more subtle one. When you create your WebGL context, you can pass an option: premultipliedAlpha. By default, it's true. This setting tells the browser's compositor how to blend your canvas with the rest of the page.

• premultipliedAlpha: true (Default): Expects your fragment shader's output color to have its R, G, and B components already multiplied by its A (alpha) component. (e.g., gl_FragColor = vec4(color.rgb * color.a, color.a);)
• premultipliedAlpha: false: Expects your shader to output straight, un-multiplied colors. (e.g., gl_FragColor = color;)

A mismatch between what your shader outputs and what the context expects can lead to black boxes around transparent objects or other blending artifacts. If you're drawing a fully opaque scene, this often won't matter. But if you're blending with a transparent canvas background, getting this wrong can sometimes result in a black output. Ensure your asset pipeline and your shaders are consistent with your context creation setting.

A Systematic Debugging Workflow

When faced with the black screen, don't panic. Follow a process of elimination:

1. Connect to the Device: Use Chrome's Remote Devices (chrome://inspect) or a similar tool for your browser. This gives you access to the console on your Android device.
2. Check for Logs: Look for the shader compilation/linking errors you implemented above. This is the most likely culprit.
3. Simplify, Simplify, Simplify: If there are no logs, start stripping your scene down. Can you draw a single, hardcoded color? Try a shader that just outputs red: gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);. If that works, the problem is in your shader logic or uniforms.
4. Isolate the Draw Call: If the solid color works, re-introduce your real shaders but try to draw a single, simple triangle. Does it appear? If so, the issue might be with your model data or buffers.
5. Check Textures: If a simple colored triangle works but a textured one doesn't, the issue is your texture. Check the POT/NPOT issue, texture format, and ensure it loaded correctly.
6. Use a Frame Debugger: Tools like Spector.js can be invaluable. It's a browser extension that allows you to capture a single frame of your WebGL application and inspect every single GL call, view the state of your buffers and textures, and see how your scene is built step-by-step.

Quick Reference Checklist

Here’s a quick table to reference when you're in the thick of it.

Check	Potential Issue	Common Solution
Shader Precision	highp not supported in fragment shader.	Default to mediump float and check for highp support if needed.
Shader Logs	Silent shader compilation or program linking failure.	Implement error checking using getShaderInfoLog and getProgramInfoLog.
Textures	Using Non-Power-of-Two (NPOT) textures with mipmaps/repeat.	Use POT textures or set wrapping to CLAMP_TO_EDGE and disable mipmaps.
Context State	Context lost after app is backgrounded.	Listen for webglcontextlost and webglcontextrestored events to re-initialize.
Initial GL State	Drawing without clearing the canvas or setting a viewport.	Ensure you call gl.viewport() and gl.clear() at the start of your render loop.
Alpha Blending	Mismatch between premultipliedAlpha context setting and shader output.	Ensure shader output matches the context setting, or explicitly set it to false.

Conclusion: From Black Screen to Brilliance

The WebGL black screen on Android is a formidable but beatable foe. It rarely stems from a single, obvious bug but rather from a fundamental misunderstanding of the constraints and diversity of the mobile hardware ecosystem. By embracing defensive programming—checking for errors, understanding precision limits, respecting texture requirements, and handling context loss—you can build robust WebGL applications that delight users on any device.

So the next time you see that black screen, don't despair. See it as a puzzle. Start with your shaders, check your textures, and work through the possibilities systematically. With the right knowledge and a methodical approach, you'll turn that void back into the vibrant, interactive experience you worked so hard to create.