Texture coordinates in opengl android showing image reversed

Question

I wrote opengl android code to show a bitmap on a square. But bitmap was drawn in reverse. When i change texture array combination to the commented code it is drawn correctly. But i insist my texture array must be as below . Am i thinking wrong ?

  /** The initial vertex definition */
  private float vertices[] = { 
                      -1.0f, 1.0f, 0.0f,      //Top Left
                      -1.0f, -1.0f, 0.0f,     //Bottom Left
                      1.0f, -1.0f, 0.0f,      //Bottom Right
                      1.0f, 1.0f, 0.0f        //Top Right
                                      };
  /** Our texture pointer */
  private int[] textures = new int[1];

  /** The initial texture coordinates (u, v) */
  private float texture[] = {         
          //Mapping coordinates for the vertices
//            1.0f, 0.0f,
//            1.0f, 1.0f,
//            0.0f, 1.0f,
//            0.0f, 0.0f,
          0.0f, 1.0f,
          0.0f, 0.0f,
          1.0f, 0.0f,
          1.0f, 1.0f,
                              };

    /** The initial indices definition */ 
    private byte indices[] = {
                      //2 triangles
          0,1,2, 2,3,0,           
                                          };

did you load the image with GLUtils.texImage2D?

AndrewBloom
– AndrewBloom

2019-12-17 14:11:39 +00:00
Commented Dec 17, 2019 at 14:11 — AndrewBloom
– AndrewBloom, Commented Dec 17, 2019 at 14:11

Will Kru · Accepted Answer · 2010-12-23 23:07:47Z

5

Whereas Android uses the top-left corner as being 0,0 of the coordinate system, OpenGL uses the bottom-left corner being 0,0 which is why your texture gets flipped.

A common solution to this is to flip your texture at load time,

Matrix flip = new Matrix();
flip.postScale(1f, -1f);
Bitmap bmp = Bitmap.createBitmap(resource, 0, 0, resource.getWidth(), resource.getHeight(), flip, true);

answered Dec 23, 2010 at 23:07

Will Kru

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3 Comments

kml_ckr Over a year ago

I know how opengl coordinate system is but there a confuse where is the 0,0 of the image. I want to understand how opengl reads the image for texture mapping. In my texture array noncommented texture elements must be correct combination according to opengl coordinate system but the image is drawn reversed vertically. It is correct in combination of commented texture array.

Mariux Over a year ago

Thanks, worked like a charm. However still I don't get why the image is drawn reversed, tried everything, changed vertex order to anything possible, but still the issue remains!!

Learn OpenGL ES Over a year ago

Instead of flipping the bitmap you can also just flip the texture coordinate.

dcordes · Accepted Answer · 2013-07-11 20:48:24Z

0

Actually, I think Will Kru's solution should have flipped the background around both axes

flip.postScale(-1f, -1f);

That solution worked for me!

answered Jul 11, 2013 at 20:48

dcordes

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Comments

Tuan Chau · Accepted Answer · 2016-09-30 05:08:12Z

0

This worked for me. (no need to create another bitmap and scale)

 private float vertices[] = { 
                   1.0f, -1.0f, 0.0f,     //Bottom Right
                   1.0f,  1.0f, 0.0f,     //Top Right
                  -1.0f,  1.0f, 0.0f,     //Top Left
                  -1.0f, -1.0f, 0.0f,     //Bottom Left
              };

answered Sep 30, 2016 at 5:08

Tuan Chau

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