FloatBuffer 绘制纹理四边形，但 ShortBuffer 不绘制

Question

I am trying to draw a textured quad. Since it is a quad I want to use glDrawElements and a VEO, but that requires a ShortBuffer instead of a FloatBuffer . I tried changing my code, but now nothing draws. Old code: Uploading and drawing:

 public void flush() {
    if (numVertices > 0) {
        vertices.flip();

        if (vao != null) {
            vao.bind();
        } else {
            vbo.bind(GL_ARRAY_BUFFER);
            specifyVertexAttributes();
        }
        program.use();

        /* Upload the new vertex data */
        vbo.bind(GL_ARRAY_BUFFER);
        vbo.uploadSubData(GL_ARRAY_BUFFER, 0, vertices);

        /* Draw batch */
        glDrawArrays(GL_TRIANGLES, 0, numVertices);

        /* Clear vertex data for next batch */
        vertices.clear();
        numVertices = 0;
    }
}

Adding texture to buffer:

if (vertices.remaining() < 7 * 6) {
        /* We need more space in the buffer, so flush it */
        flush();
    }

    float r = c.getRed();
    float g = c.getGreen();
    float b = c.getBlue();
    float a = c.getAlpha();

    vertices.put(x1).put(y1).put(r).put(g).put(b).put(a).put(s1).put(t1);
    vertices.put(x1).put(y2).put(r).put(g).put(b).put(a).put(s1).put(t2);
    vertices.put(x2).put(y2).put(r).put(g).put(b).put(a).put(s2).put(t2);

    vertices.put(x1).put(y1).put(r).put(g).put(b).put(a).put(s1).put(t1);
    vertices.put(x2).put(y2).put(r).put(g).put(b).put(a).put(s2).put(t2);
    vertices.put(x2).put(y1).put(r).put(g).put(b).put(a).put(s2).put(t1);

    numVertices += 6;

Updated code: Uploading and Drawing:

public void flush() {
    if (numVertices > 0) {
        vertices.flip();

        if (vao != null) {
            vao.bind();
        } else {
            vbo.bind(GL_ARRAY_BUFFER);
            specifyVertexAttributes();
        }
        program.use();

        /* Upload the new vertex data */
        vbo.bind(GL_ARRAY_BUFFER);
        vbo.uploadSubData(GL_ARRAY_BUFFER, 0, vertices);

        /* Draw batch */
        glDrawArrays(GL_TRIANGLES, 0, numVertices);

        /* Clear vertex data for next batch */
        vertices.clear();
        numVertices = 0;
    }
}

Adding texture to buffer:

if (vertices.remaining() < 7 * 6) {
        /* We need more space in the buffer, so flush it */
        flush();
    }

    short r = (short) c.getRed();
    short g = (short) c.getGreen();
    short b = (short) c.getBlue();
    short a = (short) c.getAlpha();

    short sx1 = (short) Math.round(x1), sx2 = (short) Math.round(x2), sy1 = (short) Math.round(y1), sy2 = (short) Math.round(y2), ss1 = (short) Math.round(s1), ss2 = (short) Math.round(s2), st1 = (short) Math.round(t1), st2 = (short) Math.round(t2);
    vertices.put(sx1).put(sy1).put(r).put(g).put(b).put(a).put(ss1).put(st1);
    vertices.put(sx1).put(sy2).put(r).put(g).put(b).put(a).put(ss1).put(st2);
    vertices.put(sx2).put(sy2).put(r).put(g).put(b).put(a).put(ss2).put(st2);

    vertices.put(sx1).put(sy1).put(r).put(g).put(b).put(a).put(ss1).put(st1);
    vertices.put(sx2).put(sy2).put(r).put(g).put(b).put(a).put(ss2).put(st2);
    vertices.put(sx2).put(sy1).put(r).put(g).put(b).put(a).put(ss2).put(st1);

    numVertices += 6;

There were no other changes to the code, except replacing FloatBuffer with ShortBuffer in my uploadSubData method. The VBO class is just a wrapper for the OpenGL methods, so uploadSubData is glUploadSubData , etc... What am I missing? Why doesn't glDrawArrays draw the ShortBuffer ? If I left out anything, please tell me, I didn't have much time to write this.

Answer 1

You are confusing indices and vertex coordinates. The coordinates are tuples of type GL_FLOAT in an GL_ARRAY_BUFFER . But the indices are a list of integral indices (eg type GL_SHORT ) in a GL_ELEMENT_ARRAY_BUFFER which refer to the vertex coordinates.

A quad can be be drawn by 2 triangles. You can define 6 vertex coordinates and attributes and use glDrawArrays .

In the following vertices is of type FloatBuffer :

vertices.put(x1).put(y1).put(r).put(g).put(b).put(a).put(s1).put(t1);
vertices.put(x1).put(y2).put(r).put(g).put(b).put(a).put(s1).put(t2);
vertices.put(x2).put(y2).put(r).put(g).put(b).put(a).put(s2).put(t2);

vertices.put(x1).put(y1).put(r).put(g).put(b).put(a).put(s1).put(t1);
vertices.put(x2).put(y2).put(r).put(g).put(b).put(a).put(s2).put(t2);
vertices.put(x2).put(y1).put(r).put(g).put(b).put(a).put(s2).put(t1);

numVertices += 6;

vbo.bind(GL_ARRAY_BUFFER);
vbo.uploadSubData(GL_ARRAY_BUFFER, 0, vertices);

glDrawArrays(GL_TRIANGLES, 0, numVertices);

Or you can define 4 vertex coordinates respectively attributes and 6 indices and use glDrawElements .

In the following vertices still is of type FloatBuffer , but indices is of type ShortBuffer :

vertices.put(x1).put(y1).put(r).put(g).put(b).put(a).put(s1).put(t1);
vertices.put(x1).put(y2).put(r).put(g).put(b).put(a).put(s1).put(t2);
vertices.put(x2).put(y2).put(r).put(g).put(b).put(a).put(s2).put(t2);
vertices.put(x2).put(y1).put(r).put(g).put(b).put(a).put(s2).put(t1);

numVertices += 4;

indices.put(0).put(1).put(2);
indices.put(0).put(2).put(3);

numIndices += 4;

vbo.bind(GL_ARRAY_BUFFER);
vbo.uploadSubData(GL_ARRAY_BUFFER, 0, vertices);
ibo.bind(GL_ELEMENT_ARRAY_BUFFER);
ibo.uploadSubData(GL_ELEMENT_ARRAY_BUFFER, 0, indices);

glDrawElements(GL_TRIANGLES, GL_SHORT, numIndices, null);

So the key is that you'll need 2 uploadSubData methods. The former hast to deal with FloatBuffer and the later has to deal with ShortBuffer .
Note, in common the vertex attributes are floating point values. Colors are often floating point values in the range [0, 1]. Texture coordinates are in range [0, 1]. Of course it is possible to encode this to an integral data type, but at least for the vertex coordinates this would cause a loss of accuracy.