Sign in to follow this  
Servant of the Lord

OpenGL OpenGL using one texture as the alpha mask of another

Recommended Posts

If I'm drawing one texture, and I want to use another texture as the alpha channel for it, and permit the alpha mask texture to be rotated 90 degrees and flipped horizontally or vertically, how would I go about doing that?

 

I can pass in the mask texture to the fragment shader, and pass in a float with the 90 degree rotations in it, and manually rotate in the GLSL shader like this:

vec2 RotateTexCoord(vec2 pos, vec2 rotation, bool flipHorizontally, bool flipVertically)
{
    //Mirror (horizontally, vertically, or both).
    if(flipHorizontally)
    {
        pos.x = (1.0 - pos.x);
    }
    if(flipVertically)
    {
        pos.y = (1.0 - pos.y);
    }

    //Rotate in 90 degree increments.
    
    if(rotation > (270 - 45))
    {
        //Swap x and y (same reason as above).
        float temp = pos.y;
        pos.y = pos.x;
        pos.x = temp;

        pos.x = (1.0 - pos.x);
    }
    else if(rotation > (180 - 45))
    {
        pos.x = (1.0 - pos.x);
        pos.y = (1.0 - pos.y);
    }
    if(rotation > (90 - 45))
    {
        //Swap width and height (since image is turned on side)
        //int alteredWidth = this->GetHeight();
        //int alteredHeight = this->GetWidth();

        float temp = pos.x;
        pos.x = pos.y;
        pos.y = temp;

        pos.y = (1.0 - pos.y);
    }
}

Questions:

A) Does OpenGL already have functionality built-in for using one texture as the alpha mask of another?

B) When passing in a second texture to GLSL, are there features built in for rotating the texture coordinate for the second texture separate from the texture coordinate of the first texture?

C) This simple RotateTexCoord() function has 5 if() statements... that's 5 branches per fragment drawn for a trivial operation. How could it be optimized?

Share this post


Link to post
Share on other sites
C0lumbo    4411


A) Does OpenGL already have functionality built-in for using one texture as the alpha mask of another?

B) When passing in a second texture to GLSL, are there features built in for rotating the texture coordinate for the second texture separate from the texture coordinate of the first texture?

C) This simple RotateTexCoord() function has 5 if() statements... that's 5 branches per fragment drawn for a trivial operation. How could it be optimized?

 

A) Don't think so

B) Don't think so

C) Use a matrix to represent the transformation. Calculate the 3x2 transformation matrix on the CPU and pass it into the vertex shader as a couple of vec3's, transforming the UVs should boil down to a couple of dot products. If you're not already doing so, make sure you do the transformation on the UVs in the vertex shader, not in the pixel shader.

Share this post


Link to post
Share on other sites

C) Use a matrix to represent the transformation. Calculate the 3x2 transformation matrix on the CPU and pass it into the vertex shader as a couple of vec3's, transforming the UVs should boil down to a couple of dot products.

 

I don't know matrix math yet - what's the matrix for a 180 degree rotation (around the Z axis I guess, since they are texture coordinates)? Can matrices be used for flipping horizontally or vertically as well? Is that a 180 rotation around the X axis for vertical flipping and 180 around Y axis for horizontal?

 

If you're not already doing so, make sure you do the transformation on the UVs in the vertex shader, not in the pixel shader.

 

Well, the texture being drawn is already rotated by OpenGL - I don't yet have a need for a vertex shader. Are you saying rotate the texture coordinates of the alpha mask image in the vertex shader? So the vertex shader would rotate two separate textures at once?

Edited by Servant of the Lord

Share this post


Link to post
Share on other sites
SeanMiddleditch    17565

I don't know matrix math yet


I can't stress enough how much you should stop everything you're doing and learn this. A game developer that doesn't have at least a decent grasp of linear algebra in Euclidean space is much akin to a surgeon who doesn't know how to hold a scalpel. smile.png
 

what's the matrix for a 180 degree rotation (around the Z axis I guess, since they are texture coordinates)?


Assuming 2D texture coordinates and no need to apply an offset, you can use a 2x2 matrix here (offsets would require a 3x3 affine transformation matrix):

| -1, 0 |
| 0, -1 |
 

Can matrices be used for flipping horizontally or vertically as well? Is that a 180 rotation around the X axis for vertical flipping and 180 around Y axis for horizontal?


Yes.

vertical mirror:
| 1 0 |
| 0 -1 |

or

horizontal mirror:
| -1 0 |
| 0 1 |

You can perform a great many operations with linear transformations which is what a matrix models.
 

Well, the texture being drawn is already rotated by OpenGL - I don't yet have a need for a vertex shader. Are you saying rotate the texture coordinates of the alpha mask image in the vertex shader? So the vertex shader would rotate two separate textures at once?


You must have a vertex shader to use a pixel shader. Your vertex shader is likely just doing passthru (copying its inputs directly to the outputs). The pixel shader is then receiving the original UVs post-interpolation and rotating them. There's usually no reason you can't do that rotation in the vertex shader so that you don't have to re-evaluate the rotation for every pixel fragment.

