CODEWITHSUNDEEP
assemblybitmapmipsmars-simulator
user366312
user366312

Reputation: 17008

Loading the red component of a 24-bit bitmap image into an array

Suppose, I have a 200 x 200 pix 24-bit BMP image.
I want to load the red component into an array named list. 

image: .space 120054
list: .space 8160  

# ... ... ...

LoadGreenComponentToList:
    sub $sp, $sp, 4     #push $ra to the stack
    sw $ra,4($sp)

    la $t1, image + 10  #adress of file offset to pixel array
    lw $t2, ($t1)       #file offset to pixel array in $t2

    li  $t5, BMP_FILE_SIZE
    sub $t5, $t5, $t2   # ? 
    div $t5, $t5, 3     # ?

    la $t1, image       # adress of bitmap
    add $t2, $t1, $t2   # adress of pixel array in $t2

    #fill the array with red values
    add $t2, $t2, 1 # ? 
    li $t3, 0       # $t3 is the counter of loop
    li $t4, 0

loop_through_pixels:
    beq $t3, $t5, get_pixel_end
    la $t6, list        # $t6 = array address

    mul $t4, $t3, 3     # $t4 = 3* $t3
    add $t4, $t2, $t4   # $t4 = $t4 + $t2
    lb $t1,($t4)        # ?

    #save to array
    #sb $t1, ($t6)
    #add $t6, $t6, 4

    #inc array
    mul $t1, $t1, 4 #or 4
    add $t6, $t6, $t1
    lw $t7, ($t6)   
    addi $t7, $t7, 1
    sw  $t7, ($t6)

    add $t3, $t3, 1
    j loop_through_pixels 

get_pixel_end:                                              
    lw $ra, 4($sp)      #restore (pop) $ra
    add $sp, $sp, 4
    jr $ra

This code loads the green component.

How can I convert it to operate on Red?

Also, I think, the size of list is not correct.

Upvotes: 0

Views: 190

Answers (1)

Simon Whitehead
Simon Whitehead

Reputation: 65077

Assuming they're stored <red_byte><green_byte><blue_byte> or <blue_byte><green_byte><red_byte> then I imagine this is your issue:

la $t1, image       # adress of bitmap
add $t2, $t1, $t2   # adress of pixel array in $t2

#fill the array with red values
add $t2, $t2, 1 # ?

Essentially that is the equivalent of this pseudo-code:

PixelBuffer * buf = GetBitmap();
buf++;

So all of a sudden you're starting off the pixel iteration by not being at the base of the pixel buffer (you're pointing at the second byte).

[r][g][b][r][g][b][r][g][b][r][g][b][r][g][b][r][g][b][r][g][b]
    ^ -- pointing here at the start of the loop

This line further down:

mul $t4, $t3, 3     # $t4 = 3* $t3
add $t4, $t2, $t4   # $t4 = $t4 + $t2

This is jumping by 3's - to skip into each next full pixel component:

[r][g][b][r][g][b][r][g][b][r][g][b][r][g][b][r][g][b][r][g][b]
    ^ ------ ^ ------ ^ ------ ^ ------ ^ ------ ^ ------ ^

So, in either of the two cases (I can't remember what order the pixel data is stored in ..), either remove that first block of code:

########### REMOVED: add $t2, $t2, 1 # ?

...if its RGB. Or, change it to this:

add $t2, $t2, 2 # ?

...if its BGR.

I am almost certain BMPs are stored BGR, so option 2 is probably the correct call.

Upvotes: 2

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