X86 bmp fading with linear interpolation

Challenge: Thicken the top of the 24 Bpp.BMP image to white so that each pixel color is linearly interpolated between its original color and white based on its distance from the top edge. Distance pixels> = dist have not disappeared.

I created all the code and tried to guess something. It only seems to work for distances that have a power of 2, so 32,64,128, etc. I am using linear interpolation equality and I have no idea why it doesn't work for other numbers. Any ideas?:.)

Image example:

After fading away, I get ...

global fadetop

; parm.
%define     img         [ebp+8]
%define     width       [ebp+12]
%define     height      [ebp+16]
%define     dist        [ebp+20]
; local
%define     row_bytes      [ebp-4]
%define     dist_counter   [ebp-8]

fadetop:
; create stack frame
push    ebp
mov     ebp, esp
sub     esp, 8

; push register on stack
push    ebx
push    esi
push    edi

; calculate size of row 
mov     edx, width              
lea     edx, [edx+edx*2]        
add     edx, 3                  
and     edx, 0fffffffch         
mov     row_bytes, edx          

; address of datas -calculations to write pixels from top left corner
mov eax,height
mul edx
sub eax ,row_bytes
add eax,3
mov     esi, img
add esi,eax

;line couter 

xor edx, edx
mov dist_counter , edx
line:    
; pixel counter in line 
mov     edi, width

; index rexister 
xor     ebx, ebx

; INTERPOlATION STARTS NOW !!!


convert:                                  
movzx   eax, byte [esi+ebx+0]   ; take color 1
sub     eax, 255        ; sub white value
mov     ecx, dist_counter   
imul    ecx         ;color * actual position  
cdq
mov     ecx, dist       ; devide by full fade operation distance 
idiv    ecx
add     eax, 255        ; add 255 
mov     [esi+ebx+0], al     ;put in this place  

movzx   eax,byte [esi+ebx+1]    ; take color 2
sub     eax, 255
mov     ecx , dist_counter
mul     ecx
cdq
mov     ecx, dist
div     ecx
add     eax, 255
mov     [esi+ebx+1], al

movzx   eax,byte [esi+ebx+2]    ; take color 3
sub     eax, 255
mov     ecx , dist_counter
mul     ecx
cdq
mov     ecx, dist
div     ecx
add     eax, 255
mov     [esi+ebx+2], al

;________________________________________________________________
                    ;if still in one line 
    add     ebx, 3
    dec     edi
    jnz     convert

                        ; if next line 
sub     esi, row_bytes
mov     edx,dist_counter
inc     edx
mov     dist_counter,edx
mov     eax, dist
cmp     eax, edx      
jnz     line


    ; pop registers
    pop     edi
    pop     esi
    pop     ebx

    ; return trace         
    mov     esp, ebp
    pop     ebp
    ret

      

        
        
        
      

    

code in C for those who would like to run it and try fading out:

#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

void fadetop(void* img, int width, int height ,int dist );

int main (int argc, char** argv) {

    char* buff;

    if (argc != 3 ) {
        printf("Use file: %s [file]\n",argv[0]);
        return 1;

    }

    struct stat st;
    stat(argv[1], &st);

    buff = (char *) malloc(st.st_size);
    if (buff == NULL) {
        printf("Memory error!!\n");

        return 1;
    }

    int fd = open(argv[1], O_RDONLY, 0);
    if (fd == -1) {
        printf("File access error\n");
        free(buff);

        return 1;
    }

    int size = read(fd, buff, st.st_size);
    uint32_t offset    = *(uint32_t *) (buff + 0x0a);
    uint32_t width    = *(uint32_t *) (buff + 0x12);
    uint32_t height    = *(uint32_t *) (buff + 0x16);
    uint16_t bpp    = *(uint16_t *) (buff + 0x1c);

    if (bpp == 24) {
        int fd_out;
        printf("worked dist: %d , heigh: %d\n", atoi(argv[2]), height);
        fadetop(buff + offset, width , height , atoi(argv[2]) );
        fd_out = creat("fade.bmp", 0644);
        write(fd_out, buff, size);
        close(fd_out);

        printf("Image faded.\n");
    }
    else {
        printf("Invalid BMP\n");
    }

    close(fd);
    free(buff);

    return 0;
}