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I am learning open GL ES 2.0 and I just made 1 easy game for 1.0

I am following tutorials but I cant understand this thing

In OpenGL ES 1.0 if I define a square that way:

private float[] vertices = { // Vertices for a face
0.0f, 0.0f, 0.2f, // 0. left-bottom-front
1.0f, 0.0f, 0.2f, // 1. right-bottom-front
0.0f, 1.0f, 0.2f, // 2. left-top-front
1.0f, 1.0f, 0.2f // 3. right-top-front
};

and draw it that way it Works

public void draw(GL10 gl)
{

    gl.glFrontFace(GL10.GL_CCW); // Front face in counter-clockwise
                                    // orientation
    gl.glEnable(GL10.GL_CULL_FACE); // Enable cull face
    gl.glCullFace(GL10.GL_BACK); // Cull the back face (don't display)

    gl.glEnableClientState(GL10.GL_VERTEX_ARRAY);
    gl.glVertexPointer(3, GL10.GL_FLOAT, 0, vertexBuffer);
    gl.glEnableClientState(GL10.GL_TEXTURE_COORD_ARRAY); // Enable
                                                            // texture-coords-array
                                                            // (NEW)
    gl.glTexCoordPointer(3, GL10.GL_FLOAT, 0, texBuffer); // Define
                                                            // texture-coords
                                                            // buffer (NEW)

    gl.glEnable(GL10.GL_BLEND);
    gl.glBlendFunc(GL10.GL_SRC_ALPHA, GL10.GL_ONE_MINUS_SRC_ALPHA);

    // front
    gl.glBindTexture(GL10.GL_TEXTURE_2D,TextureLoader.TeclaBlancatextureIDs[0]);


    gl.glDrawArrays(GL10.GL_TRIANGLE_STRIP, 0, 4);

    gl.glDisableClientState(GL10.GL_TEXTURE_COORD_ARRAY); // Disable
                                                            // texture-coords-array
                                                            // (NEW)
    gl.glDisableClientState(GL10.GL_VERTEX_ARRAY);
    gl.glDisable(GL10.GL_CULL_FACE);
    gl.glDisable(GL10.GL_BLEND);
}

but in OpenGL ES 2.0 if I do this that way( with the same vertex) it only write a triangle:

private void drawSuelo(Suelo suelo)
{       
    // Pass in the position information
    suelo.mPositions.position(0);       
    GLES20.glVertexAttribPointer(mPositionHandle, mPositionDataSize, GLES20.GL_FLOAT, false,
            0, suelo.mPositions);        

    GLES20.glEnableVertexAttribArray(mPositionHandle);        

    // Pass in the color information
    suelo.mColors.position(0);
    GLES20.glVertexAttribPointer(mColorHandle, mColorDataSize, GLES20.GL_FLOAT, false,
            0, suelo.mColors);        

    GLES20.glEnableVertexAttribArray(mColorHandle);

    // Pass in the normal information
    cubo.mCubeNormals.position(0);
    GLES20.glVertexAttribPointer(mNormalHandle, mNormalDataSize, GLES20.GL_FLOAT, false, 
            0, cubo.mCubeNormals);

    GLES20.glEnableVertexAttribArray(mNormalHandle);

    // This multiplies the view matrix by the model matrix, and stores the result in the MVP matrix
    // (which currently contains model * view).
    Matrix.multiplyMM(mMVPMatrix, 0, mViewMatrix, 0, mModelMatrix, 0);   

    // Pass in the modelview matrix.
    GLES20.glUniformMatrix4fv(mMVMatrixHandle, 1, false, mMVPMatrix, 0);                

    // This multiplies the modelview matrix by the projection matrix, and stores the result in the MVP matrix
    // (which now contains model * view * projection).
    Matrix.multiplyMM(mMVPMatrix, 0, mProjectionMatrix, 0, mMVPMatrix, 0);

    // Pass in the combined matrix.
    GLES20.glUniformMatrix4fv(mMVPMatrixHandle, 1, false, mMVPMatrix, 0);

    // Pass in the light position in eye space.        
    GLES20.glUniform3f(mLightPosHandle, mLightPosInEyeSpace[0], mLightPosInEyeSpace[1], mLightPosInEyeSpace[2]);

    // Draw the cube.
    GLES20.glDrawArrays(GLES20.GL_TRIANGLES, 0, 6);                               
}   

It seems to need all of this vertex in order to draw a square

final float[] PositionData = {

// Top face
-1.0f, 1.0f, -1.0f, 
-1.0f, 1.0f, 1.0f,
1.0f, 1.0f, -1.0f, 
-1.0f, 1.0f, 1.0f, 
1.0f, 1.0f, 1.0f,
1.0f, 1.0f, -1.0f };

Why is happening this? Why I am missing? Is there any way in Open GL ES 2.0 to write a square with 4 vertex?

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  • \$\begingroup\$ where is the glsl shader ? as far as i know gles2 does not have fixed function pipeline so you need shader to draw something \$\endgroup\$
    – Raxvan
    Mar 20, 2015 at 12:16

1 Answer 1

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The correct answer was answered by Reto Koradi

You're using different primitive types. In the working ES 1.0 code, you have this:

gl.glDrawArrays(GL10.GL_TRIANGLE_STRIP, 0, 4); and in the ES 2.0 code that renders only a triangle when you give it 4 vertices:

GLES20.glDrawArrays(GLES20.GL_TRIANGLES, 0, 6); So in the ES 1.0 code, you're specifying that you want to draw a triangle strip (primitive type GL_TRIANGLE_STRIP) with 4 vertices. This corresponds to 2 triangles. While in the ES 2.0 code, you specify that you want to draw 2 separate triangles (primitive type GL_TRIANGLES), which indeed needs 6 vertices.

If you want to use the original 4 vertices for the ES 2.0 code, you simply have to use the equivalent draw call with the same primitive type you used in the ES 1.0 version:

GLES20.glDrawArrays(GLES20.GL_TRIANGLE_STRIP, 0, 4);

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