Add OpenGL Troubleshooting steps/hints

blog/2024/2024-11-01-my-screen-is-black/index.md

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+---
+title: 'My Screen is Black'
+slug: 'my-screen-is-black'
+description: 'This shall be a little guide to help you troubleshoot graphics problems. Focussing mainly on OpenGL.'
+date: '2024-11-01'
+authors: ['deccer']
+tags: ['opengl', 'troubleshoot', 'guide', 'article', 'debug']
+image: 'https://raw.githubusercontent.com/graphicsprogramming/blog/main/blog/2024/2024-11-01-my-screen-is-black/arrested-for-opengl-crimes-full.png'
+---
+
+![](arrested-for-opengl-crimes-full.png)
+
+<!-- truncate -->
+
+Those were my findings over the months and years of exposing myself to OpenGL. I hope it helps anyone.
+
+### First Things To Do
+
+Hook up [RenderDoc](https://renderdoc.org). If you have not read its getting-started, read it before anything else.
+There is also a plugin available "Where is my drawcall" hook it up by following the instructions on RenderDoc's site.
+
+Setup `glDebugMessageCallback` see [here](https://deccer.github.io/OpenGL-Getting-Started/02-debugging/02-debug-callback/) for example code.
+
+If you use `glGetError` with or without macros like `GLCALL` or `GLCHECK` or rolled your own, get rid of it. `glDebugMessageCallback` will replace those. There is a high chance that you used it incorrectly anyway because you copy pasted it from some questionable source. 
+
+Make sure you check that shader compilation _and_ linking was successful. See `glGetShaderiv` & `glGetProgramiv` on compile and link status.
+
+### You are on a Mac 
+
+Seriously port your engine to `metal`, or `webgpu` at least. There is no support for `KHR_debug` and you cannot use anything > gl 4.1 is enough reason
+
+You are getting `UNSUPPORTED (log once): POSSIBLE ISSUE: unit 0 GLD_TEXTURE_INDEX_2D is unloadable and bound to sampler type (Float) - using zero texture because texture unloadable`. You need to call glGenerateMipmap(GL_TEXTURE_2D) after glTexImage2D() or set max level to 0 if you dont need/want mips.
+
+### RenderDoc is crashing when trying to run your application
+
+This is most likely **not** RenderDoc's fault, but yours. Something in your code is fishy and RenderDoc doesnt like it. Use a debugger/code-sanitizer to figure out
+what is going on in your application. Most likely some problem around memory allocation/freeing/UB related thing.
+
+Another reason why RenderDoc is crashing is that it doesnt like certain extensions you might be using in your code. RenderDoc used to tell you usually and not just
+crash, but that behaviour has changed since 1.36 or so. I don't know why. I have not bothered asking baldur yet. But what you can do is check your code for
+things involving bindless textures, and bindless buffers. Stuff like `glMakeTextureResidentARB`, `glProgramUniform...64NV`, `glGetTextureHandleARB`. RenderDoc also does
+not support legacy OpenGL. Make sure you arent using those either `glVertexXy`, `glNormalXy` etc. To debug old school stuff, use `apitrace` or nVidia's NSight Graphics.
+Older versions of `gEDebugger` or `CodeXL` might work too.
+
+### You use GLAD but it is giving you a hard time about symbols not found and multiple definitions or the likes
+
+It is most likely that the headers you are using are just outdated. Regenerate the header on dav1d's site. Or check your build system that it is
+pulling a recent version of glad.
+
+### Debug Callback Says...
+
+- `GL_INVALID_OPERATION error generated. Array object is not active.`:
+  You didn't bind a VAO. Core Context OpenGL requires a VAO bound at all times. Bind one.
+
+### Shader Compiler Log Says...
+
+- `function "main" is already defined`
+  You probably compile your fragment shader as vertex shader or other way around
+
+### You are unable to figure out if an extension is supported
+
+`GL_NV_conservative_raster` for example despite calling `glGetString(GL_EXTENSIONS)` and all that.
