Point Sprite Alpha Blending Issue

Question

I'm trying to render a load of stars in 3D space using point sprites. My texture image is definitely 'good' as far as the alpha channel is concerned. It renders perfectly fine as a quad but when I render a lot of point sprites, I can see the square border of the image overwriting some of the images.

The above image shows the star being rendered nicely over the top of my cube. In the bottom right I would expect to see a continuous trail of star images in the 'flower' pattern that they're been drawn in.

What am I doing wrong?

Fragment shader:

#version 140

#extension GL_ARB_explicit_attrib_location : enable

precision mediump float;

uniform sampler2D uTexture;

void main( void )
{    
  gl_FragColor = texture2D( uTexture, gl_PointCoord );
}

Vertex shader:

#version 140

#extension GL_ARB_explicit_attrib_location : enable

precision mediump float;

layout (location = 0) in float aTheta; 

uniform float uK;

uniform mat4 model;
uniform mat4 view;
uniform mat4 projection;

void main( void )
{
  float x = cos( uK * aTheta ) * sin( aTheta );
  float y = cos( uK * aTheta ) * cos( aTheta );

  gl_Position  = projection * view * model * vec4( x, y, 0.0, 1.0 );
  gl_PointSize = 64.0;
}

Code:

void CPoint::Render( void )
{
  g_PointProgram.UseProgram();

  glEnable( GL_PROGRAM_POINT_SIZE );
  glEnable( GL_POINT_SPRITE );

  // Set the alpha blending function
  glEnable( GL_BLEND );
  glBlendFunc( GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA );
  glBlendEquation( GL_FUNC_ADD );

  glBindBuffer( GL_ARRAY_BUFFER, m_VBO );

  glm::mat4 Model = glm::mat4( 1.0 );

  Model = glm::translate( Model, glm::vec3( 2.0f, 0.0f, 4.0f ) );

  glm::mat4 Projection = g_Camera.GetProjection();
  glm::mat4 View       = g_Camera.GetView();

  g_PointProgram.SetUint( "uTexture", g_ImgStar.m_Texture );

  g_PointProgram.SetMatrix4fv( "model", Model );
  g_PointProgram.SetMatrix4fv( "view", View );
  g_PointProgram.SetMatrix4fv( "projection", Projection );

  g_PointProgram.SetFloat( "uK", m_Emitter.k );

  glActiveTexture( GL_TEXTURE0 );
  glBindTexture( GL_TEXTURE_2D, g_ImgStar.m_Texture );
  glEnable( GL_TEXTURE_2D );

  // Attributes
  glVertexAttribPointer( 0,                // 1st attribute array (only have 1)
                         1,                                        // One theta angle per particle
                         GL_FLOAT,                                 // Data is floating point type
                         GL_FALSE,                                 // No fixed point scaling
                         sizeof( Particle ),                       // No gaps in data
                         (void*) 0 );      // Start from "theta" offset within bound buffer


  glEnableVertexAttribArray( 0 );

  glDrawArrays( GL_POINTS, 0, NUM_PARTICLES );

  glDisableVertexAttribArray( 0 );
  glBindBuffer( GL_ARRAY_BUFFER, 0 );

  glDisable( GL_TEXTURE_2D );
}

Answer 1

Perhaps depth culling is playing tricks on you? Try glDepthMask(false) while rendering the sprites