Blitter line drawing: nothing happens

[Crank]

I'm stuck with getting the Blitter to draw a simple line. I'm not sure what I'm doing wrong for not getting anything visible. Here's the C code:

while (!(REG_DMACON_R & (1 << 6))); // Wait for blitter to be ready

const unsigned int dx = 40;
const unsigned int dy = 20;
const unsigned int x1 = 7;

REG_BLTADAT_W = 0x8000;
REG_BLTBDAT_W = 0xFFFF; // Solid line
REG_BLTAFWM_W = 0xFFFFFFFF;

REG_BLTAMOD_W = 4*(dy - dx);
REG_BLTBMOD_W = 4*dy;
REG_BLTCMOD_W = 320/8; // Width of bitplane (bytes)
REG_BLTDMOD_W = 320/8; // Width of bitplane (bytes)

REG_BLTAPTH_W = 4*dy - 2*dx;
REG_BLTCPTH_W = bitplane_0_address + (32*320/8); // First word (16bit) where the drawing starts
REG_BLTDPTH_W = bitplane_0_address + (32*320/8); // First word (16bit) where the drawing starts

REG_BLTCON0_W = ((x1 & 0x0F) << 12) |
((1 << 8) | (1 << 9) | (1 << 11)); // SRCA/SRCC/SRDD

REG_BLTCON1_W = 1 | // Line mode
(0 << 2); // Octant number: bits 2-4

REG_BLTSIZE_W = ((dx + 1) << 6) | 2;

// ----------------

And here are the register definitions, just in case:

#define REG_BPL1PT_W *((volatile unsigned int*) 0xDFF0E0)
#define REG_BPL2PT_W *((volatile unsigned int*) 0xDFF0E4)

#define REG_BLTCON_W *((volatile unsigned int*) 0xDFF040)
#define REG_BLTCON0_W *((volatile unsigned short*) 0xDFF040)
#define REG_BLTCON1_W *((volatile unsigned short*) 0xDFF042)

#define REG_BLTAFWM_W *((volatile unsigned int*) 0xDFF044)

#define REG_BLTAPTH_W *((volatile unsigned int*) 0xDFF050)
#define REG_BLTBPTH_W *((volatile unsigned int*) 0xDFF04C)
#define REG_BLTCPTH_W *((volatile unsigned int*) 0xDFF048)
#define REG_BLTDPTH_W *((volatile unsigned int*) 0xDFF054)

#define REG_BLTAMOD_W *((volatile unsigned short*) 0xDFF064)
#define REG_BLTBMOD_W *((volatile unsigned short*) 0xDFF062)
#define REG_BLTCMOD_W *((volatile unsigned short*) 0xDFF060)
#define REG_BLTDMOD_W *((volatile unsigned short*) 0xDFF066)

#define REG_BLTSIZE_W *((volatile unsigned short*) 0xDFF058)

#define REG_BLTADAT_W *((volatile unsigned short*) 0xDFF074)
#define REG_BLTBDAT_W *((volatile unsigned short*) 0xDFF072)
#define REG_BLTCDAT_W *((volatile unsigned short*) 0xDFF070)

#define REG_BLTAFWM_W *((volatile unsigned int*) 0xDFF044)

// ----------------

What am I doing wrong? I've tried all 8 possible value choices for the octant but nothing is visible on the screen.

[StingRay]

First question that springs to mind (without checking the code): is blitter DMA enabled?

[Crank]

Hmm, I have a feeling that it is not. Now I have to check how to enable it.

But then again, if I'm able to use blitter to clear rectangular areas, does this mean the DMA is on?

[Crank]

Aha!

If I fill the screen with white color and see what the line drawing routine does, I get the two black boxes (see the attached image).

[Photon]

Looks like the low byte of BLTCON0 (the minterm) is empty? One normal value is $fa.

[Crank]

Hey, now it works! Thanks!

My next stop is to check if there is a way to draw the line with subpixel accuracy. I think this is possible, based on some website which showed that subpixel accuracy is possible when doing polygon filling (used blitter for line drawing IIRC).

[hooverphonique]

Originally Posted by Crank

Hey, now it works! Thanks!

