Sign in to follow this  

OpenGL Aliasing confusion

This topic is 1678 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

If you intended to correct an error in the post then please contact us.

Recommended Posts

Hello,

 

I am a bit confused on the aliasing problem in computer graphics(OpenGL), specifically:

 

1. In the picture below, the checkerboard texture is laid on a rectangle consisting of 2 triangles. Please note at the bottom of the image the jagged  edges of black and white blocks of the texture:

 

http://www.arcsynthesis.org/gltut/Texturing/BasicCheckerboardPlane.png

 

Is that problem called aliasing?

 

2. In the picture below, the same jagged effect can be seen on the edges of the right-most two triangles:

 

http://slizerboy.files.wordpress.com/2009/11/anti-alias.png?w=343&h=150

 

Is that the same aliasing problem?

 

I'm asking this because in first case, texture sampling is involved but in the second case there is no texture. 

 

 

3. If in the first example the issue is called aliasing, can we say that using linear filtering as magnification/minification filters in sampling the texture represent an anti-aliasing technique? I have searched the web for OpenGL anti-aliasing and I mostly read about "multisampling", so I'm wondering if linear and mipmap filtering can be considered anti-aliasing techniques?

Edited by vladic2000x

Share this post


Link to post
Share on other sites
  1. yes
  2. yes
  3. no and no

Linear filtering is texture filter. If you attempt to take color between 2 pixels linear filter will interpolate them.

Aliasing is... Like in your 2nd picture. You draw white line on black background, so pixels are completely white or completely black. It doesn't look like a line since you can see pixels, it's more like bunch of rectangles. Anti-aliasing is supposed to smooth out these pixels, to make it look like an actual line. Here's some example: http://international.download.nvidia.com/webassets/en_US/shared/images/articles/introducing-the-geforce-gtx-680-gpu/TXAA.png

Share this post


Link to post
Share on other sites

1. Yes, that's aliasing. Essentially you have some signal that's stored in a texture, and the fragment shader samples that signal. If the fragment shader doesn't sample at an adequate rate relative to the rate at which the signal changes (the frequency of the signal), then you get aliasing. A black and white checkerboard is basically a series of step functions, and sampling step functions will always result in aliasing because they have an infinite rate of change.

 

2. Yes, that's a similar case of aliasing. In this case the aliasing stems from the rasterizer, which samples some signal (in this case a triangle) at a fixed set of sample positions (usually aligned to the center of pixels). A triangle edge is also a step function, so you get the same aliasing problem.

3. Yes, texture filtering is a very important means of anti-aliasing. Filtering reduces aliasing by attenuating high-frequency components of a signal, but can also remove detail that's present in those higher frequencies. However if you oversample by sampling at a higher rate than your screen resolution and then filter the result when downsampling, you can reduce aliasing while still preserving details. This is the basic premise behind both texture filtering and MSAA. The former works with texture sampling, and the later works with the rasterizer. With texture sampling you typically take 4 texture samples in a 2x2 grid and perform linear filtering, which allows you to avoid aliasing as long as your sampling rate is no less than 1/2 the resolution of your texture. So if you had a 512x512 texture, bilinear filtering won't alias as long as the texture is mapped to an area of the screen that's >= 256x256 pixels. If you need to go lower, you need to use pre-filter by creating a chain of mip maps. This "no less than 1/2 the texture resolution" rule is why you create each mip level to be 1/2 the size of the previous level.

If you're interested in this sort of thing, I have a rather lengthy series of articles on my blog regarding sampling and aliasing. It may put you to sleep, but there's a lot of (hopefully useful) info in there.

Edited by MJP

Share this post


Link to post
Share on other sites

Regarding the apparent disagreement in previous answers, filtering and mipmapping reduce aliasing in texture lookups (approximate sampling from wrong texture coordinates) which is separate from aliasing in rendering (approximate drawing to wrong frame buffer positions) but conceptually similar. They are addressed in different ways for technological reasons; for example, filtering texture lookups has been a cheap commoditized basic feature since the period of fixed function pipelines, while the same kind of filtering applied to rendering is a luxury because it would need expensive additional fragments as input.

Edited by LorenzoGatti

Share this post


Link to post
Share on other sites

This topic is 1678 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

If you intended to correct an error in the post then please contact us.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

Sign in to follow this  

  • Similar Content

    • By xhcao
      Does sync be needed to read texture content after access texture image in compute shader?
      My simple code is as below,
      glUseProgram(program.get());
      glBindImageTexture(0, texture[0], 0, GL_FALSE, 3, GL_READ_ONLY, GL_R32UI);
      glBindImageTexture(1, texture[1], 0, GL_FALSE, 4, GL_WRITE_ONLY, GL_R32UI);
      glDispatchCompute(1, 1, 1);
      // Does sync be needed here?
      glUseProgram(0);
      glBindFramebuffer(GL_READ_FRAMEBUFFER, framebuffer);
      glFramebufferTexture2D(GL_READ_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,
                                     GL_TEXTURE_CUBE_MAP_POSITIVE_X + face, texture[1], 0);
      glReadPixels(0, 0, kWidth, kHeight, GL_RED_INTEGER, GL_UNSIGNED_INT, outputValues);
       
