OpenGL repeated calls to glTexImage2D and alpha blending - opengl

Question

This is more out of curiosity than for any practical purpose: is there anything in the OpenGL specification that suggests that calling glTexImage2D many times (e.g., once per frame) is illegal? I mean illegal as in 'it could produce wrong results', not just inefficient (suppose I don't care about the performance impact of not using glTexSubImage2D instead).
The reason I'm asking is that I noticed some very odd artifacts when drawing overlapping, texture-mapped primitives that use a partly-transparent texture which is loaded once per every frame using glTexImage2D (see the attached picture): after a few seconds (i.e., a few hundred frames), small rectangular black patches appear on the screen (they're actually flipping between black and normal between consecutive frames).
I'm attaching below the simplest example code I could write that exhibits the problem.
#include <stdio.h>
#ifndef __APPLE__
# include <SDL/SDL.h>
# include <SDL/SDL_opengl.h>
#else
# include <SDL.h>
# include <SDL_opengl.h>
#endif
/* some constants and variables that several functions use */
const int width = 640;
const int height = 480;
#define texSize 64
GLuint vbo;
GLuint tex;
/* forward declaration, creates a random texture; uses glTexSubImage2D if
update is non-zero (otherwise glTexImage2D) */
void createTexture(GLuint label, int update);
int init()
{
/* SDL initialization */
if (SDL_Init(SDL_INIT_VIDEO) < 0)
return 0;
SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
if (!SDL_SetVideoMode(width, height, 0, SDL_OPENGL)) {
fprintf(stderr, "Couldn't initialize OpenGL");
return 0;
}
/* OpenGL initialization */
glClearColor(0, 0, 0, 0);
glEnable(GL_TEXTURE_2D);
glEnable(GL_BLEND);
glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
glViewport(0, 0, width, height);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0, width, height, 0, -1, 1);
glMatrixMode(GL_MODELVIEW);
/* creating the VBO and the textures */
glGenBuffers(1, &vbo);
glBindBuffer(GL_ARRAY_BUFFER, vbo);
glBufferData(GL_ARRAY_BUFFER, 1024, 0, GL_DYNAMIC_DRAW);
glGenTextures(1, &tex);
createTexture(tex, 0);
glEnableClientState(GL_VERTEX_ARRAY);
glEnableClientState(GL_TEXTURE_COORD_ARRAY);
return 1;
}
/* draw a triangle at the specified point */
void drawTriangle(GLfloat x, GLfloat y)
{
GLfloat coords1[12] = {0, 0, 0, 0, /**/200, 0, 1, 0, /**/200, 150, 1, 1};
glLoadIdentity();
glTranslatef(x, y, 0);
glBufferSubData(GL_ARRAY_BUFFER, 0, sizeof(coords1), coords1);
glVertexPointer(2, GL_FLOAT, 4*sizeof(GLfloat), (void*)0);
glTexCoordPointer(2, GL_FLOAT, 4*sizeof(GLfloat),
(char*)0 + 2*sizeof(GLfloat));
glDrawArrays(GL_TRIANGLES, 0, 3);
}
void render()
{
glClear(GL_COLOR_BUFFER_BIT);
drawTriangle(250, 50);
createTexture(tex, 0);
drawTriangle(260, 120);
SDL_GL_SwapBuffers();
}
void cleanup()
{
glDeleteTextures(1, &tex);
glDeleteBuffers(1, &vbo);
SDL_Quit();
}
int main(int argc, char* argv[])
{
SDL_Event event;
if (!init()) return 1;
while (1) {
while (SDL_PollEvent(&event))
if (event.type == SDL_QUIT)
return 0;
render();
}
cleanup();
return 0;
}
void createTexture(GLuint label, int update)
{
GLubyte data[texSize*texSize*4];
GLubyte* p;
int i, j;
glBindTexture(GL_TEXTURE_2D, label);
for (i = 0; i < texSize; ++i) {
for (j = 0; j < texSize; ++j) {
p = data + (i + j*texSize)*4;
p[0] = ((i % 8) > 4?255:0);
p[1] = ((j % 8) > 4?255:0);
p[2] = ((i % 8) > 4?255:0);
p[3] = 255 - i*3;
}
}
if (!update)
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, texSize, texSize, 0, GL_RGBA,
GL_UNSIGNED_BYTE, data);
else
glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, texSize, texSize, GL_RGBA,
GL_UNSIGNED_BYTE, data);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
}
Notes:
I'm using SDL, but I've seen the same happening in wxWidgets, so it's not an SDL-related problem.
If I use glTexSubImage2D instead for every frame (use update = 1 in createTexture), the artifacts disappear.
If I disable blending, there are no more artifacts.
I've been testing this on a late 2010 MacBook Air, though I doubt that's particularly relevant.

Answer 1

This clearly an OpenGL implementation bug (just calling glTexImage2D in a loop should not cause this to happen).