Implementation: Textured Cube

Overview 

In the previous discussions, we covered sending vertex data - Position and color. In this post we will try to send texture vertex data. It's implemented in TexturedCube class.

It looks as shown at the bottom. It looks elongated because aspect ratio is not applied.

Details

A Texture is 2D image that can be wrapped around a 3D object like a gift wrapper. For example, resources\textures\bricks2.jpg is a 2D Texture file. 

Unlike cartesian coordinates, Texture follow UV  System as shown below.

TextureUtil Class is used for loading textures. 

Loading Texture

OpenGL supports up to 80 Textures and each with its own identifier. Loading textures is implemented in LoadTexture method in TextureUtil class.

SOIL_load_OGL_texture(filename.c_str(), SOIL_LOAD_AUTO, 0, SOIL_FLAG_MIPMAPS | SOIL_FLAG_INVERT_Y);

MIPMAP

Mipmapping is a technique where a high-resolution texture is downscaled and filtered so that each subsequent mip level is a quarter of the area of the previous level. This means that the texture and all of its generated mips requires no more than 1.5 times the original texture size. An example is shown below.

Inversion of Y Axis

After loading the image needs to be inverted since the V axis of the texture and Y axis of OpenGL  have opposite polarity.

Wrapping

As shown below, determines how the texture is wrapped.

Texture Filtering

Texture filtering is a method that is used to improve the texture quality in a scene. Without texture filtering, artifacts like aliasing generally look worse. Texture filtering makes textures look better and less blocky.

Applying Texture

As discussed previously, Texture coordinates are passed during rendering. The following mapping in relation to the UxV axes is used as texture coordinates for the vertices consecutively.

        //first triangle
		{  0.0,   1.0,   0.0 },
		{ -1.0,   0.0,   0.0 },
		{  0.0,  -1.0,   0.0 },
        //second triangle
		{  1.0,   0.0,   0.0 },
		{  0.0,   0.0,  -1.0 },
		{  0.0,   0.0,   1.0 },

System  class diagram

Every 3D object such as IndexedCube  derives from BaseGeometry class. It overrides mesh with an instance of CubeMesh to generate geometry. It uses a helper class texutl to help render texture.

TextureUtil

TextureUtil implements handling textures from image files and bitmaps. It's used by multiple purposes such as rendering 3D objects or text.

Members

Name	Description
textureID	Unique texture handle returned after creating the texture.
texunit	Unique texture unit. It's value should be one of the 80 values supported by the system.

Methods

Name	Description
Cleanup	Releases resources.
Init	Assigns unique texture unit to texunit.
LoadTextTexture	Used to render text. It generates texture from GDI bitmap.
LoadTextTextureImage	Used to render text. It wraps 2D image from GDI bitmap.
LoadTexture	Loads texture from an image file.
MakeActive	Makes the texture active.

TexturedCube

TexturedCube is derived from BaseGeometry class. It assembles vertex data generated by mesh in VBO and EBO buffers. Later  sent to GPU using shader programs.

Members

Name	Description
filename	File name of the image file containing the texture.
texutl	Texture utility to aid rendering texture.

Methods

Name	Description
Init	This method overrides the base class Init method. It creates the mesh object. Generates the vertices and texture data in non indexed mode and later sets them up in VBO buffers of the GPU. It later loads the texture from the image file.
UpdateUniforms	This method overrides the base class UpdateUniforms method. The base class method updates the "transform" matrix uniform by calling GetTransformationMatrix method. It updates the uniform "tex" containing the texture coordinates from to the loaded texture.
vertexShaderSource	This method overrides the base class vertexShaderSource method. It returns the vertex shader code to render the cube object.
fragmentShaderSource	This method overrides the base class fragmentShaderSource method. It returns the fragment shader code to render the cube object.
Cleanup	Releases resources. Calls base class method and the texutl.

Output