      Compute shader is very simple, imageLoad content from texture[0], and imageStore content to texture[1]. Does need to sync after dispatchCompute?
    • By Jonathan2006
      My question: is it possible to transform multiple angular velocities so that they can be reinserted as one? My research is below:
      // This works quat quaternion1 = GEQuaternionFromAngleRadians(angleRadiansVector1); quat quaternion2 = GEMultiplyQuaternions(quaternion1, GEQuaternionFromAngleRadians(angleRadiansVector2)); quat quaternion3 = GEMultiplyQuaternions(quaternion2, GEQuaternionFromAngleRadians(angleRadiansVector3)); glMultMatrixf(GEMat4FromQuaternion(quaternion3).array); // The first two work fine but not the third. Why? quat quaternion1 = GEQuaternionFromAngleRadians(angleRadiansVector1); vec3 vector1 = GETransformQuaternionAndVector(quaternion1, angularVelocity1); quat quaternion2 = GEQuaternionFromAngleRadians(angleRadiansVector2); vec3 vector2 = GETransformQuaternionAndVector(quaternion2, angularVelocity2); // This doesn't work //quat quaternion3 = GEQuaternionFromAngleRadians(angleRadiansVector3); //vec3 vector3 = GETransformQuaternionAndVector(quaternion3, angularVelocity3); vec3 angleVelocity = GEAddVectors(vector1, vector2); // Does not work: vec3 angleVelocity = GEAddVectors(vector1, GEAddVectors(vector2, vector3)); static vec3 angleRadiansVector; vec3 angularAcceleration = GESetVector(0.0, 0.0, 0.0); // Sending it through one angular velocity later in my motion engine angleVelocity = GEAddVectors(angleVelocity, GEMultiplyVectorAndScalar(angularAcceleration, timeStep)); angleRadiansVector = GEAddVectors(angleRadiansVector, GEMultiplyVectorAndScalar(angleVelocity, timeStep)); glMultMatrixf(GEMat4FromEulerAngle(angleRadiansVector).array); Also how do I combine multiple angularAcceleration variables? Is there an easier way to transform the angular values?
    • By dpadam450
      I have this code below in both my vertex and fragment shader, however when I request glGetUniformLocation("Lights[0].diffuse") or "Lights[0].attenuation", it returns -1. It will only give me a valid uniform location if I actually use the diffuse/attenuation variables in the VERTEX shader. Because I use position in the vertex shader, it always returns a valid uniform location. I've read that I can share uniforms across both vertex and fragment, but I'm confused what this is even compiling to if this is the case.
       
      #define NUM_LIGHTS 2
      struct Light
      {
          vec3 position;
          vec3 diffuse;
          float attenuation;
      };
      uniform Light Lights[NUM_LIGHTS];
       
       
    • By pr033r
      Hello,
      I have a Bachelor project on topic "Implenet 3D Boid's algorithm in OpenGL". All OpenGL issues works fine for me, all rendering etc. But when I started implement the boid's algorithm it was getting worse and worse. I read article (http://natureofcode.com/book/chapter-6-autonomous-agents/) inspirate from another code (here: https://github.com/jyanar/Boids/tree/master/src) but it still doesn't work like in tutorials and videos. For example the main problem: when I apply Cohesion (one of three main laws of boids) it makes some "cycling knot". Second, when some flock touch to another it scary change the coordination or respawn in origin (x: 0, y:0. z:0). Just some streng things. 
      I followed many tutorials, change a try everything but it isn't so smooth, without lags like in another videos. I really need your help. 
      My code (optimalizing branch): https://github.com/pr033r/BachelorProject/tree/Optimalizing
      Exe file (if you want to look) and models folder (for those who will download the sources):
      http://leteckaposta.cz/367190436
      Thanks for any help...

    • By Andrija
      I am currently trying to implement shadow mapping into my project , but although i can render my depth map to the screen and it looks okay , when i sample it with shadowCoords there is no shadow.
      Here is my light space matrix calculation
      mat4x4 lightViewMatrix; vec3 sun_pos = {SUN_OFFSET * the_sun->direction[0], SUN_OFFSET * the_sun->direction[1], SUN_OFFSET * the_sun->direction[2]}; mat4x4_look_at(lightViewMatrix,sun_pos,player->pos,up); mat4x4_mul(lightSpaceMatrix,lightProjMatrix,lightViewMatrix); I will tweak the values for the size and frustum of the shadow map, but for now i just want to draw shadows around the player position
      the_sun->direction is a normalized vector so i multiply it by a constant to get the position.
      player->pos is the camera position in world space
      the light projection matrix is calculated like this:
      mat4x4_ortho(lightProjMatrix,-SHADOW_FAR,SHADOW_FAR,-SHADOW_FAR,SHADOW_FAR,NEAR,SHADOW_FAR); Shadow vertex shader:
      uniform mat4 light_space_matrix; void main() { gl_Position = light_space_matrix * transfMatrix * vec4(position, 1.0f); } Shadow fragment shader:
      out float fragDepth; void main() { fragDepth = gl_FragCoord.z; } I am using deferred rendering so i have all my world positions in the g_positions buffer
      My shadow calculation in the deferred fragment shader:
      float get_shadow_fac(vec4 light_space_pos) { vec3 shadow_coords = light_space_pos.xyz / light_space_pos.w; shadow_coords = shadow_coords * 0.5 + 0.5; float closest_depth = texture(shadow_map, shadow_coords.xy).r; float current_depth = shadow_coords.z; float shadow_fac = 1.0; if(closest_depth < current_depth) shadow_fac = 0.5; return shadow_fac; } I call the function like this:
      get_shadow_fac(light_space_matrix * vec4(position,1.0)); Where position is the value i got from sampling the g_position buffer
      Here is my depth texture (i know it will produce low quality shadows but i just want to get it working for now):
      sorry because of the compression , the black smudges are trees ... https://i.stack.imgur.com/T43aK.jpg
      EDIT: Depth texture attachment:
      glTexImage2D(GL_TEXTURE_2D, 0,GL_DEPTH_COMPONENT24,fbo->width,fbo->height,0,GL_DEPTH_COMPONENT,GL_FLOAT,NULL); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE); glFramebufferTexture2D(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_TEXTURE_2D, fbo->depthTexture, 0);
  • Popular Now