Drawing In a Linear Framebuffer: Difference between revisions
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Remove potentially confusing statement and also indicate the length of the framebuffer in bytes |
m use pitch and pixelwidth in the examples |
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Finally, in VESA modes, you usually have truecolor or hicolor, and in both of them, you have to give independent red, green and blue values for each pixel. modeinfo will (again) instruct you of how the RGB components are organized in the pixel bits. E.g. you will have <tt>xRRRRRGGGGGBBBBB</tt> for 15-bits mode, meaning that #ff0000 red is there <tt>0x7800</tt>, and #808080 grey is <tt>0x4210</tt> (pickup pencil, draw the bits and see by yourself)
<source lang="c">
static void putpixel(unsigned char* screen, int x,int y, int color) {
unsigned where = x*
screen[where] = color & 255; // BLUE
screen[where + 1] = (color >> 8) & 255; // GREEN
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=== Optimizations ===
It can be tempting from here to write fill_rect, draw_hline, draw_vline, etc. from calls to putpixel ... don't. Drawing a filled rectangle means you access successive pixels and then advance by "pitch - rect_width" to fill the next line. If you do a "for(y=100;y<200;y++) for(x=100;x<200;x++) putpixel (screen,x,y,RED);" loop, you'll recompute 'where' about 10,000 times. Even if the compiler has done good job to translate y*
<source lang="c">
static void fillrect(unsigned char *vram, unsigned char r, unsigned char g, unsigned char b, unsigned char w, unsigned char h) {
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for (j = 0; j < h; j++) {
//putpixel(vram, 64 + j, 64 + i, (r << 16) + (g << 8) + b);
where[j*
where[j*
where[j*
}
where+=
}
}
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