59 lines
1.2 KiB
C
59 lines
1.2 KiB
C
#include <display.h>
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/* dreverse() - Reverse Video RAM area */
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void dreverse(display_t *disp, int x, int y, int width, int height)
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{
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int vram_offset_y;
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int j;
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// Check error.
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if (width < 0 || height < 0)
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return;
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// Get "real" X position and area width.
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if (x < 0)
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{
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width = width + x;
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x = 0;
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} else {
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if (x + width >= (int)disp->display.width)
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width = disp->display.width - x;
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}
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// Get "real" Y position and area height.
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if (y < 0)
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{
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height = height + y;
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y = 0;
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} else {
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if (y + height >= (int)disp->display.height)
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height = disp->display.height - y;
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}
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// Check potential error.
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// @note we do not check height because the while()
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// while do the job for us.
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if (width < 0)
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return;
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// Generate VRAM offset for Y axis.
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// @note:
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// The screen width size is always 128 and we
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// use 4-aligned Video RAM so 32 pixels per "slot"
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// and 128 / 32 = 4.
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// y * 4 can be optimised by used shift operator,
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// this is why we use y << 2 because 2^2 = 4.
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vram_offset_y = (y + height - 1) << 2;
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// Reverse area
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while (--height >= 0)
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{
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j = width + x;
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while (--j >= x)
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{
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disp->vram[(j >> 5) + vram_offset_y] ^= 0x80000000 >> (j & 31);
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}
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vram_offset_y = vram_offset_y - 4;
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}
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}
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