I use glTexSubImage2D to render characters of a font to a texture when needed. This works almost everywhere, but on some computers, the font just disappears after a while. This is what seems to happen on those computers:

-initial characters are rendered to font using glTexSubImage2D
-font looks good in the game during gameplay and such
-game requests additional characters and these are rendered to the texture
-font entirely disappears in the game

This suggests that the problem may have something to do with the previously rendered frame. Or it may be accidental and may in fact be caused by the phase of the moon.

I am kind of stuck with this, especially since it only fails on some rare computers, so I am hoping somewhere here can help me find the problem.

I'll post the related code below.

Thanks in advance! :)

This is the code I use to create the font texture:

	glGenBuffers(1, &pixelBufferObjectID);	glBindBuffer(GL_PIXEL_UNPACK_BUFFER_ARB, pixelBufferObjectID);	glBufferData(GL_PIXEL_UNPACK_BUFFER_ARB, maximumGlyphDataSize, 0, GL_STREAM_DRAW);	glBindBuffer(GL_PIXEL_UNPACK_BUFFER_ARB, 0);		const GLuint bytesPerPixel = 2;	const GLuint bufferSize = textureSize * textureSize * bytesPerPixel;		// create a zero filled buffer	GLubyte* imageData = new GLubyte[bufferSize];	memset(imageData, 0, bufferSize);		// generate and bind texture	unsigned int textureID;	glGenTextures(1, &textureID);	glBindTexture(GL_TEXTURE_2D, textureID);		glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);	glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);		// write the image data to the texture to create an empty texture	glTexImage2D(GL_TEXTURE_2D, 0, GL_LUMINANCE_ALPHA, textureSize, textureSize,				 0, GL_LUMINANCE_ALPHA, GL_UNSIGNED_BYTE, imageData);		// clean up after usage	delete[] imageData;

This is the code that renders to the font texture:

	glPixelStorei(GL_UNPACK_ALIGNMENT, 1);	glBindBuffer(GL_PIXEL_UNPACK_BUFFER_ARB, pixelBufferObjectID);	glBufferData(GL_PIXEL_UNPACK_BUFFER_ARB, maximumGlyphDataSize, 0, GL_STREAM_DRAW);	void* buffer = glMapBuffer(GL_PIXEL_UNPACK_BUFFER_ARB, GL_WRITE_ONLY);		if (buffer)	{		// copy pixels from glyph to the pixel buffer		writeGlyphToPixelBuffer(glyph, buffer);				// we are done with copying, unmap the buffer so glTexSubImage2D can use it		glUnmapBuffer(GL_PIXEL_UNPACK_BUFFER_ARB);				// write to this texture, use linear filtering, 0 = from the pixel buffer		glBindTexture(GL_TEXTURE_2D, texture.getTextureId());		glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);		glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);		glTexSubImage2D(GL_TEXTURE_2D, 0,						slot->pixelPositionLeft,						texture.height - slot->pixelPositionTop,						maxGlyphSizeInPixels, maxGlyphSizeInPixels,						GL_LUMINANCE_ALPHA, GL_UNSIGNED_BYTE, 0);				// bind pixel buffer object to update pixel values		glBindBuffer(GL_PIXEL_UNPACK_BUFFER_ARB, pixelBufferObjectID);	}		glBindBuffer(GL_PIXEL_UNPACK_BUFFER_ARB, 0);

And this is the information on drivers and extensions from one failing computer:

