Trying To Make Alienrain In Python Using The Opengl Function Glmapbufferrange

Just 4 little lines causing a problem with the alien rain program that I ported from the OpenGL Superbible. It seems I am having issues trying to write to memory after using the f

Solution 1:

First of all, the 2nd parameter of glBufferData is the buffer data in bytes.

Since the glsl data structure is

structdroplet_t
{float x_offset;
    float y_offset;
    float orientation;
    float unused;
};

layout (std140) uniform droplets
{
    droplet_t droplet[256];
};

the buffer size is 4*4*256, because the size of a float is 4, t he structure has 4 elements of type flaot and the array has 256 elements

glBufferData(GL_UNIFORM_BUFFER, 256*4*4, None, GL_DYNAMIC_DRAW)

The instruction

droplet = glMapBufferRange(GL_UNIFORM_BUFFER, 0, 256*4*4, GL_MAP_WRITE_BIT | GL_MAP_INVALIDATE_BUFFER_BIT)

returns a pointer to a allocated memory region with the proper size. You've to fill this memory with data. The easiest solution is to use the (in python built-in) library ctypes, which has the handy function .from_address():

This method returns a ctypes type instance using the memory specified by address.

so the instruction

float_array = ((ctypes.c_float * 4) * 256).from_address(droplet) 

"wraps" a 2 dimensional array to the memory addressed by droplet, with 256 elements and each element has 4 elements of type float.

The values can be set to the array, by a simple assignment statement. e.g.:

import random
import math
import ctypes

glBindBufferBase(GL_UNIFORM_BUFFER, 0, rain_buffer);
droplet = glMapBufferRange(GL_UNIFORM_BUFFER, 0, 256*4*4, GL_MAP_WRITE_BIT | GL_MAP_INVALIDATE_BUFFER_BIT)
float_array = ((ctypes.c_float * 4) * 256).from_address(droplet) 
for i inrange(0, 256):
    float_array[i][0] = random.random() * 2 -1
    float_array[i][1] = random.random() * 2 -1
    float_array[i][2] = random.random() * math.pi * 2
    float_array[i][3] = 0.0