Cel-Shading对BMP模型纹理的影响？

Question

I have stumbled across Nehe's Cel-Shading Tutorial and I was wondering if it was possible to have this technique applied to textured geometry....

I understand that in that code the effect is obtained by applying one uni-color texture to the model, the relevant code from the tutorial I think is the following for loading the shader effect:

In = fopen ("Data\\shader.txt", "r");           // Open The Shader File

if (In)                         // Check To See If The File Opened
{
    for (i = 0; i < 32; i++)         // Loop Though The 32 Greyscale Values
    {
        if (feof (In))              // Check For The End Of The File
            break;

        fgets (Line, 255, In);          // Get The Current Line

// Copy Over The Value
        shaderData[i][0] = shaderData[i][1] = shaderData[i][2] = atof (Line);
    }

    fclose (In);                    // Close The File
}

else
    return FALSE;  

glGenTextures (1, &shaderTexture[0]);           // Get A Free Texture ID

glBindTexture (GL_TEXTURE_1D, shaderTexture[0]);    // Bind This Texture. From Now On It Will Be 1D

// For Crying Out Loud Don't Let OpenGL Use Bi/Trilinear Filtering!
glTexParameteri (GL_TEXTURE_1D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri (GL_TEXTURE_1D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);

// Upload
glTexImage1D (GL_TEXTURE_1D, 0, GL_RGB, 32, 0, GL_RGB , GL_FLOAT, shaderData);

And the following for enabling the effect for drawing

// Cel-Shading Code
glEnable (GL_TEXTURE_1D);               // Enable 1D Texturing
glBindTexture (GL_TEXTURE_1D, shaderTexture[0]);    // Bind Our Texture

glColor3f (1.0f, 1.0f, 1.0f);  

....

Is there a way to run that on a model textured with a BMP GL_TEXTURE_2, and obtain a cel-shaded look of that texture?

Answer 1

You don't actually need a 1D texture for cel shading. You can make it procedural by:

// GLSL fragment shader
vec3 colour = /* a value in range 0..1, possibly from a texture */
colour -= mod(colour, 0.2) /* limit the colour range for cel-shading */

This works because x - mod(x, M) results in a nice stepping function, M controlling the step height.

Answer 2

In his tutorial on gamedev.net MENTAL gives an approach on how to apply Cel-shaded look to a textured model.

However that approach doesn't give the desired results - the lighting appears to be very "smooth" - that is because OpenGL interpolates color values along the polygon when switched to GL_SMOOTH mode. I am using an old-style OpenGL 1.1 immediate mode and do not have any clue about shaders.

My approach is either to use multitexturing from ARB extentions or to play with alpha channel and blending. Conceptually you just set up 2 textures - one 2D for object's texture and second is 1D - for sharp lighting.

You compute the second's texture coordinate by dot product and the you either use multi-texturing through extentions (haven't tried it myself though) or render your object in 2 paths applying alpha-test and blending (or just a blending). I'll try to post a source code for my experiments shortly after to exeplify this approach.