Question

I am having a problem to upgrade a program to shaders in OGLES 2.

This is the data structure:

namespace packt {
class GraphicsService {
public:
 ...
   struct ESMatrix{ GLfloat m[4][4]; };

    typedef struct
    {  // Handle to a program object
       GLuint programObject;
       // Attribute locations
       GLint  positionLoc;
       // Uniform locations
       GLint  mvpLoc;
       // Vertex data
       GLfloat  *vertices;
       GLuint   *indices;
       int       numIndices;
       // Rotation angle
       GLfloat   angle;
       // MVP matrix
       ESMatrix  mvpMatrix;
    } UserData;
...
  }
}

In android_main the context is set:

packt::GraphicsService lGraphicsService(pApplication,&lTimeService);
packt::Context lContext = { &lGraphicsService, &lTimeService };

But then instead of using glColor4f, which is not part of OGLES 2, now we are trying to replace that command by shaders. Here is how it goes to the setup method:

void GraphicsService::setup() {
    glEnable(GL_TEXTURE_2D);
        GLbyte vShaderStr[] =
           "uniform mat4 u_mvpMatrix;                   \n"
           "attribute vec4 a_position;                  \n"
           "void main()                                 \n"
           "{                                           \n"
           "   gl_Position = u_mvpMatrix * a_position;  \n"
           "}                                           \n";

        GLbyte fShaderStr[] =
           "precision mediump float;                            \n"
           "void main()                                         \n"
           "{                                                   \n"
           "  gl_FragColor = vec4( 1.0, 0.0, 0.0, 1.0 );        \n"
           "}                                                   \n";
        // Load the shaders and get a linked program object

        // replaced :: //glColor4f(1.0f, 1.0f, 1.0f, 1.0f); 
        UserData *userData = mContext->userData;
        userData->programObject = esLoadProgram (vShaderStr, fShaderStr);
    glDisable(GL_DEPTH_TEST);
}

Which gives the following compiler’s message:

error: UserData *userData = mContext->userData;
    - 'void*' is not a pointer-to-object type
error: userData->programObject = esLoadProgram (vShaderStr, fShaderStr);
    - invalid conversion from 'GLbyte*' to 'const char*'
    - initializing argument 1 of 'GLuint esLoadProgram(const char*, const char*)'
    - initializing argument 2 of 'GLuint esLoadProgram(const char*, const char*)'

Any suggestion how to fix it? All comments are highly appreciated.

Answer 1

1.

The context object, pointed to by mContext, contains a “userData” field, which is probably declared to be a void*. You are storing a pointer to your actual UserData object in that field. Conversion from UserData* to void* works implicitly, but to get a UserData* from a void*, you need an explicit cast:

UserData *userData = reinterpret_cast<UserData*>( mContext->userData );

Note that reinterpret_cast is potentially dangerous; when you use it, make sure that your void* really came from a UserData* in the first place, otherwise Bad Things Will Happen.

2.

The error message says it all: esLoadProgram expects arguments of type const char*, and you are passing arguments of type GLbyte*. Declare your vShaderStr and fShaderStr appropriately, as in:

const char *vShaderStr =
    "uniform ...