My next stop is to check if there is a way to draw the line with subpixel accuracy. I think this is possible, based on some website which showed that subpixel accuracy is possible when doing polygon filling (used blitter for line drawing IIRC).

This? https://scalibq.wordpress.com/2011/1...t-real-part-5/

[LaBodilsen]

This might be of interest
[ Show youtube player ]

With asm source here:
https://github.com/niklasekstrom/blitter-subpixel-line

[Crank]

@hooverphonique:
Yes, that was the page

@LaBodilsen:
Nice, thanks! I gotta read that carefully.

I'll report back here if/when I get something working

[Crank]

BTW:
Is there some fast way to clip the lines to screen borders using hardware or do I need to just calculate the clip locations myself?

[StingRay]

There is no built-in hardware support for clipping lines. Go and code your clipping routine.

[Crank]

OK, I will

(attached is an image of a buggy version, I was going to ask about the bug but realized I had a buggy "wait for blitter to finish" routine)

[LaBodilsen]

Originally Posted by StingRay

There is no built-in hardware support for clipping lines. Go and code your clipping routine.

That is something i never got around to myself.

Any hints on how to approach this, with limited math skils (goes as far as rotating a point and pythagoras).

[StingRay]

One hint: Cohen-Sutherland. It's a pretty straightforward clipping algorithm which can be easily implemented. Shouldn't take more than an afternoon of work to have a completely working 68k asm version.

[LaBodilsen]

Originally Posted by StingRay

One hint: Cohen-Sutherland. It's a pretty straightforward clipping algorithm which can be easily implemented. Shouldn't take more than an afternoon of work to have a completely working 68k asm version.

I was afraid you where gonna say that. I have looked at it before, but it never clicked with me, however i just found an example in C in the wikipage, which makes it a little more clear. So i might give it a go now.

[Crank]

Originally Posted by LaBodilsen

I was afraid you where gonna say that. I have looked at it before, but it never clicked with me, however i just found an example in C in the wikipage, which makes it a little more clear. So i might give it a go now.

Think of it like this:

1. Take one of the four screen borders and test if line vertices are on different sides of that border. If they are, clip the line.

2. Repeat #1 with the clipped line using the next screen border.

Once you've done it with all four screen borders, you're done.

Cohen-Sutherland algorithm optimises the number of borders you actually need to test/clip against.

[Niklas]

Originally Posted by Crank

Hey, now it works! Thanks!

My next stop is to check if there is a way to draw the line with subpixel accuracy. I think this is possible, based on some website which showed that subpixel accuracy is possible when doing polygon filling (used blitter for line drawing IIRC).

Hi, I have the repository https://github.com/niklasekstrom/blitter-subpixel-line that was mentioned above.

I just updated it with the document "Sub-pixel accurate rasterization of polygons using the Amiga blitter.pdf" that describes how to calculate the values that the blitter should be programmed with in order to do sub-pixel accurate rasterization. It builds on the document "Drawing lines using the Amiga blitter.pdf" so you probably have to read both documents to understand what is going on, but as you have already gotten line drawing using the blitter to work you will probably understand most things quickly.

I'm happy to answer any questions!

[ross]

Originally Posted by Niklas

Hi, I have the repository https://github.com/niklasekstrom/blitter-subpixel-line that was mentioned above.

I just updated it with the document "Sub-pixel accurate rasterization of polygons using the Amiga blitter.pdf" that describes how to calculate the values that the blitter should be programmed with in order to do sub-pixel accurate rasterization. It builds on the document "Drawing lines using the Amiga blitter.pdf" so you probably have to read both documents to understand what is going on, but as you have already gotten line drawing using the blitter to work you will probably understand most things quickly.

I'm happy to answer any questions!

Hi Niklas and välkommen

Your documents are a good read, thanks!

[EDIT]
Are you the ex-user Scali?
http://eab.abime.net/showthread.php?t=62607

[Niklas]

Originally Posted by ross

Hi Niklas and välkommen

Your documents are a good read, thanks!

[EDIT]
Are you the ex-user Scali?
http://eab.abime.net/showthread.php?t=62607

Hi, and thanks

I'm not Scali.

[StingRay]

Originally Posted by ross

Are you the ex-user Scali?

Scali is located in Neverlands , not in Sweden.