OpenGL GL_VENDOR ATI Technologies Inc.OpenGL GL_RENDERER ATI Mobility Radeon HD 4650OpenGL GL_VERSION 2.1.8306 ReleaseOpenGL GL_EXTENSIONS	GL_AMDX_vertex_shader_tessellator	GL_AMD_performance_monitor	GL_AMD_texture_texture4	GL_ARB_color_buffer_float	GL_ARB_depth_buffer_float	GL_ARB_depth_texture	GL_ARB_draw_buffers	GL_ARB_draw_instanced	GL_ARB_fragment_program	GL_ARB_fragment_program_shadow	GL_ARB_fragment_shader	GL_ARB_half_float_pixel	GL_ARB_half_float_vertex	GL_ARB_instanced_arrays	GL_ARB_multisample	GL_ARB_multitexture	GL_ARB_occlusion_query	GL_ARB_pixel_buffer_object	GL_ARB_point_parameters	GL_ARB_point_sprite	GL_ARB_shader_objects	GL_ARB_shader_texture_lod	GL_ARB_shading_language_100	GL_ARB_shadow	GL_ARB_shadow_ambient	GL_ARB_texture_border_clamp	GL_ARB_texture_compression	GL_ARB_texture_cube_map	GL_ARB_texture_env_add	GL_ARB_texture_env_combine	GL_ARB_texture_env_crossbar	GL_ARB_texture_env_dot3	GL_ARB_texture_float	GL_ARB_texture_mirrored_repeat	GL_ARB_texture_non_power_of_two	GL_ARB_texture_rectangle	GL_ARB_transpose_matrix	GL_ARB_vertex_buffer_object	GL_ARB_vertex_program	GL_ARB_vertex_shader	GL_ARB_window_pos	GL_ATI_draw_buffers	GL_ATI_envmap_bumpmap	GL_ATI_fragment_shader	GL_ATI_meminfo	GL_ATI_separate_stencil	GL_ATI_texture_compression_3dc	GL_ATI_texture_env_combine3	GL_ATI_texture_float	GL_EXT_abgr	GL_EXT_bgra	GL_EXT_blend_color	GL_EXT_blend_equation_separate	GL_EXT_blend_func_separate	GL_EXT_blend_minmax	GL_EXT_blend_subtract	GL_EXT_compiled_vertex_array	GL_EXT_copy_texture	GL_EXT_draw_buffers2	GL_EXT_draw_range_elements	GL_EXT_fog_coord	GL_EXT_framebuffer_blit	GL_EXT_framebuffer_multisample	GL_EXT_framebuffer_object	GL_EXT_framebuffer_sRGB	GL_EXT_gpu_program_parameters	GL_EXT_gpu_shader4	GL_EXT_multi_draw_arrays	GL_EXT_packed_depth_stencil	GL_EXT_packed_float	GL_EXT_packed_pixels	GL_EXT_point_parameters	GL_EXT_rescale_normal	GL_EXT_secondary_color	GL_EXT_separate_specular_color	GL_EXT_shadow_funcs	GL_EXT_stencil_wrap	GL_EXT_subtexture	GL_EXT_texgen_reflection	GL_EXT_texture3D	GL_EXT_texture_compression_latc	GL_EXT_texture_compression_rgtc	GL_EXT_texture_compression_s3tc	GL_EXT_texture_cube_map	GL_EXT_texture_edge_clamp	GL_EXT_texture_env_add	GL_EXT_texture_env_combine	GL_EXT_texture_env_dot3	GL_EXT_texture_filter_anisotropic	GL_EXT_texture_lod_bias	GL_EXT_texture_mirror_clamp	GL_EXT_texture_object	GL_EXT_texture_rectangle	GL_EXT_texture_sRGB	GL_EXT_texture_shared_exponent	GL_EXT_transform_feedback	GL_EXT_vertex_array	GL_KTX_buffer_region	GL_NV_blend_square	GL_NV_texgen_reflection	GL_SGIS_generate_mipmap	GL_SGIS_texture_edge_clamp	GL_SGIS_texture_lod	GL_WIN_swap_hint	WGL_EXT_swap_control

I'm still wondering what went wrong here, but I fixed this by working around it: instead of updating part of the texture, I now keep the entire texture in normal memory and whenever I change something in it, I delete the texture from video memory and reload it from memory. Not very good for performance, but since this is not done a lot, it isn't noticeable in practice.