OpenGL texture shows as a white square

Question

I am trying to draw a simple 32x32 PNG image using SharpGL and OpenGL 2.1. Despite many hours of searching, reading answers to similar questions, nothing could solve my issue. The image is showed as a white square.

I tried debugging OpenGL using glGetError() but it always returns 0. I have no idea of what could be the problem.

Some help would be very appreciated!

Loading method

public static GLImage LoadImagePNG(OpenGL gl, Bitmap bmp)
{
    bmp = ResizeTo(bmp, 32, 32);

    uint[] textures = new uint[1];
    gl.GenTextures(1, textures);

    uint texture = textures[0];

    gl.Enable(OpenGL.GL_TEXTURE_2D);

    BitmapData data = bmp.LockBits(
        new Rectangle(0, 0, bmp.Width, bmp.Height),
        ImageLockMode.ReadOnly,
        PixelFormat.Format32bppArgb
    );

    gl.BindTexture(OpenGL.GL_TEXTURE_2D, texture);

    gl.TexImage2D(
        OpenGL.GL_TEXTURE_2D,
        0,
        4,
        bmp.Width,
        bmp.Height,
        0,
        OpenGL.GL_RGBA,
        OpenGL.GL_UNSIGNED_BYTE,
        data.Scan0
    );

    Console.WriteLine(gl.GetError());

    bmp.UnlockBits(data);

    gl.TexParameter(OpenGL.GL_TEXTURE_2D, OpenGL.GL_TEXTURE_WRAP_S, OpenGL.GL_CLAMP_TO_EDGE);
    gl.TexParameter(OpenGL.GL_TEXTURE_2D, OpenGL.GL_TEXTURE_WRAP_T, OpenGL.GL_CLAMP_TO_EDGE);
    gl.TexParameter(OpenGL.GL_TEXTURE_2D, OpenGL.GL_TEXTURE_MAG_FILTER, OpenGL.GL_LINEAR);
    gl.TexParameter(OpenGL.GL_TEXTURE_2D, OpenGL.GL_TEXTURE_MIN_FILTER, OpenGL.GL_LINEAR);

    // not disposing the Bitmap on purpose (for now)

    return new GLImage(texture, bmp);
}

Render method

public static void DrawImage(OpenGL gl, GLImage img, double x, double y)
{
    gl.PushMatrix();
    gl.Translate(x, y, 0.0D);

    gl.ActiveTexture(OpenGL.GL_TEXTURE0);
    gl.BindTexture(OpenGL.GL_TEXTURE_2D, img.TextureID);
    gl.Enable(OpenGL.GL_TEXTURE_2D);

    gl.Begin(BeginMode.Quads);
        gl.TexCoord(0.0f, 0.0f); gl.Vertex(0, 0);
        gl.TexCoord(1.0f, 0.0f); gl.Vertex(img.Width, 0);
        gl.TexCoord(1.0f, 1.0f); gl.Vertex(img.Width, img.Height);
        gl.TexCoord(0.0f, 1.0f); gl.Vertex(0, img.Height);
    gl.End();

    gl.Disable(OpenGL.GL_TEXTURE_2D);

    gl.PopMatrix();
}

and GLImage being a custom class

GLImage

public class GLImage
{
    public uint TextureID { get; private set; }

    public Bitmap Bitmap { get; private set; }

    public double Width
    {
        get
        {
            return this.Bitmap.Width;
        }
    }

    public double Height
    {
        get
        {
            return this.Bitmap.Height;
        }
    }

    public GLImage(uint textureID, Bitmap bmp)
    {
        this.TextureID = textureID;
        this.Bitmap = bmp;
    }
}

Thank you!

Answer 1

I'm the OP. As I posted as a visitor and registered afterwards, I don't seem to be able to answer any comment or edit my post. So I am going to answer here.

---

EDIT: Solution !

Alright, I finally made it work. The issue was quite simple (as always, right?). I did only one little thing differently between home and work: I was loading the image in a different thread. Just that. Now it works.

---

Thanks for your answers ebv and justix. Concerning justix, as Maximus Minimus said, I'm using OpenGL 2.1 so your answer doesn't apply to my case. I still appreciate your help!

I had the issue at work, got home and tried the same code there. It didn't work at first and then figured that my resizing method ResizeTo wasn't working properly: it was returning a full transparent image. It doesn't really explain why it showed a white square at work, because it drew a black square here at home which made more sense to me (R, G, B and A at 0 and blending disabled).

I fixed my method ResizeTo, enabled OpenGL's blending and it worked! I will try that code tomorrow at work and keep you guys updated.

---

New method ResizeTo

private static Bitmap ResizeTo(Bitmap bmp, int width, int height)
{
    return new Bitmap(bmp, new Size(width, height));
}

Drawing

gl.Enable(OpenGL.GL_BLEND);
gl.BlendFunc(OpenGL.GL_SRC_ALPHA, OpenGL.GL_ONE_MINUS_SRC_ALPHA);
DrawImage(img, 20, 20);
gl.Disable(OpenGL.GL_BLEND);

---

@ebv: I checked the internal format which I set to 4, and changed it to the constant GL_RGBA8. Both worked just as well.

Answer 2

And check the "internal format" parameter of glTexImage2D() (now 4). Should be something like GL_RGBA8 if it is a 4 channel texture with byte values.