Discrad black parts in texture

bonanza

Apprentice

Posts: 9
Joined: 2010.02

Post: #1

Oct 26, 2011 — 01:51 AM

Hi,
i like to discard (without shaders) any black parts in a texture. Blending don't work, here is what i have so far:

Code:

glEnable(GL_BLEND);

    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

    glEnableClientState(GL_VERTEX_ARRAY);

    glEnableClientState(GL_TEXTURE_COORD_ARRAY);

    GLfloat boxBackground[]={0,0,320, 0,0, 480,320, 480,};

    GLfloat uvBackground[]={0,0,1,0,0,1,1,1};

    glBindTexture(GL_TEXTURE_2D, textureBackground);

    glVertexPointer(2, GL_FLOAT, 0, boxBackground);

    glTexCoordPointer(2, GL_FLOAT, 0, uvBackground);

    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

What i want to do is, to delete some parts (on the fly) of the foreground texture out (draw black lines for example), so that the background is visible trough the "deleted" parts of the foreground texture.
In principle, masking on the fly.

Any suggestions ?

backslash

Member

Posts: 244
Joined: 2005.11

Post: #2

Oct 26, 2011 — 06:16 AM

I'm not an OpenGL genius, but I think if you're not going to use shaders then you will need to generate an alpha channel for the texture. The logic to read through pixel by pixel and set the alpha to 0 for the black pixels is fairly trivial.

http://www.EchoMist.co.uk

bonanza

Apprentice

Posts: 9
Joined: 2010.02

Post: #3

Oct 26, 2011 — 07:09 AM

I think, read the pixel at realtime is a performance problem.

ThemsAllTook

Moderator

Posts: 1,552
Joined: 2003.10

Post: #4

Oct 26, 2011 — 07:27 AM

You don't need to read back the pixels from the texture itself. You can just keep the pixel data in memory after uploading it to GL, modify it there, and call glTexSubImage2D to reupload the modified parts. Also, there's no way to know for sure if it's a performance problem until you try it out.

If you describe the effect you're trying to create in more detail, we might be able to suggest more alternatives.

bonanza

Apprentice

Posts: 9
Joined: 2010.02

Post: #5

Oct 26, 2011 — 07:54 AM

What i like to do is a simple rubber (like in a painting program). I move my finger over the top most texture that portion of this texture should become transparent and the background texture are visible at this parts.

ThemsAllTook

Moderator

Posts: 1,552
Joined: 2003.10

Post: #6

Oct 26, 2011 — 08:24 AM

Fair enough. Sounds like modifying your pixels in memory and calling glTexSubImage2D is exactly what you'd want for that case.

bonanza

Apprentice

Posts: 9
Joined: 2010.02

Post: #7

Oct 26, 2011 — 08:37 AM

Mhh, i don't no, how to do that.

I have found this: http://stackoverflow.com/questions/34396...16#4238716

seems to be the same, what i want.

Code:

// RTT

glBindFramebuffer(GL_FRAMEBUFFER, [(EAGLView *)self.view fbo]);

    glViewport(0, 0, 512, 512);

//  Bind the foreground texture and render this

    glBindTexture(GL_TEXTURE_2D, textureForeground);

    glVertexPointer(2, GL_FLOAT, 0, boxBackground);

    glTexCoordPointer(2, GL_FLOAT, 0, uvBackground);

    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

// the erase part

    if(vertexBuffer!=NULL)

    {

            glEnable(GL_BLEND);

            glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);

            //glBlendFunc(1.0,0.0);

            glEnable(GL_POINT_SPRITE_OES);

            glTexEnvf(GL_POINT_SPRITE_OES, GL_COORD_REPLACE_OES, GL_TRUE);

            glPointSize(64.0 / kBrushScale);

            glBindTexture(GL_TEXTURE_2D, textureSprite);

            glVertexPointer(2, GL_FLOAT, 0, vertexBuffer);

  // only draw in the alpha-component         

 glColorMask(0,0,0,1);

            glDrawArrays(GL_POINTS, 0, vCount);

            glDisable(GL_POINT_SPRITE_OES);

            glDisable(GL_BLEND);

    }

    glBindFramebuffer(GL_FRAMEBUFFER, 0);

    glColorMask(1,1,1,0);

 // switch back to the normal frame buffer   

    [(EAGLView *)self.view setFramebuffer];    

    [(EAGLView *)self.view setViewPort];

    glClear(GL_COLOR_BUFFER_BIT);

// bind the fbo-texture and blend it over the background-texture    

    GLuint tmpTexture=[(EAGLView *)self.view img];

    glBindTexture(GL_TEXTURE_2D, tmpTexture);

    glVertexPointer(2, GL_FLOAT, 0, boxBackground);

    glTexCoordPointer(2, GL_FLOAT, 0, uvBackground);

   // glEnable(GL_ALPHA_TEST);

    //glAlphaFunc(GL_GREATER, 0.0);

    glEnable(GL_BLEND);

    glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);

    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

    glDisable(GL_ALPHA_TEST);

    glDisable(GL_BLEND);

    [(EAGLView *)self.view presentFramebuffer];

bonanza

Apprentice

Posts: 9
Joined: 2010.02

Post: #8

Oct 27, 2011 — 01:05 AM

This should work:
http://www.zeuscmd.com/tutorials/opengle...asking.php

Here my code

Code:

glBindTexture(GL_TEXTURE_2D, textureBackground);

    glVertexPointer(2, GL_FLOAT, 0, boxBackground);

    glTexCoordPointer(2, GL_FLOAT, 0, uvBackground);

    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

    glEnable(GL_BLEND);

    glDisable(GL_DEPTH_TEST);    

    glBlendFunc(GL_DST_COLOR, GL_ZERO);

    // black lines from the RTT

    GLuint tmpTexture=[(EAGLView *)self.view img];

    glBindTexture(GL_TEXTURE_2D, tmpTexture);

    glVertexPointer(2, GL_FLOAT, 0, boxBackground);

    glTexCoordPointer(2, GL_FLOAT, 0, uvBackground);

    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

    glBlendFunc(GL_ONE, GL_ONE);

    glBindTexture(GL_TEXTURE_2D, textureForeground);

    glVertexPointer(2, GL_FLOAT, 0, boxBackground);

    glTexCoordPointer(2, GL_FLOAT, 0, uvBackground);

    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

    glDisable(GL_BLEND);

    glEnable(GL_DEPTH_TEST);

The result is masked but it's somehow overbright-ed. Any ideas?

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