The vertex shader can use whatever vertex attributes it wants so there's no reason it couldn't rotate both or either of the incoming UVs (assuming they're separate). If the source texture and mask are both rotated the same then you only need a single UV attribute.

Vertex shader pseudocode:

vsout_position = vsin_position
vsout_texcoord0 = rotate(vsin_texcoord0)
vsout_texcoord1 = rotate(vsin_texcoord1)

Pixel shader pseudocode:

psout_color = sample(source_texture, psin_texcoord0)
psout_color *= sample(mask_texture, psin_texcoord1)

Again, just using a single texcoord if both textures use the same coordinates.

Share this post


Link to post
Share on other sites

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 cebugdev
      hi all,

      i am trying to build an OpenGL 2D GUI system, (yeah yeah, i know i should not be re inventing the wheel, but this is for educational and some other purpose only),
      i have built GUI system before using 2D systems such as that of HTML/JS canvas, but in 2D system, i can directly match a mouse coordinates to the actual graphic coordinates with additional computation for screen size/ratio/scale ofcourse.
      now i want to port it to OpenGL, i know that to render a 2D object in OpenGL we specify coordiantes in Clip space or use the orthographic projection, now heres what i need help about.
      1. what is the right way of rendering the GUI? is it thru drawing in clip space or switching to ortho projection?
      2. from screen coordinates (top left is 0,0 nd bottom right is width height), how can i map the mouse coordinates to OpenGL 2D so that mouse events such as button click works? In consideration ofcourse to the current screen/size dimension.
      3. when let say if the screen size/dimension is different, how to handle this? in my previous javascript 2D engine using canvas, i just have my working coordinates and then just perform the bitblk or copying my working canvas to screen canvas and scale the mouse coordinates from there, in OpenGL how to work on a multiple screen sizes (more like an OpenGL ES question).
      lastly, if you guys know any books, resources, links or tutorials that handle or discuss this, i found one with marekknows opengl game engine website but its not free,
      Just let me know. Did not have any luck finding resource in google for writing our own OpenGL GUI framework.
      IF there are no any available online, just let me know, what things do i need to look into for OpenGL and i will study them one by one to make it work.
      thank you, and looking forward to positive replies.
    • By fllwr0491
      I have a few beginner questions about tesselation that I really have no clue.
      The opengl wiki doesn't seem to talk anything about the details.
       
      What is the relationship between TCS layout out and TES layout in?
      How does the tesselator know how control points are organized?
          e.g. If TES input requests triangles, but TCS can output N vertices.
             What happens in this case?
      In this article,
      http://www.informit.com/articles/article.aspx?p=2120983
      the isoline example TCS out=4, but TES in=isoline.
      And gl_TessCoord is only a single one.
      So which ones are the control points?
      How are tesselator building primitives?
    • By Orella
      I've been developing a 2D Engine using SFML + ImGui.
      Here you can see an image
      The editor is rendered using ImGui and the scene window is a sf::RenderTexture where I draw the GameObjects and then is converted to ImGui::Image to render it in the editor.
      Now I need to create a 3D Engine during this year in my Bachelor Degree but using SDL2 + ImGui and I want to recreate what I did with the 2D Engine. 
      I've managed to render the editor like I did in the 2D Engine using this example that comes with ImGui. 
      3D Editor preview
      But I don't know how to create an equivalent of sf::RenderTexture in SDL2, so I can draw the 3D scene there and convert it to ImGui::Image to show it in the editor.
      If you can provide code will be better. And if you want me to provide any specific code tell me.
      Thanks!
    • By Picpenguin
      Hi
      I'm new to learning OpenGL and still learning C. I'm using SDL2, glew, OpenGL 3.3, linmath and stb_image.
      I started following through learnopengl.com and got through it until I had to load models. The problem is, it uses Assimp for loading models. Assimp is C++ and uses things I don't want in my program (boost for example) and C support doesn't seem that good.
      Things like glVertexAttribPointer and shaders are still confusing to me, but I have to start somewhere right?
      I can't seem to find any good loading/rendering tutorials or source code that is simple to use and easy to understand.
      I have tried this for over a week by myself, searching for solutions but so far no luck. With tinyobjloader-c and project that uses it, FantasyGolfSimulator, I was able to actually load the model with plain color (always the same color no matter what I do) on screen and move it around, but cannot figure out how to use textures or use its multiple textures with it.
      I don't ask much: I just want to load models with textures in them, maybe have lights affect them (directional spotlight etc). Also, some models have multiple parts and multiple textures in them, how can I handle those?
      Are there solutions anywhere?
      Thank you for your time. Sorry if this is a bit confusing, English isn't my native language
    • By dpadam450
      FINALLY, upgrading my engine to openGL 4. I was having some trouble so I started with a stripped down application and was wondering if VAO's are required, because I have a sample working, but if I remove the VAO then it doesn't seem to like drawing my triangle.
  • Popular Now