+You either need to query extensions with a forward compatible context or you switch to query `GL_NUM_EXTENSIONS` first and
+then iterate over all of them with `glGetStringi` and then check if the extension is part of that list. The latter requires a core OpenGL context.
+
+### Exception when calling glDrawElements - aka "0xC0000005" 
+
+You most likely have no indexbuffer is bound. Or it is not associated to/with the current VAO.
+
+### Exception when calling glDrawArrays / or worse the driver is crashing
+
+You probably want to draw more primitives than you have in your vertexbuffer, check arguments of your `glDrawArrays` call.
+Potentially you might not have set the vertex count variable and that contains an ununitialized value because you used c/c++ and are a doofus.
+
+### Textures are black
+
+- no textures bound
+- are your textures complete? if not should produce an output in debug callback, so, go check that (hmm this might not be the case for glXXXTextureStorage i might take this one off the list)
+- no samplers bound (check your sampler objects if you use them as separate objects)
+- fucked uvs
+- you might be sampling from a corner of your texture where its actually black, check uvs
+
+### Your Tringle is black
+
+- smells like your vao is fucked. make sure you setup the attributes (and stride when binding le vbo using DSA)
+- fucked uvs?
+- forgot to bind a texture?
+
+### Screen is Black
+
+- check if your screen is on/connected properly
+- camera (projection/view matrices are fucked) is not looking at the scene in question
+- no texture is sampled due to missing or wrong uvs => default value is 0 aka black (depends on the driver)
+- no shader bound (especially fragment shader)
+- fragment shader doesnt write anything to its output
+- no viewport is set/is too small
+- you might be rendering to a framebuffer, but not blitting that framebuffer to the default one
+- let clearcolor be something else than pure black
+- are you doing MRT?
+  - if yes, check that you called the right `gl(Named)DrawBuffers` and not n times `gl(Named)DrawBuffer` one per render target
+- are you playing with depth pre pass-isms?
+  - make sure the gl state between passes is the same, face winding, cullmode, etc, see Appending A.3 in the gl spec for more clues
+- check winding order and cullmode, you might be looking at the wrong side of your faces
+- you check renderdoc and wonder why the vertex list contains the same (perhaps even first element) only, for all vertices => make sure your `glDrawElements(..., ..., GL_UNSIGNED_INT, ...)` or whatever datatype your indexbuffer consists of matches that parameter 
+
+### Textures look funny, like a garbled version of the actual image
+
+- make sure your internalformat and actual pixel format match. You probably used stb_image to load, but used 0 as the last parameter, 
+  and pixel data has 3 components, instead of the 4 (GL_RGBA) you told OpenGL about -> request 4 channels from stb_image
+
+### Textures look like one color component is more prominent than others
+
+- Colors are more shifted towards blue
+
+  - You probably messed up the format and asked for GL_BRG.. of sorts => make sure they match
+
+- Colors are more shifted towards red
+
+  - Original pixeldata was probably in BGR... but you asked for GL_RGB... of sorts => make sure they match
+
+### Textures seem to work, but mesh also appear to be shaded weirdly as if black Fog is on
+
+Did you generate mipmaps?
+
+### Render artifacts like small missing tiles on a floor
+
+- synchronization issues - perhaps a missing glMemoryBarrier at the right spot
+- ubo/ssbo alignment issue - check std140/430 rule in the glsl spec
+- binding multiple textures to the same slot - check your glBindTextureUnit calls
+- you might be using a float to index into a buffer to grab material/texture info, use a flat int
+
+### Depth buffer not cleared
+
+- Despite calling `glClear(GL_DEPTH_BUFFER_BIT)` => check if `glDepthMask` was set to `GL_FALSE`. When you use FBOs migrate to glClearNamedFramebuffer() if you havent already (still requires glDepthMask set properly)
+
+### Weird "Z-fighting"
+
+- check your depth buffer, near and far planes... try near 0.1f and 512/1024 as farplane
+- your depth buffer might be too small and is set to D16 only, set it to something D24 or D32
+- you use SDL2 and on your platform the default might be set to D16, find the SDL2_GL_Set_Attribute which sets the depth bits for the default fbo