How to manually draw to an OpenGL ES texture

// Create steam texture. It's just a transparent circle

texture = 0;

GLubyte *textureData = malloc(sizeof(GLubyte) * 4 * (radius * 2) * (radius * 2));

memset(textureData, 0, sizeof(GLubyte) * 4 * (radius * 2) * (radius * 2));

unsigned int index = 0;

for (int y = 0; y <>

for (int x = 0; x <>

float d = sqrt((x - radius) * (x - radius) + (y - radius) * (y - radius));

if (d <= radius) {

GLubyte alpha = 255 * (int)(radius - d) / radius;

textureData[index++] = 250;

textureData[index++] = 250;

textureData[index++] = 255;

textureData[index++] = alpha;

} else {

index += 4;

}

}

}

// allocate a texture name

glGenTextures(1, &texture);

// select our current texture

glBindTexture(GL_TEXTURE_2D, texture);

glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);

glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);

glTexEnvf( GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE );

// build our texture mipmaps

glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, radius * 2, radius * 2,

0, GL_RGBA, GL_UNSIGNED_BYTE, textureData);

// free buffer

free(textureData);