Notes

Most OpenGL mistakes fail silently: no crash, just a black or empty window. Errors have to be requested explicitly.

How GL reports errors

• gl calls return nothing on failure. They set an internal error flag and keep going.
• glGetError() reads the flag and clears it. The flag sticks until read, so one check at the end can’t tell which call failed.
• To locate a failure: check after every suspect call; the first check that fires sits right after the culprit. Put a breakpoint in the error branch and read the call stack.

glBindBuffer(...);              printGLErrorLog();
glEnableVertexAttribArray(-1);  printGLErrorLog();  // stops here

hw04’s printGLErrorLog() ends with a bare throw;, which terminates the program. Run under the debugger to stop there instead of exiting.

Remove per-frame checks once things work; they cost performance.

Error	Usual cause
GL_INVALID_VALUE	Bad number, e.g. location -1
GL_INVALID_OPERATION	Not allowed in current state: uniform without glUseProgram, uniform type mismatch
GL_INVALID_ENUM	Wrong constant for that parameter

Symptom → cause

Symptom	Check
Nothing drawn	Attribute enabled? bind before pointer? index count right? glUseProgram?
Shape is a dot or missing	glVertexAttribPointer component count, buffer byte size
Only one triangle	Hard-coded count in glDrawElements instead of the index count
Location -1	Name typo, or unused variable optimized out
Uniform does nothing	Set before glUseProgram, wrong setter type, location -1
Shape stretched	Missing aspect correction, or integer division
Shader pattern is an ellipse	Math done in the aspect-corrected space
Translation ignored	2D vertex z = 0 instead of 1
Rotation barely moves	This old glm rotate(mat3, angle) takes degrees unless GLM_FORCE_RADIANS
GLSL won’t compile	Console log; file missing from .qrc; assigning to an in
Crash on first gl call	Before initializeOpenGLFunctions() or outside a current context

Related

OpenGL Hub · OpenGL API naming and call order · Undefined behavior · Exception handling