Direct Texture Manipulation

[Jae686]

Good Evening.

I've been attempting to directly modify the texture returned glutCopyBufferToTexture(); function, but I've been unable to access and modify its contents via the pvr_ptr_t.

In this example I'm attempting to convert a a RGB 565 texture to gray scale (without changing the texture format of course)

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void pvr_to_grayscale(pvr_ptr_t texture_ptr, long unsigned int w, long unsigned int h)
{
    uint16 bytemask_r = 0xF800 ;
    uint16 bytemask_g = 0x7E0 ;
    uint16 bytemask_b = 0x1F ;

    uint16 r = 0 ;
    uint16 g = 0 ;
    uint16 b = 0 ;

    uint16 grey = 0 ;  

    unsigned int i, j = 0;

    unsigned int pixel_count = h * w ;

    for ( ; i < w ; i++)
    {
        for( ; j < h ; j++)
        {
            r =   ((uint16**) texture_ptr[i][j] & bytemask_r ) >> 11 ;
            g = ( ((uint16**) texture_ptr[i][j]) & bytemask_g ) >> 5 ;
            b = ( ((uint16**) texture_ptr[i][j]) & bytemask_b ) ;

            grey = (r+g+b)/3 ;

            (uint16**)texture_ptr[i][j] = (grey << 11) ;
            (uint16**)texture_ptr[i][j] = ((uint16**)texture_ptr[i][j] | (grey << 5)) ;
            (uint16**)texture_ptr[i][j] = (uint16**)texture_ptr[i][j] | grey ;
        }
    }

}

But I cant access it texture that way since i get the following error on compile :

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jaerder@jaerder-G50V ~/development/Projectos Software/vidfdc/dreamcast $ make
rm -f bin/main.elf romdisk.*
kos-cc  -c src/dc_render.c -o src/dc_render.o
src/dc_render.c: In function ‘pvr_to_grayscale’:
src/dc_render.c:63:42: warning: dereferencing ‘void *’ pointer [enabled by default]
src/dc_render.c:63:45: error: subscripted value is neither array nor pointer nor vector
src/dc_render.c:64:42: warning: dereferencing ‘void *’ pointer [enabled by default]
src/dc_render.c:64:45: error: subscripted value is neither array nor pointer nor vector
src/dc_render.c:65:42: warning: dereferencing ‘void *’ pointer [enabled by default]
src/dc_render.c:65:45: error: subscripted value is neither array nor pointer nor vector
src/dc_render.c:69:34: warning: dereferencing ‘void *’ pointer [enabled by default]
src/dc_render.c:69:37: error: subscripted value is neither array nor pointer nor vector
src/dc_render.c:70:34: warning: dereferencing ‘void *’ pointer [enabled by default]
src/dc_render.c:70:37: error: subscripted value is neither array nor pointer nor vector
src/dc_render.c:70:65: warning: dereferencing ‘void *’ pointer [enabled by default]
src/dc_render.c:70:68: error: subscripted value is neither array nor pointer nor vector
src/dc_render.c:71:34: warning: dereferencing ‘void *’ pointer [enabled by default]
src/dc_render.c:71:37: error: subscripted value is neither array nor pointer nor vector
src/dc_render.c:71:64: warning: dereferencing ‘void *’ pointer [enabled by default]
src/dc_render.c:71:67: error: subscripted value is neither array nor pointer nor vector

Any suggestions ?

[BlueCrab]

You need to cast the pointer before you try to index it. Something like this should work:

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*(((uint16 *)texture_ptr) + (i * w) + j) = (grey << 11) | (grey << 5) | grey;

[Jae686]

Thank you BlueCrab!
It worked after this edit was suggested :

Code: Select all

void pvr_to_grayscale(pvr_ptr_t texture_ptr, long unsigned int w, long unsigned int h)
{
    uint16** txt_ptr = texture_ptr;

    uint16 bytemask_r = 0xF800 ;
    uint16 bytemask_g = 0x7E0 ;
    uint16 bytemask_b = 0x1F ;

    uint16 r = 0 ;
    uint16 g = 0 ;
    uint16 b = 0 ;

    uint16 grey = 0 ;  

    unsigned int i, j = 0;

  

    for ( ; i < w ; i++)
    {
        for( ; j < h ; j++)
        {
            r =   (txt_ptr[i][j] & bytemask_r ) >> 11 ;
            g =   (txt_ptr[i][j] & bytemask_g ) >> 5 ;
            b =   (txt_ptr[i][j] & bytemask_b ) ;

            grey = (r+g+b)/3 ;

            txt_ptr[i][j] = (grey << 11) ;
            txt_ptr[i][j] = ( txt_ptr[i][j] | (grey << 5)) ;
            txt_ptr[i][j] = txt_ptr[i][j] | grey ;
        }
    }

}

I wonder why the texture continues to be in color. Should I bind the texture again or create another texture ?
If I have to create another texture in run-time I suppose it would not be very efficient....

[BlueCrab]

Short version:
I'm pretty sure that the way you're doing it (with a uint16 **) is not going to work. Try it the way I suggested and see if that works.

Longer version:
I believe that the C standard treats a double-pointer like what you have way differently than you'd be expecting. The compiler doesn't know the width of a row, so it can't treat things like it would a 2D array. With a double-pointer, you can think of it doing something like this:

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uint16 **ptr = /* SOMETHING */;

uint16 *ptr2 = ptr[i]; /* i.e, it's expecting to find another pointer here -- this will NOT be the case in your code. */
ptr2[j] = /* SOMETHING ELSE */;

I'm not entirely sure that really explains it well to anyone but me...

[Quzar]

For sure double pointers do not work that way. A double pointer is a pointer to a pointer. You need to feed the row length*rows as an offset in order for it to know.