OpenEuphoria

2. Re: Graphics...

• Posted by Pete Eberlein <xseal at HARBORSIDE.COM> May 11, 1999
• 436 views

> If I want to draw a rectangle with vertices
> {0,0},{640,0},{640,480},{0,480}, would I need to translate that into
> {0,0},{1280,0},{1280,480},{0,480} while using a 65k mode? 24-bit?
> 32-bit? 15-bit?

vesa20.e sets the global variable "bytes_per_pixel" to the correct value
for each mode.  You only have to multiply each x-value by bytes_per_pixel
in your program to get the correct position.

bits_per_pixel  bytes_per_pixel
15              2
16              2
24              3
32              4

> How do I change colors in these bits per pixel modes:

All these modes use colors based on the red+green+blue intensities that
make up the color.  The following code examples assume red, green & blue
values are in the range from 0 to 255.

> 15?

Use the upper 5 bits of red and
    the upper 5 bits of green and
    the upper 5 bits of blue...

first_byte  = floor(blue / 8) + and_bits(green * 4, #E0)
second_byte = floor(green / 64) + and_bits(red / 2, #7C)

> 16?

Use the upper 5 bits of red and
    the upper 6 bits of green and
    the upper 5 bits of blue...

first_byte  = floor(blue / 8) + and_bits(green * 8, #E0)
second_byte = floor(green / 32) + and_bits(red, #F8)

> 24?

first_byte  = blue
second_byte = green
third_byte  = red

> 32?

first_byte  = blue
second_byte = green
third_byte  = red
fourth_byte = 0


> How many bits are there to a byte? Isn't it 8?  So in 16-bit modes it
> would be 2 bytes per pixel, 24-bit would be 3, and 32 would be 4,
> correct?  So to correct the problem from above, I could set a
> 'difference' variable to multiply the lengths of lines by...right?

The vesa20.e global variable "bytes_per_line" will always have the exact
length of the lines.  Please use it instead of trying to calculate it
yourself.  Some graphics cards will use 1024 instead of 640 for the
scanline length even though the rightmost 384 bytes are off the edge of
the screen.

> Thank you all,
> Mike Hurley

I should probably go put some of this stuff in the docs now... 
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